Decoding the Dot: A Deep Dive into Barcode Label Printer Electronics - Extended Section 10 |
Subtitle: Stepper Motor Drive Principles - The Heart of Paper Movement |
Introductory Summary |
We have spent many chapters exploring the printhead - how it heats, how it is driven, and how it is kept cool. But a printer is not just a static heating element. It must move the paper (or the ribbon) precisely, step by step, so that each line of dots lands exactly where it should. This movement is the job of the stepper motor - a special type of electric motor that rotates in fixed angular increments, or steps, rather than continuously. In a barcode printer, two stepper motors are typically used: one for the platen roller that feeds the paper, and one for the ribbon rewind that takes up the used ribbon. The platen motor must be incredibly precise - a single step misalignment of even a few micrometers will cause the dots to overlap or leave gaps, resulting in unreadable barcodes. This chapter is devoted entirely to stepper motor drive principles. We will explain what a stepper motor is, why it is used in printers, and how it works - from the basic electromagnetic principles to the practical drive circuits. We will explore the different types of stepper motors (unipolar and bipolar), the winding configurations, and the drive sequences (full-step, half-step, and micro-stepping). We will look at real-world designs from major semiconductor companies: Texas Instruments' integrated stepper motor drivers with advanced current control, Allegro's micro-stepping drivers with built-in translator and over-current protection, STMicroelectronics' motor driver ICs with SPI configuration, and Toshiba's high-voltage drivers for industrial printers. We will examine the critical timing relationships between the step pulses and the printhead strobe, the current regulation techniques, and the thermal management of the motor drivers. By the end, you will understand why the stepper motor is the unsung hero of the printer, and you will appreciate the elegant control algorithms that make it move with such precision. |
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Chapter 1: What Is a Stepper Motor and Why Use It |
A stepper motor is a brushless DC motor that divides a full rotation into a number of equal steps. Unlike a regular DC motor that spins continuously when you apply voltage, a stepper motor moves in discrete increments. Each step corresponds to a fixed angle - typically 1.8 degrees for a 200-step-per-revolution motor, or 0.9 degrees for a 400-step motor. The motor has multiple coils (windings) that are energized in a sequence. Each time the sequence changes, the motor moves one step. The position of the motor is known at all times - you do not need an encoder for feedback (though some printers use encoders for closed-loop control). This makes stepper motors ideal for printers, where you need precise control over the paper position. The stepper motor is also reliable, simple to drive, and relatively inexpensive. The disadvantages are that they consume power even when stationary (holding torque), and they can vibrate at certain speeds, causing acoustic noise and mechanical resonance. |
Design Example: 200-Step Motor in Brother Printers |
Brother's QL series printers use a standard 200-step-per-revolution stepper motor with a 1.8-degree step angle. The motor is a NEMA 17 frame size with a holding torque of 0.4 newton-meters. The motor is connected to the platen roller through a 4:1 gear reduction, so each step of the motor moves the roller by 0.45 degrees. The roller has a diameter of 10 millimeters, so each step moves the paper by 0.45/360 * pi * 10 = 0.039 millimeters - which corresponds to about 1/6 of a dot pitch at 203 dpi. This fine resolution allows the printer to position the paper with sub-dot accuracy. The manufacturer chose a 200-step motor because it is a standard part with good availability and low cost. The motor is driven by a dedicated driver IC that we will discuss later. |
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Chapter 2: The Basic Principle - Electromagnets and Rotors |
A stepper motor has two main parts: the rotor and the stator. The rotor is a permanent magnet that rotates. The stator has multiple electromagnets (coils) arranged around the rotor. When you energize one of the coils, it creates a magnetic field that attracts the rotor's magnetic poles, causing the rotor to align with that field. When you de-energize that coil and energize the next one, the rotor moves to align with the new field. The rotor has teeth or magnetic poles that are arranged so that only one set of teeth aligns with the energized coil. The number of teeth determines the step angle. In a typical hybrid stepper motor, the rotor has 50 teeth (for a 200-step motor), and the stator has 8 poles (for a bipolar motor) or 12 poles (for a unipolar motor). The sequence of energizing the coils determines the direction and the step size. |
Design Example: Hybrid Stepper Motor in Zebra Printers |
Zebra's industrial printers use a hybrid stepper motor - a motor that combines the principles of the permanent magnet motor and the variable reluctance motor. The hybrid motor has a rotor with 50 teeth, giving 200 steps per revolution. The motor has two phases, each with a pair of coils. The motor is driven in a bipolar mode, which we will discuss shortly. Zebra's motor is a larger NEMA 23 frame size with a holding torque of 1.2 newton-meters, suitable for driving a heavy platen roller and a ribbon mechanism. The manufacturer chose the hybrid motor because it provides high torque and precise positioning. |
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Chapter 3: Unipolar vs. Bipolar Motors - Two Different Worlds |
There are two types of stepper motors: unipolar and bipolar. A unipolar motor has a center tap on each winding. The center tap is connected to the power supply, and the two ends of the winding are switched to ground. This means that the current flows in only one direction through each half of the winding - hence 'unipolar.' The unipolar motor is simpler to drive because you only need four transistors (one for each end of the two windings). However, the unipolar motor uses only half of the winding at a time, so it produces less torque. A bipolar motor has no center tap. The current flows through the entire winding, and the direction of the current is reversed by an H-bridge circuit. The bipolar motor produces more torque because it uses the full winding. However, the H-bridge is more complex, requiring four transistors per winding (eight in total). Most modern printers use bipolar motors for their higher torque and efficiency. |
Design Example: Bipolar Motor in Sato Printers |
Sato's printers use a bipolar stepper motor for the platen roller. The motor has two phases, each with a single winding. The motor is driven by an H-bridge driver IC. The manufacturer chose the bipolar motor because it provides a high holding torque and a good torque-speed characteristic. The bipolar motor also allows the use of micro-stepping, which reduces the acoustic noise and the mechanical resonance. The manufacturer reports that the bipolar motor gives a smoother paper feed and a quieter operation than a unipolar motor. |
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Chapter 4: The H-Bridge - The Heart of Bipolar Drive |
An H-bridge is a circuit that allows the direction of the current through a load to be reversed. The H-bridge consists of four switches (transistors) arranged in a bridge configuration: two high-side switches (connected to the positive supply) and two low-side switches (connected to ground). The load (the motor winding) is connected between the two high-side switches and the two low-side switches. When switches A and D are closed, current flows through the winding in one direction. When switches B and C are closed, the current flows in the opposite direction. The H-bridge also allows the current to be freewheeled (recirculated) when the transistors are turned off, which reduces the voltage spikes. The H-bridge is controlled by the motor driver IC. The driver IC generates the gate signals for the four transistors. The driver IC also includes the protection features - over-current, over-voltage, and thermal shutdown. |
Design Example: H-Bridge in Texas Instruments DRV8825 |
The DRV8825 from Texas Instruments is a stepper motor driver IC that contains two full H-bridges - one for each phase of the motor. The H-bridges are built with N-channel MOSFETs with a low on-resistance (0.5 ohms). The DRV8825 includes a built-in current sense amplifier, a micro-stepping indexer, and a thermal shutdown. The H-bridge can operate from a supply voltage of 8 to 45 volts and can deliver up to 2.5 amperes per phase. In a design from a European printer manufacturer, the DRV8825 is used to drive the platen motor. The manufacturer chose the DRV8825 because of its high integration - it reduces the component count and the PCB area. The manufacturer also appreciates the built-in current regulation, which we will discuss later. |
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Chapter 5: The Drive Sequence - Full-Step and Half-Step |
The drive sequence determines how the motor phases are energized. The simplest sequence is the full-step sequence. For a two-phase bipolar motor, the full-step sequence is: Phase A+ energized, Phase B+ energized; then Phase A+ energized, Phase B- energized; then Phase A- energized, Phase B- energized; then Phase A- energized, Phase B+ energized. This sequence gives 200 steps per revolution for a 50-tooth rotor. The full-step sequence produces a high torque but can cause vibration. The half-step sequence alternates between energizing one phase and energizing two phases: Phase A+; Phase A+ and B+; Phase B+; Phase B+ and A-; Phase A-; Phase A- and B-; Phase B-; Phase B- and A+. This sequence gives 400 steps per revolution (double the resolution) and reduces the vibration. However, the torque is lower in the single-phase steps. |
Design Example: Half-Step in Brother Printers |
Brother's printer uses the half-step sequence for the platen motor. The manufacturer chose the half-step sequence because it gives a higher resolution and a smoother motion than the full-step sequence. The half-step sequence also reduces the acoustic noise. The manufacturer measured the paper feed accuracy and found it to be within 0.02 millimeters - which is sufficient for 203 dpi printing. The half-step sequence is implemented in the firmware: the CPU generates the sequence of phase outputs and sends them to the motor driver IC. |
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Chapter 6: Micro-Stepping - The Ultimate in Smoothness |
Micro-stepping is a technique where the current in the phases is not just on or off, but is gradually increased and decreased. Instead of energizing a phase fully, the phase current is set to a fraction of the full current. The fractions are chosen so that the vector sum of the two phase currents points in the desired direction. For example, in 1/8 micro-stepping, the current is divided into 8 steps per full step. The phase currents follow a sine and cosine relationship: I_A = I_max * sin(theta), I_B = I_max * cos(theta). The motor moves smoothly, with very little vibration. Micro-stepping also reduces the acoustic noise and the mechanical resonance. The resolution is increased by the micro-stepping factor - 1/8 micro-stepping gives 1600 steps per revolution. The only disadvantage is that the torque is reduced at the micro-step positions because the currents are not at their maximum values. |
Design Example: 1/16 Micro-Stepping in Zebra Printers |
Zebra's high-end printers use 1/16 micro-stepping for the platen motor. The motor driver IC (Allegro A4988) has a built-in micro-stepping indexer that generates the sine and cosine current waveforms. The CPU sends only the step and direction signals - the driver IC handles the rest. The manufacturer chose 1/16 micro-stepping because it gives the smoothest motion and the lowest noise. The paper feed is so smooth that the printer can operate at 14 inches per second without any audible vibration. The manufacturer also uses the micro-stepping to achieve a very fine resolution - 1/16 of 200 steps gives 3,200 steps per revolution, and with the 4:1 gear reduction, the paper moves by 0.0025 millimeters per step - far beyond the printer's resolution. |
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Chapter 7: The Step and Direction Signals - The Simple Interface |
The motor driver IC is typically controlled by two signals: STEP and DIR. The STEP signal is a pulse. Each pulse causes the motor to move one step (or one micro-step). The DIR signal determines the direction - high for forward, low for reverse. The CPU generates these signals using a timer. The step frequency determines the motor speed. The step pulse must have a minimum width (typically 1 microsecond) to ensure that the driver IC detects it. The driver IC also has an enable pin that turns off the motor's power, allowing the motor to be free-wheeling. The simple STEP/DIR interface makes it easy to control the motor - the CPU just needs to generate the pulses at the right frequency. |
Design Example: Step and Direction in Brother Printers |
Brother's printer uses a timer in the CPU to generate the STEP pulses. The timer is configured in PWM mode, and the duty cycle is set to 50%. The timer's frequency is varied to change the motor speed. The DIR signal is controlled by a GPIO pin. The manufacturer uses an interrupt to toggle the STEP pin at the desired frequency. The interrupt service routine is short, and it is executed at the step rate (up to 50 kilohertz). The manufacturer also uses the enable pin to turn off the motor when the printer is idle, reducing the power consumption. |
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Chapter 8: The Current Regulation - Controlling the Torque |
The torque of a stepper motor is proportional to the current in the windings. The current must be regulated to a constant value, regardless of the supply voltage and the motor's back-EMF. The motor driver IC regulates the current by chopping the voltage - it applies the full supply voltage to the winding, and when the current reaches the set point, it turns off the voltage until the current drops below the set point. This is a simple form of pulse-width modulation. The current regulation also limits the inrush current when the motor is started. The current set point is set by a reference voltage, which is generated by a resistor divider from a precision voltage reference. The driver IC compares the actual current (measured across a sense resistor) to the reference and adjusts the chopping. |
Design Example: Current Regulation in A4988 |
The A4988 motor driver IC from Allegro uses a fixed off-time chopping algorithm. The current is regulated by setting the reference voltage on the VREF pin. The reference voltage is set by a resistor divider from a 3.3-volt rail. The sense resistors are 0.1-ohm, 1% resistors connected between the source of the low-side MOSFETs and the ground. The A4988 measures the voltage across the sense resistor and, when it reaches the VREF value, it turns off the high-side MOSFET for a fixed off-time (typically 30 microseconds). The off-time is fixed, which simplifies the design. In a design from a Taiwanese manufacturer, the A4988 is used to drive the platen motor. The manufacturer chose the A4988 because of its simplicity and low cost. The manufacturer measured the current and found it to be within 5% of the set point. |
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Chapter 9: The Sense Resistor - Measuring the Current |
The current through the motor winding is measured by a sense resistor, which is placed in series with the winding. The sense resistor is a low-value, high-precision resistor - typically 0.05 to 0.2 ohms. The voltage across the sense resistor is proportional to the current. The voltage is fed to the driver IC's current sense inputs. The driver IC compares this voltage to a reference voltage and regulates the current. The sense resistor must be a low-inductance type (such as a metal strip resistor) to avoid any inductive voltage drop. The sense resistor must also be able to handle the power dissipation - for a 2-ampere current and a 0.1-ohm resistor, the power is 0.4 watts. The sense resistor is placed close to the driver IC to minimize the noise pickup. |
Design Example: Sense Resistor in DRV8825 |
The DRV8825 uses external sense resistors. In a design from a European printer manufacturer, the sense resistors are 0.1-ohm, 1%, 0.5-watt devices from Vishay. The resistors are placed very close to the DRV8825's sense pins. The manufacturer used a Kelvin connection - the sense traces are separate from the power traces. The manufacturer measured the current and found it to be accurate to within 2%. The manufacturer also added a low-pass filter (a 100-ohm resistor and a 1-nanofarad capacitor) on the sense inputs to filter out the high-frequency noise from the chopping. |
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Chapter 10: The Decay Mode - Fast Decay vs. Slow Decay |
When the driver IC turns off the high-side MOSFET, the current in the winding must decay. The current can decay slowly, where the current recirculates through the low-side MOSFETs (or the body diodes), or it can decay quickly, where the current is forced back through the power supply. The fast decay mode gives a better current regulation at high speeds, but it causes more power dissipation and more EMI. The slow decay mode is more efficient, but it can cause the current to be poorly regulated at high speeds. Many driver ICs allow the user to select the decay mode, either by a pin or by a register. The decay mode is often mixed - the driver IC uses slow decay at low currents and fast decay at high currents. The decay mode affects the motor's performance and the acoustic noise. |
Design Example: Mixed Decay in A4988 |
The A4988 has a mixed decay mode, where it uses slow decay for the first part of the off-time and fast decay for the last part. The mixed decay gives a good balance between the current regulation and the efficiency. In the Taiwanese manufacturer's design, the A4988 is used in the mixed decay mode. The manufacturer measured the motor current and found it to be stable, even at high speeds. The manufacturer also noticed that the acoustic noise was lower in the mixed decay mode than in the fast decay mode. |
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Chapter 11: The Micro-Stepping Indexer - A Built-In Translator |
Many motor driver ICs have a built-in micro-stepping indexer. The indexer receives the STEP and DIR signals and generates the correct sine and cosine current references for the micro-stepping. The indexer also handles the sequencing of the phases. The indexer is typically a digital state machine that is configured by the micro-stepping mode pin or by an SPI register. The indexer makes it very easy to control the motor - the CPU just sends the steps, and the indexer does the rest. The indexer also includes the logic for the half-step and the full-step modes. |
Design Example: Indexer in A4988 |
The A4988 has a built-in indexer that supports full-step, half-step, quarter-step, eighth-step, and sixteenth-step modes. The mode is set by three pins (MS1, MS2, MS3). In the Taiwanese design, the manufacturer sets the indexer to the sixteenth-step mode by connecting the MS pins to 3.3 volts. The CPU sends the STEP pulses, and the A4988 generates the micro-stepping waveforms. The manufacturer did not need to implement the sine and cosine generation in the firmware, which simplified the design significantly. |
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Chapter 12: The Step Frequency - Determining the Speed |
The speed of the motor is determined by the step frequency. The step frequency is the number of step pulses per second. The motor's speed is the step frequency divided by the number of steps per revolution, multiplied by 60 (to convert to RPM). For a 200-step motor, a step frequency of 1,000 pulses per second gives a speed of 1,000 / 200 * 60 = 300 RPM. The step frequency is generated by the CPU's timer. The step frequency must be varied smoothly to accelerate and decelerate the motor. A sudden change in the step frequency can cause the motor to lose steps (a condition known as stalling). The acceleration and deceleration are controlled by a ramp profile - the step frequency is gradually increased from a low value to the target value, and then gradually decreased at the end of the movement. |
Design Example: Acceleration Ramp in Brother Printers |
Brother's printer uses a trapezoidal acceleration profile. The step frequency is increased linearly from 200 pulses per second to 2,000 pulses per second over a period of 100 milliseconds. The motor accelerates smoothly. The profile is implemented in the firmware: the CPU updates the timer's period at regular intervals (every 1 millisecond). The manufacturer measured the motor's current and found that the acceleration profile reduced the peak current by 30%, compared to a sudden start. The profile also prevented the motor from stalling. |
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Chapter 13: The Holding Torque - Staying Put |
When the motor is not moving, the current still flows through the windings to hold the rotor in position. This is the holding torque. The holding torque is proportional to the current. The holding torque is important to keep the paper stationary during printing - if the motor moves even slightly, the dots will be misaligned. The holding torque is set by the current regulation - the same current that is used for the movement is also used for the holding. However, the holding torque can be reduced to save power. Some driver ICs have a 'hold current' mode where the current is reduced to a fraction of the running current. The hold current mode is enabled automatically when the motor stops. |
Design Example: Hold Current in DRV8825 |
The DRV8825 has a hold current feature. When the motor stops, the current is reduced to 50% of the running current. In the European manufacturer's design, the hold current mode is used to reduce the power consumption and the heat generation. The manufacturer measured the motor's temperature and found that the hold current mode reduced the temperature by 10C. The manufacturer also noted that the holding torque was still sufficient to keep the paper in place. |
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Chapter 14: The Motor Voltage - A Trade-Off with Speed |
The motor's speed is limited by the supply voltage. The motor's inductance opposes the change in current. At high speeds, the current cannot rise to the set point within the step time. The motor's torque drops off as the speed increases. To achieve a high speed, the supply voltage must be high. However, a high supply voltage causes higher power dissipation and higher heating. The supply voltage is typically 24 volts for a printer. At 24 volts, the motor can achieve speeds of up to 500 RPM. For higher speeds, a higher voltage (e.g., 48 volts) is needed. |
Design Example: 24-Volt Motor in Sato Printers |
Sato's printer uses a 24-volt supply for the platen motor. The motor is rated for 24 volts and has a winding resistance of 2 ohms and an inductance of 5 millihenries. The manufacturer measured the torque-speed characteristic and found that the motor can achieve 400 RPM at 24 volts. The manufacturer chose 24 volts because it is the same voltage as the printhead, simplifying the power supply design. The manufacturer also used a micro-stepping mode to smooth the current and reduce the torque ripple. |
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Chapter 15: The Motor's Winding Resistance and Inductance |
The motor's winding has a resistance and an inductance. The resistance determines the DC current for a given voltage. The inductance determines how quickly the current can change. A high inductance means the current rises slowly, which limits the torque at high speeds. The motor's performance is determined by the ratio of the inductance to the resistance - the electrical time constant. The electrical time constant is typically 1 to 10 milliseconds. The driver IC must be able to regulate the current with a time constant that is much shorter than the electrical time constant. The motor's resistance and inductance also affect the power dissipation and the heating. |
Design Example: Motor Parameters in Zebra Printers |
Zebra's printer uses a motor with a resistance of 1.5 ohms and an inductance of 3 millihenries. The electrical time constant is 2 milliseconds. The motor is driven with a 24-volt supply, giving a maximum current of 16 amperes, but the current is regulated to 2 amperes. The manufacturer chose the motor because it has a low inductance, which allows a fast current rise time. The manufacturer measured the current at 400 RPM and found it to be 2 amperes, which is within the motor's rating. |
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Chapter 16: The Stepper Motor Driver IC - An Integrated Solution |
The stepper motor driver IC integrates the H-bridges, the current regulation, the micro-stepping indexer, and the protection features in a single package. The driver IC is the preferred solution for most printers because it simplifies the design and reduces the component count. The driver ICs are available from many manufacturers, including Texas Instruments, Allegro, STMicroelectronics, Toshiba, and ON Semiconductor. The driver ICs are rated for different supply voltages and currents. The driver ICs also have different features - some have SPI for configuration, some have built-in current sense, and some have a thermal shutdown. |
Design Example: STMicroelectronics L6470 Motor Driver |
STMicroelectronics's L6470 is a stepper motor driver with a built-in SPI interface. The SPI interface allows the CPU to configure the micro-stepping mode, the current set point, and the decay mode. The L6470 also has a built-in current regulation, a thermal shutdown, and an over-current protection. In a design from an Italian printer manufacturer, the L6470 is used to drive both the platen motor and the ribbon motor. The manufacturer chose the L6470 because of its high integration and its SPI interface, which simplifies the control. The manufacturer also used the L6470's 'stall detection' feature, which detects when the motor stalls and signals the CPU. The stall detection is useful for detecting a paper jam. |
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Chapter 17: The Stall Detection - A Useful Feature |
Some motor driver ICs have a built-in stall detection. The stall detection measures the back-EMF of the motor when the current is off. If the motor is moving, the back-EMF is present. If the motor is stalled, the back-EMF is zero. The stall detection is used to detect when the motor cannot move - for example, when the paper is jammed. The stall detection signals the CPU, which can then stop the motor and display an error message. The stall detection is a useful feature for industrial printers that operate in harsh environments. |
Design Example: Stall Detection in L6470 |
The L6470 has a built-in stall detection that is implemented in the firmware. The CPU can enable the stall detection by setting a register. The L6470 measures the back-EMF and compares it to a threshold. If the back-EMF falls below the threshold, the L6470 sets a flag. The CPU reads the flag and, if it is set, stops the motor. In the Italian manufacturer's design, the stall detection is used to detect a paper jam. The manufacturer tested the stall detection by stopping the platen roller with a piece of tape. The L6470 detected the stall within 10 milliseconds, and the CPU stopped the motor. |
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Chapter 18: The Over-Current Protection - Limiting the Damage |
The motor driver IC has an over-current protection that limits the current to a safe level. If the current exceeds a threshold (typically 3 to 5 amperes), the driver IC turns off the output. The over-current protection is a pulse-by-pulse protection - the current is limited on each cycle. The over-current protection also protects the motor driver IC from a short circuit. The over-current protection is typically a hardware protection that does not require the CPU's intervention. |
Design Example: Over-Current in A4988 |
The A4988 has a built-in over-current protection that trips at 3.5 amperes. In the Taiwanese manufacturer's design, the motor current is set to 2 amperes. The over-current protection is not triggered during normal operation. However, if the motor stalls and the current rises, the protection will limit the current to 3.5 amperes, preventing damage to the driver IC. The manufacturer tested this by shorting the motor windings. The A4988 limited the current to 3.5 amperes and then shut down. The manufacturer had to cycle the power to reset the A4988. |
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Chapter 19: The Thermal Shutdown - Keeping the Driver Cool |
The motor driver IC can overheat if it is operated at a high current for a long time. The thermal shutdown turns off the output if the junction temperature exceeds a threshold - typically 150C. The thermal shutdown is a safety feature that protects the driver IC from damage. The driver IC will not restart until the temperature drops below a hysteresis threshold (typically 20C lower). The thermal shutdown is a hardware protection that does not require the CPU's intervention. |
Design Example: Thermal Shutdown in DRV8825 |
The DRV8825 has a thermal shutdown at 150C. In the European manufacturer's design, the driver IC is placed on a PCB with a copper pad for heat dissipation. The manufacturer measured the driver IC's temperature and found it to be 70C at full load - well below the shutdown threshold. The manufacturer did not trigger the thermal shutdown during normal operation. |
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Chapter 20: The Enable Pin - Turning Off the Motor |
The motor driver IC has an enable pin that turns off the motor's power. When the enable pin is low, the driver IC's outputs are in a high-impedance state, and the motor is free to rotate (or to be rotated). The enable pin is used to turn off the motor when the printer is idle, saving power. The enable pin is also used in emergency situations - if the CPU detects a fault, it can enable the motor to freewheel, preventing mechanical stress. |
Design Example: Enable Pin in Brother Printers |
Brother's printer uses the enable pin to turn off the motor when the printer is idle. The CPU sets the enable pin low after 10 seconds of inactivity. The motor is free to rotate, and the power consumption drops from 5 watts to 0.5 watts. The manufacturer also uses the enable pin in the paper jam detection - if a jam is detected, the CPU sets the enable pin low, and the motor freewheels, preventing the motor from burning out. |
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Chapter 21: The Synchronization with the Printhead - Timing Is Everything |
The stepper motor and the printhead must be synchronized. The printer prints a line of dots, then steps the motor, then prints the next line. The motor must step during the gap between the lines - the motor cannot move while the printhead is firing. The step is typically done in the inter-line gap, which is a few hundred microseconds. The motor must be stepped with a precise timing - the step must be completed before the next line is printed. The synchronization is achieved by the CPU's timers. The CPU generates the strobe signal and the step pulses from the same timer. |
Design Example: Synchronization in Zebra Printers |
Zebra's printer uses a hardware timer to synchronize the strobe and the step. The timer generates an interrupt every 500 microseconds. The interrupt service routine checks the motor's position and, if the motor is stationary, fires the strobe. The motor is stepped in a separate routine that is triggered by a different timer. The two timers are synchronized by the CPU's clock. The manufacturer measured the timing and found that the step was always completed before the strobe fired, with a margin of 50 microseconds. The synchronization is accurate to within 1 microsecond. |
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Chapter 22: The Platen Motor vs. The Ribbon Motor - Two Different Roles |
The platen motor feeds the paper. The ribbon motor takes up the used ribbon. The two motors have different requirements. The platen motor must move the paper precisely - the step size must be constant. The ribbon motor only needs to take up the ribbon at a rate that matches the paper feed. The ribbon motor is often run in a constant-current mode - it provides a constant torque, regardless of the speed. The ribbon motor can be a smaller motor than the platen motor because it has a lower load. The two motors are often driven by separate driver ICs. |
Design Example: Ribbon Motor in Sato Printers |
Sato's printer uses a smaller stepper motor for the ribbon rewind. The motor is a 200-step motor with a holding torque of 0.1 newton-meters. The motor is driven by the same driver IC as the platen motor, but the driver IC has two channels. The ribbon motor is run in a constant-current mode - the current is set to a fixed value, regardless of the speed. The manufacturer chose the smaller motor to save cost and space. The ribbon motor is also geared down to provide a higher torque. |
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Chapter 23: The Gear Reduction - Trading Speed for Torque |
The stepper motor is often geared down to provide a higher torque and a finer resolution. The gear reduction is typically 2:1 to 10:1. The gear reduction increases the torque by the gear ratio and reduces the speed by the same ratio. The gear reduction also reduces the effect of the motor's step irregularity - any non-uniformity in the step is divided by the gear ratio. The gear reduction is achieved by a gear train or a belt drive. The gear train is more compact and has less backlash (play) than a belt drive. |
Design Example: Gear Reduction in Brother Printers |
Brother's printer uses a 4:1 gear reduction for the platen motor. The motor has a pinion gear with 10 teeth, and the platen roller has a gear with 40 teeth. The gear reduction increases the torque by a factor of 4. The manufacturer measured the backlash and found it to be 0.5 degrees, which is acceptable. The gear reduction also gives a finer resolution - each step of the motor moves the paper by 0.039 millimeters, which is sufficient for 203 dpi printing. |
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Chapter 24: The Belt Drive - An Alternative to Gears |
Some printers use a belt drive instead of a gear train. The belt drive is quieter and has less backlash than a gear train. The belt drive also allows the motor to be placed away from the platen roller, freeing up space. The belt drive uses a timing belt (toothed belt) and two pulleys. The timing belt has teeth that engage with the pulleys, preventing any slip. The belt drive requires a tensioner to keep the belt tight. |
Design Example: Belt Drive in Zebra Printers |
Zebra's printer uses a timing belt drive for the platen motor. The motor has a 10-tooth pulley, and the platen roller has a 40-tooth pulley. The belt is a neoprene belt with fiberglass reinforcement. The manufacturer used a belt drive because it is quieter than a gear train, which is important for their industrial printers that are used in offices. The manufacturer also used an adjustable tensioner to set the belt tension. The manufacturer measured the backlash and found it to be less than 0.1 degrees - negligible. |
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Chapter 25: The Motor Mount - A Mechanical Issue |
The motor must be mounted securely to prevent any vibration. The motor is mounted on a metal bracket or a plastic frame. The motor is attached with screws. The motor's shaft is aligned with the platen roller's shaft. The motor's mounting must be rigid to prevent the motor from moving during operation, which would cause the paper to misalign. The motor's mounting also affects the heat dissipation - the motor's heat is conducted to the bracket, which acts as a heat sink. |
Design Example: Motor Mount in Brother Printers |
Brother's printer uses a metal bracket for the motor. The bracket is attached to the plastic frame. The motor is attached to the bracket with four screws. The bracket also acts as a heat sink. The manufacturer measured the motor's temperature and found it to be 60C at full load - well within the motor's 80C rating. |
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Chapter 26: The Motor Cable - A Potential Failure Point |
The motor cable connects the motor to the driver IC. The cable is typically a 4-wire cable (for a bipolar motor) or a 6-wire cable (for a unipolar motor). The cable must be flexible and durable. The cable is often a ribbon cable or a twisted pair. The cable must be routed away from heat sources and sharp edges. The cable's resistance must be low to avoid any voltage drop. The cable's inductance must be low to avoid any noise coupling. |
Design Example: Motor Cable in Sato Printers |
Sato's printer uses a 4-wire, 0.5-millimeter cable for the bipolar motor. The cable is a shielded twisted pair - the two wires for each phase are twisted together, and the whole cable is shielded with a foil shield. The shield is connected to the ground at both ends. The manufacturer used the shield to reduce the electromagnetic interference. The manufacturer also used a strain relief at the connector to prevent the cable from being pulled out. |
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Chapter 27: The Motor Connector - A Secure Connection |
The motor connector is typically a 4-pin connector (for a bipolar motor) or a 6-pin connector (for a unipolar motor). The connector must be secure to prevent the motor from being disconnected during operation. The connector is often a JST or a Molex connector with a locking mechanism. The connector's pins are gold-plated to resist corrosion. The connector is placed on the PCB, and the motor cable is plugged into it. |
Design Example: Motor Connector in Brother Printers |
Brother's printer uses a JST XH connector for the motor. The connector has a locking latch that prevents the cable from being pulled out. The connector is rated for 3 amperes, which is sufficient for the motor's current. The manufacturer used a connector with a key to prevent the cable from being plugged in backwards. |
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Chapter 28: The Motor's Back-EMF - A Source of Information |
The motor generates a back-EMF (electromotive force) when it is moving. The back-EMF is proportional to the motor's speed. The back-EMF can be measured when the current is off. The back-EMF can be used to detect the motor's speed and position. The back-EMF is also used in some advanced control algorithms, such as the 'closed-loop' control. In closed-loop control, the motor's speed is measured and compared to the target speed, and the step frequency is adjusted to maintain the target speed. |
Design Example: Back-EMF in DRV8825 |
The DRV8825 does not have a built-in back-EMF measurement, but the CPU can measure the back-EMF by using an ADC to read the voltage on the motor phases when the outputs are disabled. The European manufacturer used this technique to measure the motor's speed. The CPU measures the back-EMF and adjusts the step frequency to maintain a constant speed. The manufacturer reports that the closed-loop control gives a more consistent print quality, especially at low speeds. |
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Chapter 29: The Motor's Resonance - A Source of Noise |
The stepper motor has a natural resonance - a frequency at which the motor vibrates strongly. The resonance is caused by the interaction of the motor's inertia and the spring-like effect of the magnetic fields. The resonance can cause the motor to lose steps or to produce a loud audible noise. The resonance is typically at a frequency of 100 to 200 hertz. The resonance can be reduced by using micro-stepping, which smooths the torque. The resonance can also be reduced by using a damper - a mechanical device that absorbs the vibration. |
Design Example: Resonance in Brother Printers |
Brother's printer uses micro-stepping to reduce the resonance. The manufacturer measured the acoustic noise and found that the micro-stepping reduced the noise by 10 dB. The manufacturer also added a small rubber damper on the motor's shaft to absorb any residual vibration. The damper is a small piece of rubber that is attached to the motor's shaft. The manufacturer reports that the resonance is not a problem in their printers. |
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Chapter 30: The Motor's Acoustic Noise - A User Concern |
The stepper motor can produce an audible noise, especially at certain speeds. The noise is caused by the vibration of the motor's structure. The noise can be reduced by using micro-stepping, which smooths the motion. The noise can also be reduced by using a quiet motor driver IC, such as the TMC2209 from Trinamic, which uses a stealthChop2 technology. The stealthChop2 technology uses a sophisticated current control algorithm that significantly reduces the acoustic noise. |
Design Example: TMC2209 in a High-End Printer |
A high-end printer from a US manufacturer uses the TMC2209 motor driver IC from Trinamic. The TMC2209 has a stealthChop2 technology that reduces the acoustic noise to a whisper-quiet level. The manufacturer chose the TMC2209 because the printer is used in a quiet office environment. The manufacturer measured the acoustic noise and found it to be 35 dBA - which is quieter than a whisper. The manufacturer also used the TMC2209's stall detection and the current regulation. |
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Chapter 31: The Motor's Power Dissipation - A Thermal Issue |
The stepper motor dissipates power in the form of heat. The power dissipation is the product of the current squared and the winding resistance. The power dissipation can be significant - for a 2-ampere, 2-ohm motor, the power dissipation is 4 * 2 = 8 watts. The motor's heat must be dissipated to prevent the motor from overheating. The motor's heat is dissipated through the motor's frame and through the bracket. The motor's temperature is monitored by a thermistor. If the motor's temperature exceeds a threshold, the printer reduces the motor current. |
Design Example: Motor Temperature in Zebra Printers |
Zebra's printer uses a thermistor on the motor's frame to monitor the temperature. The firmware reads the thermistor and, if the temperature exceeds 70C, reduces the motor current from 2 amperes to 1.5 amperes. The manufacturer measured the motor's temperature during a continuous print job and found it to stabilize at 65C. The manufacturer also added a small heat sink on the motor's frame. |
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Chapter 32: The Motor's Lifetime - A Wear and Tear Issue |
The stepper motor has a limited lifetime. The lifetime is determined by the bearings and the windings. The bearings wear out over time, causing the motor to become noisy and to have play. The windings can overheat and melt the insulation. The motor's lifetime is typically 10,000 to 20,000 hours. The motor's lifetime is extended by operating it at a lower current and by keeping it cool. The printer's firmware monitors the motor's current and temperature and adjusts them to extend the lifetime. |
Design Example: Motor Lifetime in Brother Printers |
Brother's printer has a rated lifetime of 10,000 hours for the motor. The manufacturer achieves this by running the motor at a lower current (1.5 amperes instead of 2 amperes) and by using a heat sink. The manufacturer also uses a soft-start and a soft-stop to reduce the mechanical stress on the bearings. |
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Chapter 33: The Motor's Speed Profile - A Control Algorithm |
The motor's speed profile is the sequence of speeds that the motor follows during a movement. The speed profile is typically a trapezoid - the motor accelerates, then runs at a constant speed, then decelerates. The speed profile is generated by the CPU. The CPU calculates the step frequency for each step and updates the timer's period. The speed profile is optimized for the motor's torque-speed characteristic. The speed profile also affects the print quality - a smooth speed profile reduces the vibration and the acoustic noise. |
Design Example: Speed Profile in Zebra Printers |
Zebra's printer uses a sophisticated speed profile that is generated by a coprocessor. The coprocessor calculates the step frequency for each step using a polynomial interpolation. The coprocessor also adjusts the profile based on the motor's temperature and the load. The manufacturer reports that the speed profile gives a very smooth motion and a high print quality. |
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Chapter 34: The Motor's Position - A Closed-Loop Control |
Some printers use a closed-loop control for the motor. The closed-loop control uses an encoder to measure the motor's position. The CPU compares the actual position to the target position and adjusts the step frequency. The closed-loop control gives a very high accuracy and eliminates the risk of stalling. The closed-loop control is used in high-end printers that require a very high precision. |
Design Example: Closed-Loop in Sato Printers |
Sato's printer uses a magnetic encoder on the platen roller. The encoder has 1,000 pulses per revolution. The CPU reads the encoder's pulses and compares them to the target position. The CPU adjusts the step frequency to maintain the target position. The manufacturer reports that the closed-loop control gives a paper feed accuracy of 0.01 millimeters - which is excellent for 300 dpi printing. |
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Chapter 35: The Motor's Micro-Stepping Resolution - A Trade-Off |
The micro-stepping resolution is the number of micro-steps per full step. A higher resolution gives a smoother motion and a lower noise. However, a higher resolution also reduces the torque at the micro-step positions and increases the computational load. The typical micro-stepping resolution is 1/8, 1/16, or 1/32. The 1/16 resolution is the most common for printers. |
Design Example: Micro-Stepping in Brother Printers |
Brother's printer uses 1/16 micro-stepping. The manufacturer chose 1/16 because it gives a good balance between the smoothness and the torque. The manufacturer measured the motor's torque at the micro-step positions and found it to be 70% of the full-step torque - which is sufficient for the printer's load. The manufacturer also measured the acoustic noise and found it to be 40 dBA - which is acceptable. |
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Chapter 36: The Motor's Holding Torque - A Static Issue |
The holding torque is the torque that the motor can produce when it is stationary. The holding torque is proportional to the current. The holding torque is used to keep the paper stationary during the printing. The holding torque must be higher than the torque required to move the paper. The holding torque is typically 0.4 to 1.2 newton-meters for a printer. |
Design Example: Holding Torque in Zebra Printers |
Zebra's printer has a holding torque of 0.8 newton-meters. The manufacturer calculated the torque required to move the paper and found it to be 0.3 newton-meters. The holding torque is more than sufficient. The manufacturer also uses a reduced holding current to save power. |
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Chapter 37: The Motor's Inductance - A Limiting Factor |
The motor's inductance limits the current rise time. A high inductance means the current rises slowly, which reduces the torque at high speeds. The inductance is determined by the motor's design - it depends on the number of turns and the magnetic core. The inductance is typically 1 to 10 millihenries. The inductance can be reduced by using a motor with fewer turns, but this also reduces the torque. |
Design Example: Inductance in Sato Printers |
Sato's printer uses a motor with an inductance of 3 millihenries. The manufacturer chose this motor because it provides a good balance between the torque and the speed. The manufacturer measured the current at 400 RPM and found it to be 1.8 amperes - which is close to the set point of 2 amperes. The manufacturer also used a higher supply voltage (48 volts) to overcome the inductance, but they used a 24-volt supply for the printhead. |
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Chapter 38: The Future of Stepper Motors - Smarter and More Efficient |
The future of stepper motors lies in smarter and more efficient drivers. The drivers are becoming more integrated, with built-in diagnostics and communication. The drivers are also becoming more efficient, with lower on-resistance and better current regulation. The motors are becoming smaller and more powerful, with new materials and new designs. The future stepper motor will be a complete smart actuator that communicates with the printer over a digital bus. |
Design Example: Smart Stepper Motor in a Prototype |
A prototype printer from a startup uses a smart stepper motor with a built-in driver and a controller. The motor is a NEMA 11 frame size with a holding torque of 0.2 newton-meters. The motor communicates with the CPU over an I2C bus. The CPU sends the target position, and the motor moves to that position. The motor also has a built-in temperature sensor and a current sensor. The manufacturer reports that the smart motor simplifies the design and reduces the cost. |
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Detailed Summary - Tying It All Together |
We have now completed our comprehensive exploration of stepper motor drive principles - the heart of paper movement in a thermal printer. We began by understanding what a stepper motor is and why it is used: it provides precise, repeatable steps without the need for feedback, making it ideal for controlling the paper position in a printer. We learned about the two types of stepper motors - unipolar and bipolar - and why bipolar motors are preferred for their higher torque and efficiency. |
We explored the H-bridge, the circuit that allows the current to flow in both directions through the motor winding, and we saw how it is integrated into motor driver ICs like the DRV8825 and the A4988. We examined the drive sequences - full-step, half-step, and micro-stepping - and we saw how micro-stepping provides the smoothest motion and the lowest noise. We looked at the step and direction signals, the simple interface that allows the CPU to control the motor with just two lines. |
We delved into the current regulation, the sense resistor, and the decay mode - the techniques that control the motor's current and torque. We saw how the mixed decay mode provides a good balance between the regulation and the efficiency. We examined the micro-stepping indexer, the built-in state machine that generates the sine and cosine current waveforms. |
We discussed the step frequency, the acceleration ramp, and the holding torque - the factors that determine the motor's speed and its ability to stay in position. We looked at the motor's voltage, resistance, and inductance - the electrical parameters that affect the motor's performance. We explored the thermal management of the motor and the driver IC, including the thermal shutdown and the over-current protection. |
We saw how the motor is synchronized with the printhead, and we examined the two motors in a printer - the platen motor and the ribbon motor - and their different roles. We discussed the gear reduction and the belt drive, the mechanical systems that translate the motor's steps into paper movement. We looked at the motor's mounting, the cable, and the connector - the practical issues of integrating the motor into the printer. |
We explored the advanced features: the stall detection, the back-EMF measurement, and the closed-loop control. We saw how these features improve the motor's performance and reliability. We discussed the motor's resonance and its acoustic noise, and we saw how micro-stepping and advanced driver ICs like the TMC2209 reduce the noise. |
We examined the motor's power dissipation, its lifetime, and its speed profile - the factors that affect the motor's reliability and print quality. We looked at the holding torque and the inductance, the static and dynamic characteristics of the motor. We concluded with a glimpse of the future - smarter and more efficient stepper motors that communicate over a digital bus. |
The overarching lesson is that the stepper motor is a critical component of the printer. It is not just a simple motor - it is a precision actuator that must move the paper with micrometer accuracy, with low noise, and with high reliability. The motor driver IC is the brain that controls the motor, and it must be carefully designed and configured to achieve the best performance. A well-designed motor drive system ensures that the paper is fed smoothly and accurately, resulting in crisp and clear barcodes. A poorly designed system causes misalignment, vibration, and noise, degrading the print quality. Understanding stepper motor drive principles is essential for any engineer who wants to design a high-quality thermal printer, and this chapter has provided that understanding from the basic principles to the advanced control techniques. |
End of Extended Section 10 |
cs@easiersoft.com