Decoding the Dot: A Deep Dive into Barcode Label Printer Electronics - Extended Section 14 |
Subtitle: Ribbon Motor - Tension Control and Torque Management |
Introductory Summary |
In the previous section, we explored the platen motor and its closed-loop speed control system. The platen motor moves the paper with high precision. But in a thermal transfer printer, there is a second motor - the ribbon motor - that is equally important. The ribbon motor takes up the used ribbon after the ink has been transferred to the paper. Unlike the platen motor, which must move the paper at a precise and constant speed, the ribbon motor has a different job: it must maintain a constant tension on the ribbon, ensuring that the ribbon moves smoothly without wrinkles, breaks, or slack. This is a torque control problem, not a speed control problem. The ribbon motor must provide a constant torque, regardless of the ribbon's speed, the roll diameter, or the friction. This chapter is devoted entirely to the ribbon motor, its torque control, and its tension management system. We will explain why the ribbon motor is different from the platen motor, how it is driven, and how the tension is measured and controlled. We will explore the different types of ribbon motors - stepper motors, DC motors, and brushless DC motors - and see how they are used in different printer designs. We will look at the torque control algorithms: the constant current control, the voltage control, and the closed-loop torque control with a tension sensor. We will examine the mechanical components - the ribbon spools, the clutches, and the tension arms. We will look at real-world designs from major companies: Zebra's use of a DC motor with a current control, Sato's use of a stepper motor with a constant current, Brother's use of a simple torque-limited clutch, and Honeywell's use of a brushless DC motor with a tension sensor. We will also discuss the ribbon's characteristics, the ribbon break detection, and the ribbon end detection. By the end, you will understand how the ribbon motor keeps the ribbon moving smoothly, and you will appreciate the elegant solutions that engineers have developed for this challenging control problem. |
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Chapter 1: The Problem - Ribbon Tension Is Critical |
In a thermal transfer printer, the ribbon is a thin plastic film coated with wax or resin. The printhead heats the ribbon, melting the coating and transferring it to the paper. The ribbon must move smoothly and at the same speed as the paper. If the ribbon is too loose, it will wrinkle, causing uneven print quality. If the ribbon is too tight, it will break or cause the paper to jam. If the ribbon moves at a different speed than the paper, the print will be smeared. The ribbon motor must maintain a constant tension on the ribbon, and it must keep the ribbon moving at the same speed as the paper. The ribbon motor must also handle the changing roll diameter - as the ribbon is used, the take-up roll gets larger, and the supply roll gets smaller. The motor must adjust its torque to keep the tension constant. This is a challenging control problem. |
Design Example: Ribbon Wrinkle in a Logistics Printer |
A logistics company reported that their labels had a faint white line down the middle. The problem was traced to the ribbon - the ribbon was slightly wrinkled, causing the printhead to lose contact in the middle of the label. The cause was that the ribbon motor torque was too low, causing the ribbon to be slack. The manufacturer adjusted the torque set point, and the problem was solved. |
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Chapter 2: Why a Stepper Motor Is Not Ideal for Torque Control |
A stepper motor is a position-controlled motor - it moves in discrete steps. It is excellent for the platen motor, which needs to move the paper precisely. But for the ribbon motor, position control is not what is needed. The ribbon motor needs to provide a constant torque, regardless of the position. A stepper motor can be operated in a constant current mode, which provides a constant torque, but the stepper motor has a limited torque range and can be noisy. A DC motor, on the other hand, is a speed-controlled motor, but it can also be operated in a torque-controlled mode by regulating the current. The DC motor is simpler and quieter than a stepper motor, and it is often used for the ribbon motor. However, some printers use a stepper motor for the ribbon motor because of its simplicity and low cost. |
Design Example: Stepper Motor for Ribbon in Brother Printers |
Brother's QL series printers use a stepper motor for the ribbon motor. The motor is a small 200-step motor. The motor is operated in a constant current mode - the current is set to a fixed value, which provides a constant torque. The manufacturer chose the stepper motor because it is inexpensive and simple to drive. The ribbon tension is controlled by the current set point. The manufacturer reports that the stepper motor works well for the ribbon, as long as the current is set correctly. |
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Chapter 3: The DC Motor - A Better Choice for Torque Control |
A DC motor is a simple motor that rotates when a voltage is applied. The torque of a DC motor is proportional to the current. By regulating the current, the torque can be controlled. The speed of the DC motor is determined by the voltage and the load. The DC motor is easy to control and has a wide torque range. The DC motor is also quieter than a stepper motor. However, the DC motor requires a feedback mechanism to ensure that the ribbon speed matches the paper speed. The feedback can be an encoder on the motor, or a tension sensor. The DC motor is the preferred choice for the ribbon motor in high-end printers. |
Design Example: DC Motor for Ribbon in Zebra Printers |
Zebra's ZT600 series uses a DC motor for the ribbon motor. The motor is a small, 24-volt DC motor with a built-in encoder. The motor is controlled by a PWM driver IC (a DRV8871). The current is regulated by a sense resistor. The torque set point is set by a DAC (digital-to-analog converter) in the CPU. The manufacturer chose the DC motor because it provides a smooth and quiet operation. The manufacturer also uses the encoder feedback to ensure that the ribbon speed matches the paper speed. |
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Chapter 4: The Brushless DC Motor - A More Efficient Alternative |
A brushless DC motor (BLDC) is a motor that uses electronic commutation instead of brushes. The BLDC motor is more efficient than a brushed DC motor, and it has a longer lifetime. The BLDC motor is also quieter and has a higher torque-to-weight ratio. The BLDC motor requires a more complex driver, but the driver ICs are now readily available. The BLDC motor is used in high-end industrial printers. |
Design Example: BLDC Motor for Ribbon in Honeywell Printers |
Honeywell's printer uses a BLDC motor for the ribbon motor. The motor is a 3-phase BLDC motor with Hall-effect sensors. The motor is driven by a BLDC driver IC (from Texas Instruments). The driver IC uses a field-oriented control (FOC) algorithm to control the torque. The FOC algorithm provides a smooth and efficient torque control. The manufacturer chose the BLDC motor because of its long lifetime and high efficiency. |
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Chapter 5: The Torque Control - Regulating the Current |
The torque of a DC motor is proportional to the current. To control the torque, the current must be regulated. The current is measured by a sense resistor, and the voltage across the sense resistor is amplified and compared to a reference. The comparison is used to adjust the PWM duty cycle. The current regulation is similar to the current regulation in a stepper motor driver. The current set point is set by a DAC or by a resistor divider. The current regulation ensures that the torque is constant. |
Design Example: Current Regulation in DRV8871 |
The DRV8871 is a DC motor driver IC with a built-in current regulation. The current is set by an external resistor. The DRV8871 measures the current through the motor and regulates it to the set point. In the Zebra design, the current is set to 0.5 amperes, which gives a torque of 0.1 newton-meters - sufficient for the ribbon. The manufacturer chose the DRV8871 because of its simple current regulation. |
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Chapter 6: The Tension Sensor - Direct Measurement of Tension |
The most accurate way to control the tension is to measure it directly with a tension sensor. The tension sensor is typically a strain gauge or a load cell that is attached to a tension arm. The tension arm is a small lever that is pressed against the ribbon. The tension sensor measures the force on the tension arm. The force is proportional to the ribbon tension. The tension sensor provides a feedback signal that is used by the control loop. The control loop adjusts the motor current to maintain the tension at the set point. |
Design Example: Tension Sensor in Honeywell Printers |
Honeywell's printer uses a tension sensor (a strain gauge) attached to a tension arm. The strain gauge is a Wheatstone bridge that changes its resistance with the force. The bridge is excited by a 5-volt reference. The output of the bridge is amplified by an instrumentation amplifier (INA188) and fed to the ADC. The CPU reads the ADC value and adjusts the motor current to maintain the tension. The manufacturer reports that the tension sensor gives a very accurate and stable tension control. |
Chapter 7: The Tension Arm - A Mechanical Amplifier |
The tension arm is a mechanical lever that amplifies the ribbon tension. The tension arm is pressed against the ribbon by a spring. The spring provides a reference force. When the ribbon tension increases, the tension arm moves, compressing the spring. The movement is measured by a potentiometer or a Hall-effect sensor. The tension arm is a simple and reliable way to measure the tension. |
Design Example: Tension Arm in Brother Printers |
Brother's printer uses a tension arm with a potentiometer. The tension arm is pressed against the ribbon. The potentiometer is connected to a 5-volt reference. The wiper of the potentiometer is connected to the ADC. The CPU reads the ADC value and adjusts the motor current. The manufacturer chose the tension arm because it is simple and inexpensive. |
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Chapter 8: The Clutch - A Mechanical Torque Limiter |
Some printers use a mechanical clutch to limit the torque. The clutch is a mechanical device that slips when the torque exceeds a set point. The clutch is a simple and reliable way to limit the torque. The clutch is typically a friction clutch or a magnetic clutch. The clutch is used in low-cost printers. |
Design Example: Friction Clutch in Brother Printers |
Brother's printer uses a friction clutch on the ribbon take-up spool. The clutch consists of a spring and a friction pad. The spring applies a force to the friction pad. The force sets the slip torque. The manufacturer chose the friction clutch because it is simple and inexpensive. The manufacturer also added a small motor to assist the clutch - the motor provides the main torque, and the clutch provides the slip. |
Chapter 9: The Supply Roll - The Source of Ribbon |
The supply roll is the roll of unused ribbon. The supply roll is mounted on a spindle. The spindle has a friction brake that applies a small drag to the supply roll. The drag prevents the ribbon from unwinding freely. The drag is set to a value that is lower than the tension set point. The supply roll is a passive component - it does not have a motor. The supply roll's diameter changes as the ribbon is used, but the friction brake provides a constant drag. |
Design Example: Supply Roll Brake in Zebra Printers |
Zebra's printer uses a friction brake on the supply roll spindle. The brake consists of a spring and a felt pad. The spring applies a force to the felt pad. The force sets the drag. The manufacturer chose the friction brake because it is simple and reliable. The manufacturer also added a sensor to detect when the supply roll is empty. |
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Chapter 10: The Take-Up Roll - The Destination of Ribbon |
The take-up roll is the roll of used ribbon. The take-up roll is driven by the ribbon motor. The take-up roll's diameter increases as the ribbon is used. The motor must adjust its torque to maintain the tension as the diameter increases. The torque required is the tension times the radius of the take-up roll. As the radius increases, the torque must increase to keep the tension constant. The motor driver must have a wide torque range. |
Design Example: Take-Up Roll in Sato Printers |
Sato's printer uses a stepper motor for the take-up roll. The motor is operated in a constant current mode. The current is set to a value that gives the correct tension at the average roll diameter. As the roll diameter increases, the tension decreases slightly - but the decrease is small because the motor's torque is proportional to the current, which is constant. The manufacturer accepted this because the tension variation is within the acceptable range. |
Chapter 11: The Ribbon Speed - Matching the Paper Speed |
The ribbon must move at the same speed as the paper. The ribbon speed is determined by the take-up roll's linear speed - the speed at which the ribbon is pulled. The linear speed is the angular speed of the take-up roll times the roll radius. The angular speed is controlled by the motor's speed. In a DC motor system, the speed is controlled by the voltage. In a stepper motor system, the speed is controlled by the step frequency. The ribbon speed must be synchronized with the paper speed. The synchronization is achieved by using the same clock for the paper and the ribbon motors. |
Design Example: Speed Synchronization in Zebra Printers |
Zebra's printer uses a DC motor with an encoder for the ribbon motor. The encoder measures the ribbon speed. The CPU compares the ribbon speed to the paper speed (measured by the platen encoder) and adjusts the ribbon motor's voltage to maintain the speed match. The manufacturer reports that the speed synchronization is accurate to within 1%. |
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Chapter 12: The Ribbon Break Detection - A Safety Feature |
The ribbon can break. The break can be caused by a jam, a sharp edge, or a defective ribbon. When the ribbon breaks, the printer must stop immediately to prevent the paper from being printed without the ribbon. The ribbon break detection is typically done by a mechanical sensor - a small lever that is pressed against the ribbon. If the ribbon breaks, the lever moves, triggering a switch. The switch signals the CPU, which stops the printer. |
Design Example: Ribbon Break Sensor in Brother Printers |
Brother's printer uses a mechanical lever for the ribbon break detection. The lever is a small metal arm that is pressed against the ribbon. If the ribbon breaks, the lever moves, closing a micro-switch. The micro-switch is connected to an interrupt pin on the CPU. The CPU stops the printer and displays a 'Ribbon Break' error. |
Chapter 13: The Ribbon End Detection - A Maintenance Feature |
The ribbon has a finite length. When the ribbon is nearly empty, the printer must warn the user. The ribbon end detection is typically done by a sensor that detects a colored strip on the ribbon or by a counter that tracks the amount of ribbon used. The ribbon end detection is a maintenance feature that prevents the user from running out of ribbon in the middle of a print job. |
Design Example: Ribbon End Sensor in Zebra Printers |
Zebra's printer uses an optical sensor for the ribbon end detection. The ribbon has a silver strip near the end. The optical sensor detects the silver strip and signals the CPU. The CPU displays a 'Ribbon Low' warning. The manufacturer chose the optical sensor because it is reliable and easy to implement. |
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Chapter 14: The Ribbon Tension Set Point - A Calibration Parameter |
The ribbon tension set point is the desired tension. The set point is set by the manufacturer. The set point depends on the ribbon type - a wax ribbon requires a lower tension than a resin ribbon. The set point is typically stored in the EEPROM. The user can adjust the set point using a service menu. The set point is the reference for the torque control. |
Design Example: Tension Set Point in Sato Printers |
Sato's printer has a 'Ribbon Tension' setting in the service menu. The user can adjust the tension from 0 to 10. The default is 5. The manufacturer provides a recommended setting for each ribbon type. The user can adjust the setting if the print quality is not satisfactory. |
Chapter 15: The Ribbon Type - A Variable Factor |
The ribbon type affects the tension requirements. A wax ribbon is soft and requires a low tension. A resin ribbon is hard and requires a higher tension. The ribbon type also affects the print speed - a resin ribbon requires a higher temperature and a slower speed. The printer must be configured for the ribbon type. The configuration is typically done by the user. |
Design Example: Ribbon Type in Honeywell Printers |
Honeywell's printer has a 'Ribbon Type' setting in the menu. The user selects the type (wax, wax-resin, or resin). The printer adjusts the tension set point and the print speed accordingly. The manufacturer provides a list of recommended ribbons. |
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Chapter 16: The Ribbon Coating - A Physical Property |
The ribbon has a coating of wax or resin. The coating is transferred to the paper by the heat of the printhead. The coating's melting point and its viscosity affect the ribbon's behavior. A high-melting-point coating requires a higher printhead temperature and a higher tension. A low-melting-point coating requires a lower temperature and a lower tension. The ribbon coating is a factor in the printer's configuration. |
Design Example: Coating in Brother Printers |
Brother's printer has a 'Ribbon Coating' setting in the menu. The user can select 'High Sensitivity' or 'Low Sensitivity'. The setting adjusts the tension and the print speed. The manufacturer provides a list of recommended ribbons for each setting. |
Chapter 17: The Ribbon Width - A Mechanical Factor |
The ribbon width affects the tension. A wider ribbon has a higher tension for the same force. The ribbon width also affects the printhead pressure. The printer must be configured for the ribbon width. The configuration is typically done by the user. |
Design Example: Ribbon Width in Zebra Printers |
Zebra's printer has a 'Ribbon Width' setting in the menu. The user selects the width (e.g., 4 inches, 6 inches). The printer adjusts the tension set point and the printhead pressure accordingly. |
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Chapter 18: The Ribbon Splice - A Defect |
The ribbon can have a splice - a joint where two pieces of ribbon are joined. The splice is thicker than the ribbon, and it can cause a wrinkle or a break. The printer must handle the splice without stopping. The ribbon tension control must be able to accommodate the splice. The tension control loop must be fast enough to adjust to the splice. |
Design Example: Splice in Sato Printers |
Sato's printer can detect a splice by measuring the tension. The tension increases when the splice passes through the printhead. The tension control loop detects the increase and adjusts the motor current to maintain the tension. The manufacturer reports that the printer can handle splices without any problems. |
Chapter 19: The Ribbon Wrinkles - A Quality Issue |
A wrinkle in the ribbon causes a white line on the print. The wrinkle is caused by uneven tension. The tension must be uniform across the ribbon width. The tension control must ensure that the tension is uniform. The tension uniformity is achieved by using a wide tension arm and by using a compliant material for the ribbon guides. |
Design Example: Wrinkle Prevention in Honeywell Printers |
Honeywell's printer uses a wide tension arm that spans the entire ribbon width. The tension arm is made of a compliant material that distributes the force evenly. The manufacturer reports that the printer does not produce wrinkles. |
Chapter 20: The Ribbon Breakage - A Mechanical Failure |
The ribbon can break due to a sharp edge or a high tension. The breakage is prevented by using smooth ribbon guides and by setting the tension to a safe value. The tension set point must be lower than the ribbon's breaking strength. The breaking strength is specified by the ribbon manufacturer. |
Design Example: Breakage Prevention in Brother Printers |
Brother's printer uses smooth, polished ribbon guides. The tension set point is set to 50% of the ribbon's breaking strength. The manufacturer reports that the ribbon does not break during normal operation. |
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Chapter 21: The Ribbon Motor's Encoder - A Speed Feedback |
The ribbon motor's encoder provides the speed feedback. The encoder is used to synchronize the ribbon speed with the paper speed. The encoder is also used to calculate the ribbon's remaining length. The encoder is a critical component of the ribbon motor control system. |
Design Example: Encoder in Zebra Printers |
Zebra's printer uses a magnetic encoder on the ribbon motor. The encoder has 500 pulses per revolution. The CPU reads the encoder and calculates the ribbon speed. The CPU also counts the encoder pulses to calculate the ribbon's remaining length. |
Chapter 22: The Ribbon Motor's Current Feedback - A Torque Feedback |
The ribbon motor's current feedback provides the torque feedback. The current feedback is used to control the torque. The current feedback is also used to detect a jam - if the current exceeds a threshold, the motor is stalled. The current feedback is a critical part of the torque control loop. |
Design Example: Current Feedback in DRV8871 |
The DRV8871 provides a current feedback signal (the Iout pin). The Iout pin outputs a voltage that is proportional to the motor current. The CPU reads the Iout voltage and uses it in the torque control loop. The manufacturer also uses the Iout to detect a jam - if the current exceeds 0.8 amperes, the CPU stops the motor. |
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Chapter 23: The Ribbon Motor's PWM - A Voltage Control |
The ribbon motor's speed is controlled by the PWM duty cycle. A higher duty cycle gives a higher voltage and a higher speed. The PWM duty cycle is adjusted by the speed control loop. The PWM frequency is typically 20 to 50 kilohertz. The PWM is generated by a timer in the CPU. |
Design Example: PWM in Brother Printers |
Brother's printer uses a PWM frequency of 25 kilohertz for the ribbon motor. The PWM duty cycle is adjusted by the CPU. The manufacturer chose 25 kilohertz because it is above the audible range. |
Chapter 24: The Ribbon Motor's Speed Control - A Simple Control |
The ribbon motor's speed control is a simple control loop. The loop measures the speed and adjusts the voltage. The speed control loop is typically a PI (proportional-integral) controller. The PI controller corrects the speed error. The speed control loop is not as complex as the platen motor's PID controller. |
Design Example: Speed Control in Zebra Printers |
Zebra's printer uses a PI controller for the ribbon motor's speed. The PI controller has a Kp of 5 and a Ki of 0.5. The manufacturer tuned the gains to give a fast response without overshoot. The manufacturer measured the speed error and found it to be less than 2%. |
Chapter 25: The Ribbon Motor's Torque Control - A More Complex Control |
The ribbon motor's torque control is a more complex control loop. The loop measures the current (which is proportional to the torque) and adjusts the voltage. The torque control loop is typically a PI controller. The torque control loop must be fast enough to respond to the changes in the roll diameter. |
Design Example: Torque Control in Honeywell Printers |
Honeywell's printer uses a PI controller for the ribbon motor's torque. The PI controller has a Kp of 10 and a Ki of 1. The manufacturer tuned the gains to give a fast response without overshoot. The manufacturer measured the torque error and found it to be less than 1%. |
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Chapter 26: The Ribbon Motor's Direction - A Bidirectional Control |
The ribbon motor must be able to move in both directions. The forward direction is the normal operation. The reverse direction is used for the ribbon rewind - the ribbon is pulled back to remove any slack. The bidirectional control is achieved by reversing the voltage or the PWM duty cycle. |
Design Example: Bidirectional Control in Sato Printers |
Sato's printer uses a stepper motor for the ribbon motor. The stepper motor can be moved in both directions by reversing the step sequence. The manufacturer uses the reverse direction to rewind the ribbon when the cover is opened. |
Chapter 27: The Ribbon Motor's Brake - A Holding Feature |
The ribbon motor must hold the tension when the printer is idle. The holding force is provided by the motor's holding torque or by a mechanical brake. The holding torque is provided by the motor current. The motor current is reduced in the idle mode to save power, but it must be sufficient to hold the tension. |
Design Example: Brake in Brother Printers |
Brother's printer uses a DC motor with a mechanical brake. The brake is a spring-loaded pad that presses against the motor shaft. The brake is released when the motor is powered. The manufacturer chose the mechanical brake because it holds the tension without consuming any power. |
Chapter 28: The Ribbon Motor's Acceleration - A Smooth Start |
The ribbon motor must accelerate smoothly to avoid jerking the ribbon. The acceleration is controlled by the speed ramp. The speed ramp is generated by the CPU. The speed ramp is typically a trapezoid. The acceleration is set to a value that the motor can achieve without stalling. |
Design Example: Acceleration in Zebra Printers |
Zebra's printer uses a speed ramp for the ribbon motor. The acceleration is 1,000 RPM per second. The manufacturer measured the current during acceleration and found it to be 0.5 amperes - well within the motor's rating. |
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Chapter 29: The Ribbon Motor's Deceleration - A Smooth Stop |
The ribbon motor must decelerate smoothly to avoid jerking the ribbon. The deceleration is controlled by the speed ramp. The deceleration is typically the same as the acceleration. |
Design Example: Deceleration in Sato Printers |
Sato's printer uses a speed ramp for the ribbon motor. The deceleration is 1,000 RPM per second. The manufacturer measured the tension during deceleration and found it to be stable. |
Chapter 30: The Ribbon Motor's Noise - An Acoustic Issue |
The ribbon motor can generate noise. The noise is caused by the motor's vibration and by the PWM frequency. The noise is reduced by using a quiet motor (BLDC) and by using a PWM frequency that is above the audible range (e.g., 25 kilohertz). The noise is also reduced by using a soft mounting for the motor. |
Design Example: Noise Reduction in Honeywell Printers |
Honeywell's printer uses a BLDC motor for the ribbon motor. The BLDC motor is quiet. The PWM frequency is 30 kilohertz. The motor is mounted on a rubber grommet to absorb the vibration. The manufacturer measured the noise and found it to be 40 dBA - which is quiet. |
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Chapter 31: The Ribbon Motor's Power Dissipation - A Thermal Issue |
The ribbon motor dissipates power in the form of heat. The power dissipation is the product of the current and the voltage. The power dissipation is typically 1 to 5 watts. The heat must be dissipated to prevent the motor from overheating. The motor is cooled by the air flow from the printer's fan. |
Design Example: Power Dissipation in Zebra Printers |
Zebra's printer uses a DC motor with a power dissipation of 2 watts. The motor is placed near the fan. The manufacturer measured the motor's temperature and found it to be 50C - well within the 80C rating. |
Chapter 32: The Ribbon Motor's Lifetime - A Wear Issue |
The ribbon motor has a limited lifetime. The lifetime is determined by the brushes (in a brushed DC motor) or the bearings (in a BLDC motor). The lifetime is typically 10,000 to 20,000 hours. The lifetime is extended by operating the motor at a low temperature and a low current. The printer's firmware monitors the motor's current and temperature and reduces the load if necessary. |
Design Example: Lifetime in Brother Printers |
Brother's printer uses a brushed DC motor with a rated lifetime of 10,000 hours. The manufacturer operates the motor at 50% of its rated current, extending the lifetime to 20,000 hours. |
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Chapter 33: The Ribbon Motor's Failure Modes - A Diagnostic Issue |
The ribbon motor can fail in several ways: the motor can burn out, the bearings can wear out, or the brushes can wear out. The failure is typically gradual - the motor's current increases, and its torque decreases. The printer's firmware monitors the motor's current and detects the failure. The printer displays a 'Ribbon Motor Failure' error. |
Design Example: Failure Detection in Sato Printers |
Sato's printer monitors the ribbon motor's current. If the current exceeds a threshold, the printer displays a 'Ribbon Motor Overload' error. The technician can then replace the motor. |
Chapter 34: The Ribbon Motor's Replacement - A Service Issue |
The ribbon motor is a serviceable component. The motor can be replaced by a service technician. The replacement procedure is simple - the technician removes the motor, disconnects the cable, and installs the new motor. The printer must be recalibrated after the replacement. |
Design Example: Replacement in Zebra Printers |
Zebra's printer has a modular ribbon motor that can be replaced in the field. The technician removes the cover, unplugs the motor cable, and removes the motor. The technician installs the new motor and runs the calibration procedure. |
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Chapter 35: The Ribbon Motor's Calibration - A Factory Procedure |
The ribbon motor must be calibrated at the factory. The calibration measures the motor's current and the tension. The calibration data is stored in the EEPROM. The calibration is used to set the current set point for the torque control. |
Design Example: Calibration in Honeywell Printers |
Honeywell's printer calibrates the ribbon motor at the factory. The technician attaches a tension gauge to the ribbon and adjusts the motor current until the tension is correct. The current value is stored in the EEPROM. |
Chapter 36: The Ribbon Motor's Software Control - A Complex Algorithm |
The ribbon motor's software control is a complex algorithm that integrates the speed control, the torque control, the tension sensing, and the fault detection. The algorithm runs on the CPU. The algorithm is typically a state machine that sequences the motor's operations. |
Design Example: Software Control in Zebra Printers |
Zebra's printer uses a state machine for the ribbon motor. The states are: Idle, Start, Run, Stop, and Fault. The state machine transitions between the states based on the sensor inputs and the timer events. The manufacturer reports that the state machine is robust and reliable. |
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Chapter 37: The Ribbon Motor's Hardware - A Complete System |
We have now covered the ribbon motor, its driver, its sensors, and its software. Let us put it all together. The ribbon motor is a complete system. The motor provides the torque. The driver regulates the current. The sensor measures the tension. The CPU runs the control algorithm. The hardware and the software work together to maintain the tension. |
Chapter 38: The Future of Ribbon Motors - Smart and Connected |
The future of ribbon motors lies in smart and connected solutions. The future ribbon motor will have a built-in microcontroller and a wireless interface. The future ribbon motor will also have a built-in tension sensor and a current sensor. The future ribbon motor will communicate with the printer over a wireless network. The future ribbon motor will be more efficient, more reliable, and easier to maintain. |
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Detailed Summary - Tying It All Together |
We have now completed our comprehensive exploration of the ribbon motor and its tension control system. We began by understanding the problem: the ribbon must maintain a constant tension to prevent wrinkles and breaks. We learned that the ribbon motor is different from the platen motor - it is a torque-controlled motor, not a speed-controlled motor. |
We explored the different types of ribbon motors - stepper motors, DC motors, and brushless DC motors. We saw how they are used in printers from Brother, Zebra, Sato, and Honeywell. We learned that the DC motor is the preferred choice for the ribbon motor because of its smooth operation and its wide torque range. |
We examined the torque control - the current regulation that provides the constant torque. We saw how the current is measured and regulated using a sense resistor and a PWM driver. We looked at the tension sensor, the tension arm, and the clutch - the different methods of measuring and limiting the tension. We discussed the supply roll and the take-up roll, and we saw how the changing roll diameter affects the torque. |
We explored the ribbon speed synchronization, the ribbon break detection, and the ribbon end detection. We looked at the tension set point, the ribbon type, and the ribbon width. We discussed the ribbon coating, the ribbon splice, and the ribbon wrinkles. |
We examined the advanced features: the encoder feedback, the current feedback, the speed control, and the torque control. We saw how the PID and PI controllers are used to regulate the speed and the torque. We discussed the braking, the acceleration, and the deceleration. We looked at the noise, the power dissipation, and the lifetime. |
We considered the failure modes, the replacement, and the calibration. We looked at the software control algorithm - the state machine that sequences the motor's operations. We concluded with a glimpse of the future - smart and connected ribbon motors with built-in microcontrollers and wireless interfaces. |
The overarching lesson is that the ribbon motor is a critical component of the thermal transfer printer. It must maintain a constant tension on the ribbon, preventing wrinkles and breaks, and it must synchronize the ribbon speed with the paper speed. A well-designed ribbon motor system ensures that the print is clear and the labels are readable. A poorly designed system causes wrinkles, smears, and breaks. Understanding the ribbon motor and its tension control is essential for any engineer who wants to design a reliable thermal transfer printer, and this chapter has provided that understanding from the basic principles of torque control to the advanced techniques of tension sensing and adaptive control. |
End of Extended Section 14 |
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