Decoding the Dot: A Deep Dive into Barcode Label Printer Electronics - Extended Section 12 |
Subtitle: Motor Current Sensing and Regulation - The Art of Torque Control |
Introductory Summary (Extended Section 12 Preview) |
In the previous two sections, we explored the principles of stepper motors and the driver ICs that control them. But a driver IC is not a magic box - it needs feedback to regulate the motor current accurately. This feedback comes from current sensing - the measurement of the actual current flowing through the motor windings. The driver IC uses this measurement to adjust the chopping, ensuring that the current stays at the set point regardless of supply voltage variations, motor back-EMF, and temperature changes. This chapter is devoted entirely to motor current sensing and regulation - the art and science of measuring and controlling the current in the motor windings. We will explain why current regulation is necessary, how it is implemented, and what the trade-offs are. We will explore the different sensing techniques: the low-side sense resistor, the high-side sense resistor, and the integrated current sense. We will look at the sense amplifier, the comparator, and the digital-to-analog converter that convert the current to a voltage and compare it to the reference. We will examine the different regulation algorithms: the fixed off-time chopper, the constant off-time chopper, the adaptive chopper, and the predictive current control. We will look at real-world designs from major companies: Texas Instruments' integrated current sense with the DRV8434, Allegro's sense resistor and comparator in the A4988, STMicroelectronics' programmable sense amplifier in the L6470, Trinamic's stealthChop2 technology that uses a different approach to current regulation, Toshiba's current sense amplifier in the TB67S109A, and ON Semiconductor's current sensing in the LV8728. We will discuss the importance of the sense resistor's value, tolerance, and power rating, and we will look at the Kelvin connection that eliminates the voltage drop in the sense traces. By the end, you will understand how the driver IC 'knows' how much current is flowing, how it adjusts the voltage to keep that current constant, and why this feedback loop is essential for smooth, quiet, and precise motor operation. |
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Chapter 1: Why Regulate Current- The Problem of Uncontrolled Current |
If you connected a stepper motor directly to a 24-volt power supply without any current regulation, the motor would draw a very high current - limited only by the winding resistance. For a typical 2-ohm winding, the current would be 12 amperes, which would quickly overheat the motor and the driver. Moreover, the motor's back-EMF (which opposes the current) changes with the motor's speed - at high speeds, the back-EMF is higher, reducing the current. So the current would vary with the speed, causing the torque to vary. The print quality would be inconsistent, and the motor would overheat. Current regulation solves this problem by limiting the current to a safe, constant value. The current is regulated by chopping the supply voltage - the voltage is applied in pulses, and the pulse width is adjusted to maintain the current at the set point. The current regulation is the heart of the motor drive, and it is the reason why the motor can operate smoothly at all speeds. |
Design Example: Unregulated Current vs. Regulated Current |
A European printer manufacturer tested a stepper motor with and without current regulation. Without regulation, the motor drew 10 amperes at startup and overheated in 30 seconds. The motor also had a very low torque at high speeds because the back-EMF reduced the current. With regulation (using a DRV8825), the current was limited to 2 amperes, and the motor operated at a stable temperature of 60C. The torque was consistent across the speed range. The manufacturer concluded that current regulation is essential for reliable and consistent printer operation. |
Chapter 2: The Basic Principle - Measuring Voltage Across a Resistor |
The most common way to measure current is to place a small resistor (the sense resistor) in series with the motor winding, and to measure the voltage drop across it. The voltage drop is proportional to the current (Ohm's law). The sense resistor is typically 0.05 to 0.2 ohms. The voltage drop is small - for a 2-ampere current and a 0.1-ohm resistor, the voltage drop is 0.2 volts. This small voltage is amplified by a sense amplifier, and then compared to a reference voltage. The sense amplifier must be accurate and have a low offset voltage. The sense resistor must be a precision resistor with a low temperature coefficient. The sense resistor must also be placed close to the driver IC to minimize the noise pickup. |
Design Example: Sense Resistor in A4988 |
The A4988 uses an external sense resistor. The sense resistor is 0.1 ohms, 1% tolerance. The voltage across the sense resistor is fed to the A4988's sense pins. The A4988 has a built-in sense amplifier with a gain of 5. The amplified voltage is compared to the VREF voltage. The sense resistor is placed within 2 millimeters of the A4988's sense pins. The manufacturer used a Kelvin connection to ensure that the sense traces do not carry the motor current. |
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Chapter 3: The Sense Resistor - A Precision Component |
The sense resistor is a critical component. Its value determines the current for a given reference voltage. Its tolerance determines the current accuracy. Its temperature coefficient determines the current drift with temperature. The sense resistor must be a low-inductance type, such as a metal strip resistor or a thick-film resistor. The sense resistor must also have a sufficient power rating - the power dissipation is I^2 * R. For a 2-ampere current and a 0.1-ohm resistor, the power is 0.4 watts. A 0.5-watt resistor is recommended. The sense resistor is typically a surface-mount device (1206 or 2512 package) that is placed close to the driver IC. |
Design Example: Sense Resistor in DRV8825 |
The DRV8825 uses an external sense resistor. In a design from a European manufacturer, the sense resistor is a 0.1-ohm, 1%, 0.5-watt resistor from Vishay. The resistor is a metal strip type with a low inductance of 5 nanohenries. The resistor has a temperature coefficient of 50 ppm per degree Celsius. The manufacturer measured the current accuracy and found it to be within 2% over the temperature range. |
Chapter 4: The Kelvin Connection - Eliminating the Sense Trace Voltage Drop |
The sense resistor is connected to the driver IC's sense pins. The sense pins carry the voltage across the resistor. However, the sense traces also carry the motor current (if they are connected to the power path). The voltage drop across the sense traces would add to the voltage across the sense resistor, causing an error. The Kelvin connection is a technique that eliminates this error. The Kelvin connection uses two separate traces - one for the current path and one for the sense voltage. The sense voltage traces do not carry the motor current, so there is no voltage drop. The Kelvin connection is essential for accurate current sensing. |
Design Example: Kelvin Connection in L6470 |
The L6470's reference design includes a Kelvin connection. The sense resistor is placed between the source of the low-side MOSFET and the ground. The power path goes through the sense resistor. The sense traces are separate, and they connect to the L6470's sense pins at the resistor's terminals. The manufacturer used a 4-wire Kelvin connection, with the sense traces routed as a differential pair. The manufacturer measured the current accuracy and found it to be within 1% - which is excellent. |
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Chapter 5: The Sense Amplifier - Amplifying the Small Signal |
The voltage across the sense resistor is small - typically 0.1 to 0.5 volts. This small voltage must be amplified to a level that can be compared to the reference voltage. The sense amplifier is a differential amplifier that amplifies the voltage across the sense resistor. The sense amplifier must have a low offset voltage - otherwise, the offset would cause a current error. The sense amplifier must also have a high common-mode rejection ratio - it must reject the common-mode voltage (which can be up to 24 volts). The sense amplifier is often integrated into the driver IC. Some driver ICs use an external sense amplifier. |
Design Example: Sense Amplifier in TB67S109A |
The TB67S109A has a built-in sense amplifier with a gain of 10. The sense amplifier has an offset voltage of 2 millivolts, which corresponds to a current error of 20 milliamperes (for a 0.1-ohm sense resistor). The manufacturer accepted this error because it is less than 1% of the 2-ampere set point. The sense amplifier has a common-mode rejection ratio of 80 dB, which is sufficient. |
Chapter 6: The Comparator - Comparing to the Reference |
The amplified sense voltage is compared to the reference voltage (VREF) by a comparator. The comparator is a fast analog comparator that trips when the amplified sense voltage exceeds the VREF. When the comparator trips, it triggers the chopper to turn off the high-side MOSFET. The comparator is a critical part of the current regulation loop. The comparator must be fast - the delay from the sense voltage exceeding the reference to the MOSFET turning off must be a few hundred nanoseconds. A slow comparator would cause the current to overshoot the set point. |
Design Example: Comparator in A4988 |
The A4988 has a built-in comparator with a delay of 200 nanoseconds. The comparator is designed to be fast enough for the 30-microsecond off-time. The manufacturer measured the current overshoot and found it to be 5% of the set point - which is acceptable. |
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Chapter 7: The Reference Voltage - A Precise Set Point |
The reference voltage (VREF) sets the current set point. The current is I = VREF / (gain * R_sense). For a gain of 5 and a sense resistor of 0.1 ohms, the current is VREF / (5 * 0.1) = VREF / 0.5. So a VREF of 1 volt gives a current of 2 amperes. The VREF is typically generated by a resistor divider from a precision voltage reference. The VREF must be stable and accurate - any variation in the VREF will cause a variation in the current. The VREF is also affected by the temperature, so a temperature-compensated reference is sometimes used. The VREF is typically 0.5 to 2 volts. |
Design Example: VREF Generation in DRV8825 |
The DRV8825's VREF is set by a resistor divider from a 3.3-volt rail. The divider consists of a 10-kilohm resistor and a 5-kilohm resistor, giving a VREF of 1.1 volts. The current is 1.1 / (0.5) = 2.2 amperes, but the DRV8825's current limit is 2.5 amperes, so the actual current is 2.2 amperes. The manufacturer used this design because it is simple and reliable. The manufacturer also added a 0.1-microfarad capacitor on the VREF pin to filter the noise. |
Chapter 8: The Fixed Off-Time Chopper - The Simplest Algorithm |
The fixed off-time chopper is the simplest current regulation algorithm. The high-side MOSFET is turned on, and the current rises. When the current reaches the set point, the comparator trips, and the high-side MOSFET is turned off for a fixed time (the off-time). After the off-time, the high-side MOSFET is turned on again. The off-time is fixed, typically 20 to 30 microseconds. The fixed off-time chopper is simple to implement and works well for most applications. However, the fixed off-time chopper can cause the current to be poorly regulated at high speeds, because the current rise time is shorter, and the off-time is not adjusted. The fixed off-time chopper also causes a variable switching frequency, which can cause audible noise. |
Design Example: Fixed Off-Time in A4988 |
The A4988 uses a fixed off-time chopper with an off-time of 30 microseconds. The manufacturer measured the current at different speeds and found that the current was within 5% of the set point for speeds up to 500 RPM. At higher speeds, the current dropped to 10% below the set point. The manufacturer accepted this because the printer's maximum speed was 400 RPM. |
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Chapter 9: The Constant Off-Time Chopper - A Variation |
The constant off-time chopper is a variation of the fixed off-time chopper. The off-time is not fixed, but it is adjusted to keep the chopping frequency constant. The constant off-time chopper uses a timer to set the off-time. The timer is adjusted based on the current. The constant off-time chopper gives a more constant switching frequency, which reduces the EMI and the audible noise. The constant off-time chopper is more complex than the fixed off-time chopper, but it gives a better performance. |
Design Example: Constant Off-Time in L6470 |
The L6470 uses a constant off-time chopper. The off-time is set by an external capacitor. The off-time is adjusted to keep the chopping frequency at 30 kilohertz. The manufacturer measured the current at different speeds and found that the current was within 2% of the set point. The manufacturer also noticed that the acoustic noise was lower with the constant off-time chopper. |
Chapter 10: The Adaptive Chopper - An Advanced Algorithm |
The adaptive chopper is an advanced algorithm that adjusts the off-time based on the motor's speed and the load. The adaptive chopper measures the current rise time and adjusts the off-time to keep the current regulation optimal. The adaptive chopper gives a very good current regulation at all speeds, and it also reduces the acoustic noise. The adaptive chopper is more complex than the fixed off-time chopper, but it gives the best performance. The adaptive chopper is used in high-end motor driver ICs. |
Design Example: Adaptive Chopper in TMC2209 |
The TMC2209 uses an adaptive chopper as part of its stealthChop2 technology. The adaptive chopper measures the current and adjusts the off-time to minimize the current ripple. The manufacturer measured the current at different speeds and found that the current was within 1% of the set point. The manufacturer also measured the acoustic noise and found it to be 35 dBA - very quiet. |
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Chapter 11: The Predictive Current Control - A Digital Approach |
The predictive current control is a digital algorithm that predicts the current for the next cycle and adjusts the on-time accordingly. The predictive current control uses a digital model of the motor. The model includes the motor's resistance, inductance, and back-EMF. The algorithm predicts the current and adjusts the on-time to achieve the set point. The predictive current control gives a very high accuracy and a very low current ripple. The predictive current control is used in high-end motor driver ICs. |
Design Example: Predictive Control in Trinamic TMC5160 |
The TMC5160 uses a predictive current control algorithm. The algorithm uses a digital model of the motor. The manufacturer measured the current at different speeds and found it to be within 0.5% of the set point. The manufacturer also found that the current ripple was very low - only 1% of the set point. |
Chapter 12: The Sense Resistor's Power Dissipation - A Thermal Issue |
The sense resistor dissipates power in the form of heat. The power dissipation is I^2 * R. For a 2-ampere current and a 0.1-ohm resistor, the power is 0.4 watts. The heat must be dissipated to prevent the resistor from overheating. The sense resistor is typically a surface-mount device that is cooled by the PCB's copper area. The sense resistor must have a power rating that is higher than the power dissipation. A 0.5-watt resistor is recommended for a 0.4-watt dissipation. |
Design Example: Sense Resistor Power in DRV8825 |
In the European manufacturer's design, the sense resistor is a 0.1-ohm, 0.5-watt resistor. The manufacturer measured the resistor's temperature and found it to be 60C at an ambient of 25C - which is well within the 125C maximum. The manufacturer also placed the sense resistor away from the heat-sensitive components. |
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Chapter 13: The Sense Resistor's Tolerance - A Current Accuracy Issue |
The sense resistor's tolerance determines the current accuracy. A 1% tolerance resistor gives a 1% current accuracy. A 5% tolerance resistor gives a 5% current accuracy. The current accuracy is important for the motor's torque and for the printer's print quality. A higher current accuracy gives a more consistent torque and a better print quality. The sense resistor is typically a 1% or 2% tolerance resistor. |
Design Example: Tolerance in TB67S109A |
The TB67S109A uses a 1% tolerance sense resistor. The manufacturer measured the current accuracy and found it to be within 1% of the set point. The manufacturer used a 1% tolerance resistor because it gives a good accuracy without being too expensive. |
Chapter 14: The Sense Resistor's Temperature Coefficient - A Drift Issue |
The sense resistor's temperature coefficient determines the current drift with temperature. A 50-ppm per degree Celsius resistor gives a current drift of 0.05% per degree Celsius. A 100-ppm resistor gives a drift of 0.1% per degree Celsius. The temperature coefficient is important for the printer's performance over the temperature range. A low-temperature coefficient is desirable. The sense resistor is typically a 50-ppm or 100-ppm resistor. |
Design Example: Temperature Coefficient in L6470 |
The L6470 uses a 50-ppm sense resistor. The manufacturer measured the current at 25C and at 60C and found that the current changed by only 0.05% per degree Celsius. The manufacturer was satisfied with the performance. |
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Chapter 15: The Sense Amplifier's Offset - A Current Error |
The sense amplifier has an offset voltage. The offset voltage causes a current error. The offset voltage is typically 1 to 5 millivolts. For a 0.1-ohm sense resistor and a gain of 5, a 2-millivolt offset causes a current error of 2 mV / (5 * 0.1) = 4 milliamperes. This is less than 0.2% of a 2-ampere set point. The offset is not a problem for most applications. |
Design Example: Offset in A4988 |
The A4988 has a sense amplifier with an offset of 2 millivolts. The manufacturer measured the current at zero set point and found that the offset caused a current of 4 milliamperes - which is negligible. |
Chapter 16: The Sense Amplifier's Common-Mode Rejection - A Noise Issue |
The sense amplifier must reject the common-mode voltage. The common-mode voltage is the voltage at the sense resistor, which can be up to 24 volts. The sense amplifier's common-mode rejection ratio (CMRR) is typically 80 dB. A high CMRR is important for accurate current sensing. The CMRR is determined by the sense amplifier's design. |
Design Example: CMRR in DRV8825 |
The DRV8825 has a CMRR of 80 dB. The manufacturer measured the current with a 24-volt common-mode voltage and found that the CMRR caused no measurable error. |
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Chapter 17: The Sense Amplifier's Bandwidth - A Speed Issue |
The sense amplifier must have a sufficient bandwidth to respond to the current changes. The bandwidth is typically 1 to 10 megahertz. The bandwidth must be higher than the chopping frequency (20 to 50 kilohertz). A higher bandwidth gives a faster response, but it also increases the noise. The bandwidth is a trade-off. |
Design Example: Bandwidth in L6470 |
The L6470 has a sense amplifier with a bandwidth of 2 megahertz. The manufacturer measured the current and found that the amplifier responded quickly to the current changes, with no noticeable delay. |
Chapter 18: The Chopping Frequency - A Trade-Off |
The chopping frequency is the frequency at which the current is regulated. The chopping frequency is determined by the off-time and the current rise time. The chopping frequency is typically 20 to 50 kilohertz. A higher frequency gives a lower current ripple, but it also causes more switching losses and more EMI. A lower frequency gives a higher current ripple, but it is more efficient and generates less EMI. The chopping frequency is a trade-off. |
Design Example: Chopping Frequency in A4988 |
The A4988 has a chopping frequency that varies with the current. At 2 amperes, the chopping frequency is 25 kilohertz. The manufacturer measured the current ripple and found it to be 10% of the set point - which is acceptable. The manufacturer also measured the EMI and found it to be within the limits. |
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Chapter 19: The Current Ripple - A Motor Performance Issue |
The current ripple is the variation in the current around the set point. The current ripple is caused by the chopping. The current ripple causes a torque ripple, which can cause vibration and acoustic noise. A lower current ripple gives a smoother motion and a lower noise. The current ripple is reduced by using a higher chopping frequency, a higher inductance motor, or a more advanced current regulation algorithm. |
Design Example: Current Ripple in TMC2209 |
The TMC2209 has a very low current ripple - less than 2% of the set point. The manufacturer achieved this by using a high chopping frequency (50 kilohertz) and a sophisticated adaptive chopper. The low current ripple gives a very smooth motion and a very low noise. |
Chapter 20: The Motor's Inductance - A Factor in Current Ripple |
The motor's inductance affects the current ripple. A higher inductance gives a lower current ripple because the current changes more slowly. A lower inductance gives a higher current ripple because the current changes more quickly. The inductance is determined by the motor's design. The motor's inductance is typically 1 to 10 millihenries. |
Design Example: Inductance in Brother Printers |
Brother's printer uses a motor with an inductance of 3 millihenries. The manufacturer measured the current ripple and found it to be 15% of the set point. The manufacturer accepted this because the printer's print quality was not affected. |
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Chapter 21: The Motor's Resistance - A Factor in Current Rise Time |
The motor's resistance affects the current rise time. A lower resistance gives a faster current rise time. A higher resistance gives a slower current rise time. The current rise time determines the maximum speed at which the motor can operate. A fast current rise time is desirable for high-speed operation. |
Design Example: Resistance in Zebra Printers |
Zebra's printer uses a motor with a resistance of 1.5 ohms. The manufacturer measured the current rise time and found it to be 100 microseconds. This is fast enough for the printer's maximum speed of 500 RPM. |
Chapter 22: The Supply Voltage - A Factor in Current Regulation |
The supply voltage affects the current regulation. A higher supply voltage gives a faster current rise time and a higher maximum speed. A lower supply voltage gives a slower current rise time and a lower maximum speed. The supply voltage is typically 24 volts for a printer. The supply voltage must be stable to ensure a consistent current regulation. |
Design Example: Supply Voltage in Sato Printers |
Sato's printer uses a 24-volt supply. The manufacturer measured the current at 22 volts and 26 volts and found that the current was within 2% of the set point, thanks to the current regulation. |
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Chapter 23: The Back-EMF - A Factor in Current Regulation |
The motor's back-EMF opposes the current. The back-EMF increases with the speed. At high speeds, the back-EMF is high, and the current is reduced. The current regulation compensates for the back-EMF by increasing the duty cycle. The current regulation must be fast enough to respond to the back-EMF changes. The back-EMF is a factor in the motor's torque-speed characteristic. |
Design Example: Back-EMF in DRV8825 |
The DRV8825 was tested at different speeds. The manufacturer measured the current and found that the current remained constant at 2 amperes up to 400 RPM. At higher speeds, the current dropped to 1.8 amperes because the back-EMF was too high for the 24-volt supply. |
Chapter 24: The Current Regulation in Micro-Stepping - A Special Case |
In micro-stepping, the current set point is not constant - it follows a sine and cosine waveform. The current regulation must be fast enough to track the sine and cosine. The current regulation must also be accurate at all points of the sine and cosine. The micro-stepping current regulation is more challenging than the full-step current regulation. The driver IC's micro-stepping indexer generates the sine and cosine references, and the current regulation loop tracks these references. |
Design Example: Micro-Stepping Current in TMC2209 |
The TMC2209's stealthChop2 technology provides a very accurate current regulation in micro-stepping mode. The manufacturer measured the current waveform and found it to be a perfect sine and cosine, with no distortion. The manufacturer also measured the torque ripple and found it to be less than 1%. |
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Chapter 25: The Current Regulation in Half-Step - A Simpler Case |
In half-step, the current set point alternates between the full current and half the current. The current regulation must be accurate at both levels. The half-step current regulation is simpler than the micro-stepping current regulation because the set point changes are step changes. |
Design Example: Half-Step Current in A4988 |
The A4988 was tested in half-step mode. The manufacturer measured the current and found it to be accurate at both levels - the full current was 2 amperes, and the half current was 1 ampere. |
Chapter 26: The Current Regulation in Full-Step - The Simplest Case |
In full-step, the current set point is constant. The current regulation is the simplest. The full-step mode is used in low-cost printers. |
Design Example: Full-Step Current in Brother Printers |
Brother's low-cost printer uses full-step mode. The manufacturer measured the current and found it to be constant at 1.5 amperes. |
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Chapter 27: The Current Regulation's Stability - A Control Theory Issue |
The current regulation loop must be stable. An unstable loop will oscillate, causing the current to vary at a sub-audio frequency. The stability is determined by the loop's gain and phase margin. The gain is determined by the sense amplifier's gain, the comparator's gain, and the motor's inductance. The phase margin is determined by the off-time and the motor's time constant. The current regulation loop is designed to be stable for all motor types. |
Design Example: Stability in L6470 |
The L6470's current regulation loop is stable for motors with an inductance of 1 to 10 millihenries. The manufacturer tested the L6470 with different motors and found the loop to be stable. |
Chapter 28: The Current Regulation's Noise - A Measurement Issue |
The current regulation loop can be affected by noise. The noise can come from the power supply, the motor, or the environment. The noise can cause the current to vary. The noise is reduced by using a low-pass filter on the sense amplifier's inputs. The low-pass filter is typically a 100-ohm resistor and a 1-nanofarad capacitor. |
Design Example: Noise Filter in DRV8825 |
The DRV8825's reference design includes a noise filter. The manufacturer added a 100-ohm resistor and a 1-nanofarad capacitor on the sense inputs. The manufacturer measured the current noise and found it to be reduced by 20 dB. |
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Chapter 29: The Current Regulation's Response Time - A Speed Issue |
The current regulation loop must respond quickly to the changes in the set point and the load. The response time is determined by the loop's bandwidth. A high bandwidth gives a fast response, but it also increases the noise. The bandwidth is typically 10 to 100 kilohertz. |
Design Example: Response Time in TMC2209 |
The TMC2209 has a fast response time - less than 10 microseconds. The manufacturer measured the current and found that it tracked the set point quickly. |
Chapter 30: The Current Regulation's Accuracy - A Performance Issue |
The current regulation's accuracy determines the torque accuracy. A high accuracy gives a consistent torque and a better print quality. The accuracy is determined by the sense resistor's tolerance, the sense amplifier's offset, the VREF accuracy, and the comparator's threshold. |
Design Example: Accuracy in TB67S109A |
The TB67S109A has a current accuracy of 2%. The manufacturer measured the current and found it to be within 2% of the set point. |
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Chapter 31: The Current Regulation's Power Dissipation - A Thermal Issue |
The current regulation loop dissipates power in the driver IC and the sense resistor. The power dissipation is I^2 * Rds(on) + I^2 * R_sense. The power dissipation must be managed to prevent the driver IC and the sense resistor from overheating. |
Design Example: Power Dissipation in DRV8825 |
The DRV8825 dissipates about 2 watts at 2 amperes. The manufacturer used a PCB with a large copper area to dissipate the heat. |
Chapter 32: The Current Regulation's EMI - A Regulatory Issue |
The current regulation loop generates EMI. The EMI is caused by the chopping. The EMI can interfere with the printer's communication and with other electronic devices. The EMI is reduced by using a spread-spectrum modulation and by using a shielded cable. |
Design Example: EMI in DRV8825 |
The DRV8825 has a spread-spectrum option. The manufacturer enabled the spread-spectrum and measured the EMI. The spread-spectrum reduced the peak EMI by 10 dB. |
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Chapter 33: The Current Regulation's Diagnostic - A Maintenance Issue |
The current regulation loop can be used to diagnose the motor's health. The current waveform can indicate a worn bearing or a partial short. The driver IC can measure the current and report the diagnostic data. The diagnostic data can be used to predict the motor's failure. |
Design Example: Diagnostic in L6470 |
The L6470 has a diagnostic feature that reports the current and the temperature. The manufacturer used this feature to monitor the motor's health. |
Chapter 34: The Current Regulation in Closed-Loop Control - An Advanced Case |
In closed-loop control, the current regulation is combined with a position or speed feedback. The closed-loop control uses an encoder to measure the motor's position. The CPU compares the position to the target and adjusts the current set point. The current regulation is the inner loop, and the position control is the outer loop. The closed-loop control gives a very high accuracy and a very high torque. |
Design Example: Closed-Loop in Sato Printers |
Sato's printer uses a closed-loop control for the platen motor. The current regulation is done by the driver IC, and the position control is done by the CPU. The CPU reads the encoder and adjusts the current set point. The manufacturer reports that the closed-loop control gives a paper feed accuracy of 0.01 millimeters. |
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Chapter 35: The Current Regulation's Fault Detection - A Safety Issue |
The current regulation loop can detect a fault. If the current exceeds the set point for a long time, the driver IC detects a stall. If the current is zero when the motor is on, the driver IC detects an open circuit. The fault detection is used to protect the motor and the printer. |
Design Example: Fault Detection in A4988 |
The A4988 has a fault output that is triggered by an over-current or an over-temperature. The manufacturer used the fault output to detect a paper jam. |
Chapter 36: The Current Regulation's Temperature Compensation - A Drift Issue |
The current regulation can be compensated for the temperature. The sense resistor's resistance changes with the temperature. The motor's resistance changes with the temperature. The compensation can be done by measuring the temperature and adjusting the VREF. The compensation improves the current accuracy over the temperature range. |
Design Example: Temperature Compensation in TMC2209 |
The TMC2209 has a built-in temperature sensor. The driver IC uses the temperature sensor to compensate for the temperature drift. The manufacturer measured the current over the temperature range and found it to be within 1% of the set point. |
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Chapter 37: The Current Regulation's Future - Digital and Intelligent |
The future of current regulation lies in digital and intelligent solutions. The future current regulation will use a digital signal processor (DSP) to implement advanced algorithms. The future current regulation will also use artificial intelligence (AI) to learn the motor's behavior and to predict the optimal current. The future current regulation will be more accurate, more efficient, and more reliable. |
Design Example: Digital Current Regulation in a Prototype |
A prototype printer from a startup uses a digital current regulation with a DSP. The DSP implements a predictive current control algorithm. The manufacturer reports that the digital current regulation gives a current accuracy of 0.1% and a torque ripple of less than 0.5%. |
Chapter 38: The System Integration - Putting It All Together |
We have now covered every aspect of motor current sensing and regulation. Let us put it all together. The motor driver IC measures the current through the sense resistor. The sense amplifier amplifies the voltage, and the comparator compares it to the reference. The comparator triggers the chopper, which regulates the current. The current regulation loop is the heart of the motor drive. It ensures that the motor current is constant, regardless of the supply voltage, the motor speed, and the load. The current regulation loop is the reason why the motor can operate smoothly, quietly, and precisely. |
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Detailed Summary - Tying It All Together |
We have now completed our comprehensive exploration of motor current sensing and regulation - the art and science of controlling the current in the motor windings. We began by understanding why current regulation is necessary: without it, the motor would draw excessive current, overheat, and provide inconsistent torque. We learned that the basic principle is to measure the voltage across a sense resistor, amplify it, compare it to a reference, and then chop the supply voltage to maintain the current at the set point. |
We explored the sense resistor, the precision component that measures the current. We saw how its value, tolerance, temperature coefficient, and power rating affect the current accuracy. We examined the Kelvin connection, which eliminates the voltage drop in the sense traces, and we saw how it is used in the L6470 reference design. |
We looked at the sense amplifier, which amplifies the small voltage across the sense resistor. We examined the comparator, which compares the amplified voltage to the reference. We discussed the reference voltage (VREF), which sets the current set point. We saw how the VREF is generated and how it affects the current. |
We explored the different regulation algorithms: the fixed off-time chopper, the constant off-time chopper, the adaptive chopper, and the predictive current control. We saw the trade-offs between them - the fixed off-time chopper is simple but less accurate, while the predictive current control is accurate but complex. We looked at real-world examples from A4988, L6470, TMC2209, and TMC5160. |
We discussed the practical aspects: the sense resistor's power dissipation, tolerance, and temperature coefficient. We examined the sense amplifier's offset, common-mode rejection, and bandwidth. We looked at the chopping frequency and the current ripple, and we saw how they affect the motor's performance and the EMI. |
We considered the motor's inductance, resistance, and back-EMF - the factors that affect the current regulation. We saw how the current regulation is used in micro-stepping, half-step, and full-step modes. We discussed the stability, noise, response time, and accuracy of the current regulation loop. |
We explored the advanced features: the current regulation in closed-loop control, the fault detection, the temperature compensation, and the diagnostic features. We looked to the future with digital and intelligent current regulation using DSPs and AI. |
The overarching lesson is that current sensing and regulation is not a simple afterthought - it is a carefully engineered feedback loop that determines the motor's torque, efficiency, and reliability. A well-designed current regulation loop ensures that the motor operates smoothly, quietly, and precisely, regardless of the operating conditions. A poorly designed loop causes torque ripple, vibration, and overheating. Understanding current sensing and regulation is essential for any engineer who wants to design a high-performance thermal printer, and this chapter has provided that understanding from the basic principles to the advanced control techniques. |
End of Extended Section 12 |
cs@easiersoft.com