Control unit and method for detecting stall or step loss of a stepping motor

Abstract

To provide a control unit and method to detect stall or step-loss while driving a stepper motor.SOLUTION: A control unit 102 is suitable to determine a first parameter from a back EMF signal, and to determine at least one second parameter from the back EMF signal corresponding to the first parameter. The control unit 102 further verifies, based on the first parameter, at least one condition containing the at least one second parameter, and detects stall or step-loss in a stepper motor 110 based on the verification. In the present invention, a pattern detection by identifying characteristic features of peaks of the back EMF signal, which are order (n), magnitude and polarity, is used. These characteristic features are independent of RPM, backlash, and load, do not require complex calculations, and are very robust even at lower RPMs.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a control unit and method for detecting stall or step loss during driving of a stepping motor.

Background Art

[0002] Sensorless stall and step loss detection in stepper-based systems is in demand in various systems. In this system, motor operation at a low rotational speed per minute (RPM) with variable load and backlash is required, and existing solutions cannot detect stall or step loss. Stall detection is necessary to achieve a reference position, and non-detection leads to over-driving of the actuator, reducing the operating time and gradually damaging the drive. Stall detection is also used to detect damage such as damage to the drive mechanism or motor of a product that can occur in a magnetic field. Existing solutions extract back electromotive force (EMF) from a non-excited coil and use a comparison based on a threshold to detect stall or step loss. The threshold is derived dynamically based on RPM, statically fixed, or calibrated for a given system.

[0003] The concept of stall detection is used in many systems. In most systems, the stepping motor operates at high RPM, in a system with little backlash, or without any dynamic load when stall detection is stable. The concept used in existing solutions is to monitor the back EMF. Stall is detected when the back EMF drops below a given threshold or when the back EMF approaches zero in the case of stall. (It is ideal if selected for torque requirements or resonance avoidance) When the load and backlash of the drive are variable at low RPM, stall detection tends to become less robust. Generally, products using stepping motors can be seen in the market. Stall detection is mainly used to detect damage in a mechanical drive or to achieve a reference position and step loss detection for rough correction.

[0004] Patent Document 1 discloses an apparatus, a system, and a method for detecting stepping motor stall. The apparatus, system, and method are disclosed for detecting stepping motor stall. The conversion module receives the back electromotive force ("EMF") waveform generated in the non-driven coil of the stepping motor by the magnetic field of the rotor of the stepping motor during full-step operation, samples the waveform at intervals starting at a predetermined time point over a predetermined time in full-step operation, and obtains a plurality of sampled data points. The rectification module rectifies the waveform near a predetermined stationary level. The comparison module accumulates the sampled data points after rectification as a statistically representative sampling value, compares the representative sampling value with a predetermined threshold level, and indicates rotor stall when the representative sampling value is less than the predetermined threshold level.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Brief Description of the Drawings

[0006] The disclosed embodiments are described with reference to the following attached drawings.

[0007]

Figure 1

[0008]

Figure 2

[0009]

Figure 3

[0010]

Figure 4

Best Mode for Carrying Out the Invention

[0011] FIG. 1 shows a block diagram of a control unit for a stepping motor according to an embodiment of the present invention. A system 100 including a control unit 102 is provided for detecting stall or step loss of a stepping motor 110. The stall of the stepping motor is due to mechanical drive. The stepping motor 110 includes a rotor 112 such as a magnet and a stator such as a field coil or pole. The described stepping motor 110 has two field coils, namely a first coil 106 and a second coil 108. The present invention is not limited to the stepping motor 110 based on two field coils, but rather is applicable to the stepping motor 110 having a large number of poles. As shown in FIG. 2, the control unit 102 is suitable for driving at least two field coils (the first coil 106 and the second coil 108) of the stepping motor 110 through a driver circuit 104 to detect / read an electrical signal including a back electromotive force (EMF). The detection is performed through a detector circuit 114 and an amplifier 116. Without departing from the scope of the present invention, other detector circuits 114 are applicable for use. The detector circuit 114 is shown as being connected to the first coil 106. The detector circuit 114 can also be connected to the second coil 108. Further, the detector circuit 114 can be connected to both the first coil 106 and the second coil 108. The detector circuit 114 extracts / reads the back EMF signal of the stepping motor 110 with an intermediate level bias.

[0012] The present invention is independent of the method by which the back-EMF signal is read or measured. The back-EMF signal of the present invention is detected from the non-driven coil of the stepping motor 110 and the same is used throughout the present disclosure. However, this is merely an example and should not be understood in a limiting sense. The control unit 102 is configured to detect a stall or a step loss for the back-EMF signal detected / read by a circuit or method well-known in the art.

[0013] FIG. 2 shows a graph relating to the back-EMF signal according to an embodiment of the present invention. The first graph 200 includes a Y-axis 202 representing the amperes of current shown as a first waveform 208 and the volts of voltage of a second waveform 210, i.e., the back-EMF signal. The X-axis 204 of the first waveform 208 and the second waveform 210 is time in appropriate units. Both waveforms are shown after the passage of the time marked by a dotted line at the start of each waveform. The back-EMF signal can be measured in a positive phase and a negative phase. Subsequently in FIG. 1, the control unit 102 further determines a first parameter from the back-EMF signal and determines at least one second parameter corresponding to the first parameter from the back-EMF signal. The control unit 102 further verifies at least one condition including at least one second parameter based on the first parameter and detects a stall or a step loss of the stepping motor 110 based on the verification.

[0014] The first parameter determined by the control unit 102 is the order (n) of the peaks in the back-EMF signal. The order (n) corresponds to the number of peaks in the back-EMF signal. The at least one second parameter is selected from the group including the polarity (P n ) of each peak and the amplitude / magnitude (M n ) of each peak. The detector circuit 114 determines the order (n), the polarity (P n ) and the magnitude (M n) is used to extract / capture the same. During capture, the back-EMF signal induced in the first coil 106 is sampled at a high frequency so that the peak is captured. Then the captured signal is amplified and an analysis is performed to detect a stall or a step loss.

[0015] The first waveform 208 corresponds to the startup signal for at least two coils. In the case of the 2-pole stepping motor 110, the pulse of the first waveform 208 means the startup of the first coil 106, and the absence of the pulse means that the first coil 106 is not being driven. However, when the first coil 106 is not being driven, the second coil 108 may or may not be driven, but this is not shown in FIG. 2 to avoid complexity. Further, the positive phase / cycle corresponds to the waveform portion above the X-axis (204, 206), and the negative phase / cycle corresponds to the waveform portion below the X-axis (204, 206). The back-EMF signal shown in FIG. 2 includes three peaks and is without a stall or a step loss. Specifically stated, the first case 230 shows a back-EMF signal of positive phase, the second case 232 shows a back-EMF signal of negative phase, and the third case 234 shows a back-EMF signal of positive phase and others.

[0016] An enlarged portion 250 of the back-EMF signal is illustrated. The enlarged portion 250 is shown to represent the scale of the peak. The first scale 224 of the first peak 212, the second scale 224 of the second peak 214, and the third scale 224 of the third peak 216 are illustrated. Still further, the time period between time point t1 218 and time point t2 220 is considered a dead zone. To overcome false detection due to inductor discharge, the control unit 102 processes the back-EMF signal after the dead zone / window / time has elapsed. Thus, the dead zone is excluded for the detection of a stall or a step loss in the back-EMF signal. The time period between time point t2 220 and time point t3 222 is a working zone, and the control unit 102 performs the detection of a stall or a step loss. Also, the peak is considered only when the scale of the peak exceeds the threshold level in both the positive and negative phases.

[0017] Figure 3 illustrates a number of cases regarding the detection of stall or step loss according to an embodiment of the present invention. Figure 3 illustrates a second graph 300 having a number of waveforms. It should be noted that the graph is not to a fixed scale and is intended to explain the present invention and should not be understood as limiting. The first waveform 208 represents the current shown in Figure 2. Similarly, the second waveform 210 represents the volts of the back-EMF signal. Another example of the back-EMF signal is shown in the third waveform 302, where there is only one peak after the dead zone, while there are two peaks in the second waveform 210, one above the axis and the other below the axis. The second waveform 210 and the third waveform 302 are shown to enable a clear understanding, and no stall or step loss is seen in the waveforms.

[0018] As shown in the fourth waveform 304, when the order is equal to zero, the control unit 102 detects a stall or step loss of the stepper motor 110. When there is no peak in the back-EMF signal, the control unit 102 detects this condition as a stall or step loss condition and records it in the memory element. The first window 318 (depicted by the dotted boundary) indicates the absence of a peak. This is also applicable to the positive phase / cycle of the back-EMF signal.

[0019] According to one embodiment of the present invention, the control unit 102 is suitable for detecting a stall or a step loss of the stepping motor 110 when the order is equal to 1. When the order is equal to 1, at least one condition verified by the control unit 102 includes that the polarity of the first peak 212 is selected from the group including negative in the positive phase of the back-EMF signal and positive in the negative phase of the back-EMF signal. The fifth waveform 306 represents a back-EMF signal having only the first peak 212, and the stall or step loss is detected in the negative phase. The second window 320 indicates the stall or step loss of the fifth waveform 306. A peak inside the second window 320 (indicated by a dotted line) above the axis is considered the first peak 212. Since it is above the axis, the polarity of the first peak 212 is positive. The two peaks below the axis are excluded because they are lower than the threshold limit. Since the polarity of the first peak 212 is positive in the negative phase, the control unit 102 detects this as a stall or step loss and records / stores it in the memory element. Similarly, the sixth waveform 308 represents a stall or step loss for a single peak in the positive phase. The third window 322 indicates that the polarity of the first peak 212 is negative in the negative phase. The control unit 102 detects this condition as a stall or step loss and stores it in the memory element.

[0020] According to another embodiment of the present invention, the control unit 102 is suitable for detecting a stall or a step loss when the order (n) is equal to 2. When the order is 2, at least one condition verified by the control unit 102 includes that the polarities of the first peak 212 and the second peak 214 are each selected from the list including negative and positive in the positive phase of the back-EMF signal and positive and negative in the negative phase of the back-EMF signal, respectively. The waveform for this condition is not shown to avoid overcrowding in FIG. 3.

[0021] According to another embodiment of the present invention, the control unit 102 is suitable for detecting a stall or a step loss when the order is equal to 2 and when one condition including the peak magnitude is satisfied. At least one condition verified by the control unit 102 includes that the magnitude 224 of the first peak 212 is smaller than the magnitude 224 of the second peak 214. The peak magnitude is considered with a predetermined tolerance. The seventh waveform 310 represents the back-EMF signal for an order equal to 2. The fourth window 324 indicates the presence of two peaks. The polarity of the first peak 212 is negative and that of the second peak 214 is positive. The condition regarding the polarity does not indicate either a stall or a step loss. However, the magnitude 224 of the first peak 212 is smaller than the magnitude 224 of the second peak 214. This condition, when consistent, is detected as a stall or a step loss, and the control unit 102 stores this detection in a memory element. Therefore, the condition including the magnitude is a substitute for the condition based on the polarity. Further, the condition of the magnitude can be verified before the condition regarding the polarity. Similarly, the eighth waveform 312 indicates a stall or a step loss in the positive phase for the back-EMF signal having two peaks. The polarity of the first peak 212 is positive and that of the second peak 214 is negative, whereby no stall or step loss is detected. However, the magnitude 224 of the first peak 212 is smaller than the magnitude 224 of the second peak 214. This condition is detected as a stall or a step loss by the control unit 102, and as a result, the control unit 102 stores this data in a memory element.

[0022] According to another embodiment of the present invention, the control unit 102 is suitable for detecting a stall or a step loss when the order (n) is equal to 3. When the order is 3, at least one condition verified by the control unit 102 is that the polarities of the first peak 212, the second peak 214, and the third peak 216 are respectively negative, positive, and negative in the positive phase of the back-EMF signal, and positive, negative, and positive in the negative phase of the back-EMF signal, and any one selected from the group including them. Again, the waveform for this condition is not shown to avoid overcrowding in FIG. 3. Before at least one condition regarding order 3 is checked, the control unit 102 checks at least one condition applicable to orders 1 and 2 for one valid peak and two peaks. When at least one condition applicable to orders 1 and 2 is met, the control unit 102 detects a stall or a step loss condition without verifying at least one condition applicable to order 3.

[0023] According to another embodiment of the present invention, the control unit 102 is suitable for detecting a stall or a step loss when the order is equal to 3 and when one condition including the scale of the peak is satisfied. When the order (n) is equal to 3, at least one condition verified by the control unit 102 includes that the scale 224 of the first peak 212 is smaller than the scale 224 of any one of the second peak 214 and the scale 224 of the third peak 216. To illustrate this condition, the ninth waveform 314 shows the back-EMF signal including the fifth window 328. Three peaks are drawn in the fifth window 328. In the negative phase, the polarity of the first peak 212 is negative, the second peak 214 is positive, and the third peak 216 is negative. This does not indicate a stall or a step loss condition. However, in the comparison of the scales, the control unit 102 detects that the scale 224 of the first peak 212 is smaller than the scale 224 of the second peak 214 but not smaller than the scale of the third peak. Therefore, the control unit 102 detects this condition as a stall or a step loss and stores it in the memory element. Similarly, the tenth waveform 316 shows a stall or a step loss of the back-EMF signal having three peaks for the positive phase. The sixth window 330 shows that the polarity of the first peak 212 is positive, the second peak 214 is negative, and the third peak 216 is positive, and does not indicate a stall or a step loss. However, the scale 224 of the first peak 212 is smaller than the scale 224 of the second peak 214 but not smaller than the scale of the third peak, which is detected by the control unit 102 as a stall or a step loss and is recorded in the memory element.

[0024] According to one embodiment of the present invention, when the order is greater than 0, the control unit 102 is suitable for verifying the polarity-based condition as the first step and then the scale-based condition. Alternatively, the control unit 102 can also be set to first verify the scale-based condition and then the polarity-based condition.

[0025] According to another embodiment of the present invention, the control unit 102 extracts the back-EMF signal from at least two coils based on which coil is non-operative or not being driven. Example: In the case of a stepper motor 110 based on two coils, when the first coil 106 is operative / driven, the second coil 108 is non-operative, and thus the control unit 102 captures the back-EMF signal from the second coil 108. Next, when the second coil 108 is operative and the first coil 106 is non-operative, the control unit 102 captures the back-EMF signal from the first coil 106. Thus, compared to the case where only the first coil 106 is used to capture the back-EMF signal, the control unit 102 is provided with more data samples and detects stall or step loss in a shorter time. The control unit 102 processes the captured back-EMF signal with the verification of at least one condition before capturing the back-EMF signal in the next phase.

[0026] In one embodiment, the stall or step loss detection is incremented / decremented up to a threshold number. When the number of stall or step loss detections exceeds the threshold number, a stall or step loss is confirmed. The threshold number can be set according to requirements. When a stall or step loss is detected, the control unit 102 activates an alert to its user or operator through at least one signal including, but not limited to, voice, display, light, tactile means, etc.

[0027] Figure 4 illustrates a method for detecting a stall or a step loss of a stepping motor according to the present invention. This method includes step 402 which includes stepping, that is, monitoring a back electromotive force (EMF) signal from the stepping motor 110. The back EMF signal is detected or measured or extracted by techniques well known in the art and is not limited to a particular method. The back EMF signal can be measured in positive and negative phases. This method is characterized by step 404 which includes determining a first parameter of the back EMF signal by the control unit 102. The next step 406 includes determining by the control unit 102 at least one second parameter of the back EMF signal corresponding to the first parameter. Step 408 includes verifying at least one condition including at least one second parameter based on the first parameter. Step 410 includes detecting by the control unit 102 a stall or a step loss of the stepping motor 110 based on the verification.

[0028] The first parameter is the order (n) of the peaks of the back EMF signal. At least one second parameter is selected from a group including the polarity (P n ) of each peak and the magnitude (M n ) of each peak.

[0029] The conditions verified following the determination of the order of the back EMF signal are the same as those described and corresponding for Figure 3. The same description is not repeated to avoid repetition. However, before the start of activation of the stepping motor 110, the current electrical phase is read from the driver circuit 104 and synchronized with the phase of the back EMF signal. The phase is further used to detect the polarity and the magnitude as required.

[0030] The control unit 102 is also suitable for identifying the stall condition and the step loss condition. In the case where the lead screw mechanism is actuated by the stepping motor 110, a stall is identified when the end position of the lead screw is reached. If a peak is observed before reaching the end position, this condition is identified as a step loss. The method of identifying the stall and the step loss is in accordance with the corresponding steps described for the control unit 102.

[0031] According to the present invention, existing solutions require tuning and the threshold of the back EMF signal is directly proportional to the revolutions per minute (RPM), while the present invention is independent of RPM and does not require tuning or simulation to determine the factor. One of the application areas of the present invention is the semi-active damping control (SDC) system of a vehicle, throttle control, instrument panel, and many linear positioning systems (such as warning displays). The present invention uses pattern detection by identifying characteristic points of the peak of the back EMF signal in terms of order (n), scale, and polarity. These characteristic points are independent of RPM, backlash, and load, do not require complex calculations, and are very robust even at low RPM.

[0032] It should be understood that the embodiments described above are merely illustrative and do not limit the scope of the present invention. Many such embodiments and other variations and modifications of the embodiments described in the description section are conceivable. The scope of the invention is limited only by the scope of the claims.

Explanation of reference numerals

[0033] 100 System 102 Control unit 104 Driver circuit 106 First field magnet coil 108 Second field magnet coil 110 Stepping motor 112 Rotor 114 Detector circuit 116 Amplifier 200 First graph 202 Y-axis 204, 206 X-axis 208 First waveform 210 Second waveform 212 First peak 214 Second peak 216 Third peak 218 Time point t1 220 Time point t2 222 Time point t3 224 Scale 230 First case 232 Second case 234 Third case 250 Enlarged part 300 Second graph 302 Third waveform 304 Fourth waveform 306 Fifth waveform 308 Sixth waveform 310 Seventh waveform 312 Eighth waveform 314 Ninth waveform 316 Tenth waveform 318 First window 320 Second window 322 Third window 324 Fourth window 328 Fifth window 330 Sixth window

Claims

1. A control unit (102) for detecting stall or step loss in a stepping motor (110), the control unit (102) being adapted to drive at least two field coils (106, 108) of the stepping motor (110) and read a back electromotive force (EMF) signal, which is an EMF signal measurable in a positive phase and a negative phase, and further, determining a first parameter from the back EMF signal; determining at least one second parameter corresponding to the first parameter from the back EMF signal; verifying at least one condition including the at least one second parameter based on the first parameter; detecting stall or step loss of the stepping motor (110) based on the verification; A control unit (102) suitable for; wherein the first parameter is the order (n) of peaks in the back EMF signal, and the at least one second parameter is selected from a group including the polarity (P ｎ ) of each peak and the magnitude (M ｎ ) of each peak, characterized by a control unit (102).

2. When the order (n) is equal to 1, the at least one condition to be verified is that the polarity of the peak is a) negative in the positive phase of the back EMF signal; b) positive in the negative phase of the back EMF signal; The control unit (102) according to claim 1, including any one of the lists.

3. When the order (n) is equal to 2, the at least one condition to be verified is that the polarities of the first peak (212) and the second peak (214) are a) negative and positive respectively in the positive phase of the back EMF signal; b) positive and negative respectively in the negative phase of the back EMF signal; The control unit (102) according to claim 1, including being any one selected from a list including

4. When the order (n) is equal to 2, the at least one condition to be verified includes that the scale of the first peak (212) is smaller than the scale of the second peak (214), the control unit (102) according to claim 3.

5. When the order (n) is equal to 3, the at least one condition to be verified is that the polarities of the first peak (212), the second peak (214), and the third peak (216) are a) negative, positive, and negative, respectively, in the positive phase of the back EMF signal, b) positive, negative, and positive, respectively, in the negative phase of the back EMF signal, The control unit (102) according to claim 1, including being any one selected from a list including

6. When the order (n) is equal to 3, the at least one condition to be verified is that the scale of the first peak (212) is smaller than the scale of either one of the second peak (214) and the third peak (216), the control unit (102) according to claim 5.

7. A control unit (102) suitable for verifying at least one corresponding condition applicable to an order (n) equal to 2 before verifying the at least one condition applicable to an order (n) equal to 3, the control unit (102) according to claim 5 or claim 6.

8. A method for detecting a stall or step loss of a stepper motor (110), including the step of monitoring a back electromotive force (EMF) signal that is a back EMF signal from the stepper motor (110), wherein the back EMF signal can be measured in a positive phase and a negative phase, Determining a first parameter of the back EMF signal by a control unit (102), The control unit (102) determines at least one second parameter of the back-EMF signal corresponding to the first parameter; Based on the first parameter, the control unit (102) verifies at least one condition including the at least one second parameter; Based on the verification, the control unit (102) detects a stall or a step loss of the stepping motor (110); The first parameter is the order (n) of peaks in the back-EMF signal, and the at least one second parameter is selected from a group including the polarity (P ｎ ) of each peak and the magnitude (M ｎ ) of each peak; A method characterized by the above.

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