The use of stepper motors is an advantageous choice aslong as the motors do not koose any steps during the operation over along period - ideally without any homing device or transducer.
Stepper motors operate open loop and the moment a stepping motor does not operate correctly in a specific situation the most frequent conclusion is that the motor is faulty. Reasons could be that it is too small, the load too high, etc.
The motor selection is of course important - however it is not the only cause of step losses.
The following shall introduce the options to analyze step losses or non operation in a methodical fashion, enabling the designer to secure his application step by step by checking:
A. Motor Selection B. Analysis of running test results C. Back-driving D. External commutation errors
A. Stepping Motor Selection
The respect of the rules is crucial:
1. Select the motor for the highest torque/speed point in the application 2. Use a 30% safety factor from the published torque speed curve 3. Assure that the application cannot be stalled by external events
If the qualification test is successful, no step loses will occur during operation. If the test is not successful, it is required to find the reason. Since the motor is normally not equipped with a feedback device and since the motor current is not providing any assistance, this is not necessarily an easy task. There is, however, a logic that may help to analyze the reason. The following overview gives a good understanding:
NOTE A step motor cannot loose during normal operation one step only! If speed is low it will loose a multiple of 4 steps (8, 12, 16 …. Steps) or, if the speed is high stop rotating.
In cases that less than 4 steps are lost the reason must be searched with the commutation.
B. Analysis of the running test results
Depending on the type of motion profile the analysis is different
A1. Start-Stop Test
For this test the motor is hooked up to the load and a fixed frequency applied to the driver. The motor has to accelerate the load (inertia and friction) within the first step to the commanded frequency. Failure modes: 1. Motor does not start
Reasons: • load to high – wrong motor • frequency too high - reduce
A2. Acceleration (trapezoidal) profile
Failure modes: 1. Motor does not start with set fStart/Stop Reasons: • load to high – wrong motor • frequency too high – reduce frequency 2. Motor does not finish the acceleration ramp.
Possible reasons • Resonance increase the acceleration select start-stop frequency above resonance point use half-stepping or micro-stepping
• Supply voltage too low increase voltage test lower impedance motor use current mode driver (if voltage driver is used)
• Top speed too high reduce top speed 3. Motor finishes acceleration but stalls when constant speed is reached. Possible reasons • The motor is operating at the limit of its ability and stalls because it develops after the acceleration much more torque than needed. This causes vibrations the motor may not be able to damp.
Reduce jerk i.e. select a smaller acceleration rate or use two different acceleration levels, high at start, lower towards top speed.
Increase torque and system stiffness when the peak speed has been reached by boosting the current for 3-5 steps. This reduces the vibrations caused by the abrupt acceleration change from Value X to Zero (jerk).
C. Back driving
This type of failure results also in the loss of at least one commutation cycle of 4 steps, mainly in applications where the motor currents are considerably reduced or switched off after the movement. Even though ARSAPE step motors are usually used with significant gear ratios which normally generate some degree of anti-back driving. It cannot be excluded that the gears wind up during the movement and return this energy back to the motor when the currents are switched off. The motor shaft may be rotated backwards. If this back driving is larger than one step, the motor will, when the current is switched on for the next movement it is possible that the motor develops not sufficient torque to complete the first step and will either not start or only after 4 full-steps.
Possible solution:
1. Program the commutation in such way that every time the motor currents are switched off the same motor current and polarity is applied as applied for the last step before phase current(s) were turned off.
D. External Commutation Errors
As explained in the note on page 1, it is not possible to explain
The shown driver represents a type which can be commanded without any additional control element directly with a 4 bit word. The commutation happens as follows
Phase 1
Enable 1
Phase 2
Enable 2
Current Phase A
Current Phase B
Sense of Rotation
X
H
X
H
OFF
OFF
H
L
X
H
POS
OFF
X
H
H
L
OFF
POS
L
L
X
H
NEG
OFF
X
H
L
L
OFF
NEG
Table 1: One Phase ON Commutation - typical driver
This sequence is normally not the reason for any failure or step losses during normal operation when current is applied all the times to the phases. This changes when power is switched off
Power off of the controller which provides the 4 bits of the example (4 bits for Phase1, Phase2, Enable1, Enable2) will loose the counter status which is required to assure an uninterrupted stepping sequence following the above table. At power up it is therefore not assured that the counter will find the position it had prior to power-off. If there is a difference between the counter position and the real rotor position the motor will execute uncontrolled steps. The maximum error may be 2 steps in CCW or CW direction Solution • before power-off memorize the 4 bit word (more for more sophisticated drives) and reload it from memory for the counter initialization. Power-On with this position again before continuing the commutation.
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Why Stepper Motors Lose Steps
