Answers to the most frequently asked questions.
This refers to non-cumulative errors encountered in machining the motor.
Repeatability indicates how precisely a previous position can be re-established. There are very few errors in the system that cause a given position to vary, so basically, the same inaccuracy is encountered on returning to that position.
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Stepper Motor FAQs
- Current flows through the motor windings even at standstill.
- PWM drive designs tend to make the motor run hotter. Motor construction, such as lamination material and riveted rotors, will also affect heating.
Most motors have class B insulation, which is rated at 130∞C. Motor case temperatures of 90C should not cause thermal problems. However, motors should be mounted away from where operators can come into contact with the motor case.
Many drives feature an automatic reduction of current at standstill set by a command or jumper link. This reduces current when the motor is at rest without positional loss.
No. The basic absolute accuracy and hysteresis of the motor remain unchanged.
Yes, but if the load inertia is more than ten times the rotor inertia, extended ringing at the end of the move may be experienced.
Friction in the system will help damp this oscillation. If a start/stop velocity is used, lowering it will help. The additional benefits of using a microstepping drive almost eliminates this problem.
An unloaded motor develops a high torque in relation to its own inertia. Steppers are optimised for loads of the same order as the rotor inertia.
This is most likely to happen with full step systems. Adding inertia lowers the resonant frequency, and friction tends to damp out resonance. Start/stop velocities higher than the resonant speed will help. Changing to a half step, ministep or microstep drive will also minimise any resonance.
An additional phenomena of stepper systems can be midrange instability. This varies in severity with the nature of the load being driven. It generally sets in at step rates of 1200 to 3000 full steps/second (6 to 15 revolutions per second). The oscillation itself is in the 50-150 Hz range, and often builds in amplitude over a number of cycles, causing a stall condition within 0.1 to 1 second. Unlike the fundamental motor resonance, half stepping and microstepping do not alleviate the problem.
Our bipolar microstepping drives completely suppress midrange resonance, by sensing the deviation from intended position and electronically introducing viscous damping to counteract the effect. This ‘injection’ allows the motor torque to be used for accelerating the load, instead of being wasted on spurious oscillations.
The motor has 200 natural detent positions, but the drive powers up in a defined state corresponding to one of only 50 positions. Movement can therefore be up to 3.6∞ in either direction.
No, the motor is still a standard 1.8 degree stepper. Microstepping is accomplished by proportioning the current in the drive to give a higher resolution.
Conventional bipolar drives alternate the current direction in one coil at every step, resulting in a rotor displacement of 1.8 degrees. In microstepping, the coil current is changed in much smaller increments, increasing in one coil as it decreases in the other. The rotor responds by swinging to its new magnetic equilibrium, which can be a small fraction of a full step.
Microstepping has two principal benefits:
- it provides increased resolution without a sacrifice in top speed
- it provides smoother low speed motion.
For example, to achieve a resolution of 5 microns with a full step system requires the use of a screw with a 1mm lead. This places substantial constraints on top speed. A shaft speed of 40 revolutions per second results in a linear velocity of only 40mm per second. Use of a divide-by-10 microstepper provides the same 5 micron resolution with a 10mm leadscrew, but the linear velocity in this case is now 400mm per second. Alternately, the resolution can be increased, to 0.5 micron with a 1mm lead, or 1.0 micron with a 2mm lead.
Since stepping motors, by definition, move in discrete angular increments, operation at low step rates (especially near the fundamental resonance) generates noise and vibration. Microstepping decreases the size of these increments, and increases their frequency for a given rotation rate. This results in significantly smoother low speed operation. A laser interferometer was used to produce the graphs in Figures 33a-d, which show the reduction in positional oscillations and velocity ripple for a positioner at a low (0.05 revolution/second) step rate.
Despite the apparent benefits of microstepping, it is frequently implemented with excessive degrees of subdivision. A key attribute of many commercial systems is ’empty resolution’, where the apparent microstepping resolution can not be achieved. The torque which any microstep generates is found as follows:
- torque per microstep = motor holding torque x sine(90 degrees/SDR)
- where SDR is the step division ratio.
In the case of 50,000 step/revolution systems, the SDR is 250, and each microstep produces a torque change of 0.3 oz-in (with a standard 53 oz-in motor). Most high repeatability, preloaded leadscrew systems have torques in the 3-6 oz-in range; accordingly, 10-20 microsteps must be taken before the torque builds to a level which results in leadscrew motion. Direction reversals behave similarly; what appears to be backlash in the positioning table is actually excessive microstepping resolution. High division ratios have the additional effect of limiting the achievable top speed, by the inability to produce very high speed pulse trains (100 revolutions per second at 50,000 steps per revolution requires 5 MHz ramped pulse trains).
Several factors could be influencing the results. The motor has magnetic hysteresis that is seen on direction changes, this is of the order of 0.03∞.
Any mechanical backlash in the system will create positional errors. A step pulse occurring before the direction signal is established will cause an error.
This allows greater flexibility. The motor can be run as a six-lead motor with unipolar drives. With bipolar drives, the windings can then be connected in either series or parallel to give different torque-speed characteristics.
With the windings in series, low-speed torque is maximised. But this also gives the highest inductance, so performance at high speed is lower than if the windings are connected in parallel.
Yes, but care must be taken to not damage the bearings. The motor must not be dismantled.
Under normal conditions, 15m for unipolar drives and 30m for bipolar drives should be OK. Shielded cables are recommended.