The resolution of microstepping drivers often drops as the rotational speed increases; the reduction in resolution is inevitable due to the limited bandwidth of both controller and driver. For argument's sake, imagine a designer attempted to operate a microstepping controller with 40,000 steps per sec at 50 rps (3,000 rpm.) It would then have to output 2,000,000 microsteps per sec to keep all the steps. Even if this were possible, a typical PWM driver only operates at 20 to 40 kHz — so the fine interpolations would never reach the motor. To address this inability to hit every microstep at higher speeds, the number of microsteps per second is often reduced as the motor speed increases. Transitions between these different resolutions can cause an impulse in torque to the motor, causing ringing that can result in lost steps.

Instability

There is another torque reduction that takes place at higher speeds, roughly corresponding to the speed at which the power-supply/driver combination becomes unable to control the current. This, in turn, corresponds to the start of the parabolic "constant power" portion of the torque curve, called mid-resonance instability. Loss of the ability to control current occurs when a motor's back-EMF rises to the point where power-supply voltage applied to the driver cannot overcome both the back-EMF and resistive and inductive drops at the requested current. The current undergoes a 90° phase lag from the commanded current as the driver switches from current control mode to voltage drive mode.

Any mechanical ringing of the rotor pendulum causes the motor to speed up and slow down, changing both the velocity and relative angle between the driven field angle and the rotor angle. This changes two things: the magnitude and angle of the back EMF with respect to the phase of the commanded current. Why does this matter? It can cause the driver to switch back and forth between voltage mode (full on) and current mode (chopping.) This can reduce the damping of the system, or even pump up the ringing until the rotor loses sync with the commanded position and loses steps, or stops spinning altogether. Operation in this speed region may require either mechanical damping or electrical damping to stabilize the operation of the motor to provide usable torque.

Sudden movements, external forces

Low-frequency resonance and midfrequency instability are not the only ways to lose steps. The rotary pendulum is also set into swinging (in other words, becomes excited) by sudden changes in commanded velocity and load.

Instabilities can also be mechanical. Shafts, couplings, and power transmission components-between motor and load also act as rotary springs.

For example, gear trains release the load when changing direction, due to backlash. While the load is uncoupled from the system, the motor accelerates (because of lower inertia) until the backlash has been taken up. When the gears engage again, the difference in velocity between the motor and load can reflect excess torque back to the motor. Thus the system cycles: The motor slows below the speed of the load, again the load decouples, and then the motor speeds up. In some cases, the change in speed may be enough for the gears to first strike on one face and then rebound and strike on the opposite side, to repeat several times. The exact timing of the reversal ringing may vary with both the position of the gear train and with the wear of the gears, making it difficult to choose a stepping sequence that compensates.

A different stability issue arises with belt-driven linear movers. These units experience a resonance, the frequency of which constantly changes. As anyone who has ever played a stringed instrument (or a rubber band stretched across a cup) can attest, pitch or resonant frequency can be varied by changing either the string's tension or its length. The motion of a linear belt mechanism varies both of these. Linear force applied to the carrier and its load is the difference between the tensions of the two belt halves, while the position of the carrier varies the length of these belt sections. (Note that these same effects change both the resonance frequency of the belt itself, and that of the belt-load system.) This means that the same move with the same load and motor may work fine with the system starting at position A but not at position B. And what if the system carries a varying load? That only complicates the matter further.

Increasing stability

Both mechanical and electrical approaches are used to stabilize stepper motors. Mechanical approaches usually involve increasing motor rotary inertia to make load variations less significant, or adding damping to the system. Rotary inertia is increased either by swapping out the motor size or design, or by coupling flywheels to the motor shaft as close to the motor as possible. A system's mechanical damping is increased by including magnetic dampers, viscous inertial dampers, ferrofluid, and elastic motor mounts, couplers, and belts.

On the other hand, electronic approaches to increase stability typically measure (directly or indirectly) motor position and speed. Then current to motor windings is varied in a way that damps the system. These electronic methods include:

• Measuring or estimating the back EMF of each winding (which includes both speed and position information) and adding a portion of the back EMF signal into the commanded current at each winding
• Modifying driver circuits
• Using position/velocity information to modify the applied pulse train to the stepper motor
• Full servo control of the stepper motor.

Relative torque vs. error angle
Error angle (Full steps)
Error angle (Electrical deg.)
The motor produces torque only when the rotor is not aligned with the stator magnetic field, varying in a roughly sinusoidal manner with error angle.