be active at the same time, the current flow would bypass the motor and “shoot-through” from the supply  voltage directly to ground, failing the FETs. Wave operation offers the advantage of not having to worry  about “break-before-make” of the power stage FETs, while with bipolar operation you must “break” the  active FET before “making” the next active FET. Therefore, the wave control method offers higher  reliability and fewer failure modes compared to the bipolar operation. This was the discriminating factor in  our selection of wave control for this space flight stepper motors.  An operational dilemma arises from the wave operation because there is an unregulated phase at each  step, resulting in underdamped operation. For example, transitioning from step one to step two under the  wave scenario, the current flowing in phase B is instantaneously cut-off from ground. At that instant, an  L•di/dt transient voltage is generated in reaction to the state change. The transient current flows through  the B+ diode back into the supply voltage. Additionally, the motor back-emf generated from the stepper  motion is allowed to flow freely in this unregulated winding, until step 3 when it is driven to the +28  voltage. The combination of the transient current and the back-emf result in an inherently underdamped  control system. For bipolar operation, at each step of the control sequence all legs are actively controlled  to either +28V or ground. There are no “free-wheeling” legs to allow transient currents to create an  underdamped condition. This difference was found to be the root cause of the significantly underdamped  condition of the mechanism.  Another observation of the various testing was the variation of performance whether the voltage supply  was regulated or not. Notice the leg voltage measurements in Figures 3, 5 and 6. These voltages came  from a non-regulated supply. You can clearly see the impact of the back-emf on the voltage during the  step transient. Now notice the phase voltages on Figures 7 and 9. These voltages are generated on the  regulated voltages in the control electronics. Notice the difference between the regulated and unregulated  voltages during the step transients. The unregulated supply voltages “bounce” with the step transients,  while the regulated voltages do not vary with the step transient. The differences in these voltage supplies  will also affect the step characteristics; therefore, the voltage supplies used during testing should match  the conditions the mechanism will see under flight conditions.  Processing the Rotary Accelerometer Signal As previously discussed, we implemented an RA in the system for pulse step verification. The associated  electronics to process the RA may be as sophisticated or as simple as desired, depending on the flexibility and capabilities required in the system. The charter for the RA in our system is pulse verification  and state of health monitoring. For basic pulse verification a simple amplifier and comparator circuit is all  that is required. An example of this circuitry is shown in Figure 11 [1]. The RA enables step verification by  producing a voltage pulse that exceeds a specified threshold voltage each time the motor steps with the  sign of the pulse indicating whether a clockwise or counter-clockwise step is taken. This method of step  verification is called Original Transition Voltage Counting [1]. Figure 12 shows a single step of an  adequately damped stepper motor system with a well-defined pulse for step verification. This trace was  actually taken by the actuator supplier of the EDU#1, driven at our nominal power , with a bipolar drive  configuration . Notice the significant damping characteristic as compared to a single step of a wave  controller, as shown in Figure 2, with equivalent power input.  42