Our first “nuts and bolts” article in the basics of Hybrid Motion Technology (TM) series is on the most important aspect of its functionality: control boundaries. The control bounds are the basic feature that enables HMT to prevent loss of synchronization and motor stalls in stepper systems.
Synchronized motion in a stepper motor requires that the lead/lag relationship between the rotor and stator be within +/- 2 motor full steps. As this relationship drifts toward the 2 step point the torque available to the load is reduced, with maximum constant torque available at the <= 1 full step point.
Conditions that can cause the stepper motor to lose synchronization and stall are:
Rotor lags stator:
- Acceleration is too rapid to apply enough torque to overcome the inertia of the load.
- Transient load condition at velocity; i.e. load being increased on a conveyor.
Rotor leads stator:
- Deceleration is too rapid to hold the load within the +/- 2 full step range.
- Overhauling load condition where the momentum of the load is greater than the torque supplied to maintain constant velocity.
With these conditions in mind, the key to preventing loss of synchronization is to somehow maintain the relationship between the rotor and stator within the +/- 2 full step boundaries.
Hybrid Motion Technology does this by closely monitoring and maintaining the rotor stator relationship within settable “control bounds”. Figure 1 shows these control bounds. HMT allows the user to configure the control bound to one of 4 settings: 1.1, 1.3, 1.5 or 1.7 motor full step. You will see greater torque performance the lower the control bound, and greater speed performance with a setting of 1.7. Best overall torque-speed performance occurs with a setting of 1.3 or 1.5. When HMT is operating in constant torque mode, the HMT automatically sets control bounds to 1 full step.
Figure 1: Control bounds
To maintain the rotor stator relationship within the control bounds, the Hybrid circuitry compares the number of step clock pulses sent with the number of encoder counts received. When the lead/lag relationship of the stator and rotor begin to drift outside the set bounds, the Hybrid circuitry will detect and adjust the step rate to compensate.
These requires a very fast reaction and response time, since HMT is hardware, rather than software based, it will respond in time to prevent loss of synchronization.
The amount of lead or lag is stored in a register, which is used with position make up to reinsert lost steps into the motion profile at the end of a move.
Loss of synchronization and motor stalling has been a serious drawback that has prevented stepper technology from being used in a number of applications, requiring a more expensive and complicated servo solution to be designed in.
The ability of Hybrid Motion Technology to be able to defeat loss of synchronization while supplying constant torque or constant speed to a load opens many new doors for saving time and money on new or retrofit machine designs.