Hybrid Motion Technology

Exploring a new technology – variable current control

Author: Clark Hummel | March 30, 2011

In the last posting we examined how Hybrid Motion Technology overcomes loss of synchronization in stepper motors. The next feature we want to explore is the ability of the technology to variably control the current to what is required to move the load.

One of the drawbacks to stepper motor systems is the low efficiency when rotating at high speeds. This causes the motor to run very hot.

In typical stepper drive systems this is mitigated by having two current settings:

  1. A run current setting, which will set the output current when the motor is turning.
  2. A reduced current setting, which will set the output current to, a lower value when the axis is not in motion.

The Hybrid when set to fixed current mode, will operate as detailed above, with programmable run and hold current settings.

The  Hybrid Motion Technology difference is, an innovative third method of controlling current is now available: Variable Current Control.

Variable Current Control, when enabled, controls the motor current to what is needed to move the load. When the motor is beginning a move and has to overcome the inertia of the load and mechanical components, more torque, thus more current is required to initiate motion. However when the load is at required velocity, less torque is required to keep the load in motion.

Hybrid Motion Technology recognizes this and will reduce the output current to what is required to keep the load in motion at the required velocity. This results in reduced energy loss and a motor that will run cooler.

As with the operation of the control bounds, the Hybrid Motion Technology efficiently controls the current by monitoring the relationship between the rotor and stator. When the

Lead/lag relationship of the motor is at or near one full step, more current will be supplied to the motor. As the lead/lag decreases, the current to the motor will also decrease accordingly (See Figure 1).

variable current control
Figure 1: Current vs Lead/Lag

The impact of this can be seen in Figures 2 and 3, which shows test results comparing the motor temperature of a motor without variable current control to one with variable current control enabled under differing duty cycle conditions.

For Figures 2 and 3, please see the footnotes for test conditions.

Figure 2 below shows the impact of Variable Current Control on a motor that is constantly running at a speed of 2000 full motor steps per second at 25% torque.

As seen in the graph, the motor at a fixed run current quickly heats to the point of thermal shutdown at 85 ºC . The motor with Variable Current enabled maintains temperature within the operating range of the device.

Figure 2: Impact of Variable Current Control – Constant velocity

In Figure 3 below, we show the comparison of the motors running with a moderate duty cycle. In this graph we see the impact of Variable Current Control manifested in a 20% difference in the temperature, thus greater power efficiency.

Figure 3: Impact of Variable Current Control – Moderate duty cycle

The overall Impact of Variable Current Control in a system will vary depending on duty cycle, load conditions and motor speed.

Variable Current Control is one more way that Hybrid Motion Technology overcomes some of the natural limitations of stepper motors by decreasing the losses of the motor, saving you energy and money.

Figure 2 test conditions
Power supply: 48 VDC regulated, Load: 26 oz-in (18.3 N-cm), Move profile: 1 rev/sec CW/CCW for 2.5 minutes each, then continuous @ 2000 fullsteps/sec – 1 minute CW, 1 minute CCW
Figure 3 test conditions
Power supply: 48 VDC regulated, Load: 26 oz-in (18.3 N-cm), Move profilez; 1 rev/sec CW 1.5 seconds, wait 10 seconds, 1 rev/sec CCW 1.5 seconds, wait 10 seconds, repeat.

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