FAQ

Servo Motor / Mechanism Considerations

1) What are the effects of mechanical backlash?

Any backlash between the angular position of the motor shaft and the position of the feedback pot may lead to instability or oscillation.

With backlash, the feedback pot can't properly indicate the motor position, allowing the motor to begin to move without any feedback, which may cause system oscillations. The greater the backlash, the greater the tendency of the system to oscillate.

In systems with a great deal of backlash, the best solution is to connect an optical encoder directly to the motor shaft in order to provide the control loop with a feedback signal as soon as the motor begins to move. At present, however, our products do not accommodate such a dual feedback system and we simply recommend reducing the backlash.

2) Is it ok to back-drive the servo?

When a servo motor is caused to rotate due to the weight and/or inertia of the mechanism, the motor will become an electrical generator and begin to deliver power back to the power supply.

Back-driving the servo may blow up the power supply and/or servo controller.

If the power supply is a battery, this will simply recharge the battery, but when an ac-to-dc power supply is the power source, the power generated by the motor will quickly charge up the output capacitor of the power supply, causing the capacitor voltage to rapidly increase. The rapid increase in the supply voltage may severely damage the power supply and/or servo controller.

If the inertial loads and/or weight of the mechanism can cause the motor to turn into a generator, it may be necessary to add a dynamic load to dissipate the power. In the simplest case, the dynamic load can be a zener diode, a zener in series a dissipating resistor, or a transistor switch that delivers the generated power to a suitable load resistor or to a battery.

3) Is a lead-screw design any advantage over belts or gears?

The biggest difference in servo performance is between systems that can back-driven versus systems that cannot be back-driven.

In many cases, lead-screw or ball-screw designs will not be back-driven. On the other hand, many designs with belts and gears can be back-driven.

4) What can I do to reduce the power required?

One of the best ways to save power is to design the system so that the servo motor can be turned off whenever the mechanism is not moving.

When a system cannot be back-driven, it is possible to completely turn off the motor when the proper position has been achieved.

In contrast, a system that can be back-driven will require continuous application of power to the motor... leading to higher power consumption.