Avoid some common servo system problems

Machine grounding

To save some wires, one might be tempted to use the rack as the return current path for DC servomotors (especially small ones). However, machine frames have a lot of resistance, especially if paint and other coatings add to the resistance. Such a high resistance loop will increase the difficulty of crosstalk between amplifiers, resulting in unstable machine operation.

Additionally, high currents flowing through mechanical structures can cause long-term, less obvious problems—galvanic effects (i.e., rust). This can severely degrade the performance of the mechanical joint.

The solution to this problem is to install a separate Vdc return line between each motor and the amplifier.

Power supply voltage drops

By their nature, DC power supplies are unregulated, so the output will vary with changes in input AC voltage and transient load. Therefore, when the AC voltage of the power supply is 15% below its nominal rating (the common industry standard for allowing low voltages) and the servo is delivering rated peak torque, a power supply must be selected that is capable of sending full rated voltage to the amplifier at full speed. Typically, a servo amplifier’s peak current is twice its maximum continuous output. This is a worst-case scenario, but is usually within the rating of the component.

A good rule of thumb is to specify a power supply that can deliver 10% more voltage than the rated voltage under these worst-case conditions. Many engineers know from experience that this 10% “headroom” is a wise investment.

duty cycle syndrome

Of all the “gotchas” engineers encounter when working with servos, the most common is duty cycle syndrome. In applications with large load variations, it is common to underestimate the worst-case speed and torque values.

For example, the printed circuit board that powers a drill undergoes many changes between rapid high-current acceleration and braking. It is rare to operate at what might be considered “normal” operating current (in this case, no-load current).

Read more: Choosing the Right Stepper or Servo Motor for Electric Actuators

Because power semiconductors have much shorter thermal time constants than motors, their use requires servo power supplies and amplifiers with continuous ratings close to or equal to their peak ratings. Without this seemingly conservative design choice, solid-state units could fail prematurely.

Winding machines, centrifuges, and similar high-inertia loads take a while to reach full speed, so they draw high currents for long periods of time. Likewise, servo selection should be guided by the requirements for abnormal duty cycles, where torque is established at a continuous rating.

regenerative braking

Servo systems can perform regenerative braking, a method of stopping loads and saving power. This is relatively simple since the servo motor can act as a motor to accelerate and run the load, or become a generator when decelerating. It is then the job of the servo to dissipate the newly generated energy.

Battery-powered systems such as golf carts, forklifts, and automated guided vehicles are often designed to return energy harvested from the vehicle’s motion back to the vehicle’s battery. This extends the device’s operating time between charges.

For servos operating on conventional AC power, the most popular methods of absorbing or dissipating energy are:

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