Servo Motors: An In-Depth Introduction, Part 2

Posted by Dave Gazda on Nov 24, 2015 9:16:26 AM

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Servo Motors in Automation Solutions

What else can a servo motor do?

In Servo Motors: An In-Depth Introduction, Part 1, we reviewed how servo motor construction and operating characteristics create this technology's advantages. But there's more to understanding how servo motors can play an important role in automation solutions. Shaft sensors and holding brakes provide 4 different kinds of feedback and can extend the life of the equipment for maximum efficiency.

Here's how:

Shaft Sensors

Servo motors are operated in a closed control loop for speed and angle of rotation. To provide feedback, a sensor is required to record the actual speed and the position angle of the rotor. The requirements on accuracy depend on the application in which the servo motor is used. So that servo motors can be optimally implemented in different applications, a variety of sensor systems are available.

Synchronous servo motors can be equipped with resolvers, incremental encoders, single-turn Sin-Cos absolute encoders, or multi-turn Sin-Cos absolute encoders.

Resolvers

Resolvers are sensors that act magnetically. They use the angle-dependent coupling between the windings in the rotor and those in the stator. The angle and the speed are determined from the relationship of the induced currents. They are characterized by a very high level of robustness. Resolvers retain absolute angle values within one revolution. The output can also be used to generate the commutation information for current regulation in the servo drive.

Incremental Encoders

In incremental encoders, a rotating disc with an incremental slot is optically scanned. The optical signal is used to generate rectangular signals, usually with 2048 or 4096 pulses per revolution. Two signals are generated with a phase difference of ½ pulse length, which enables both angle measurement and direction of rotation. Encoders only supply incremental phase signals, which means that the commutation information required for operating synchronous motors is not provided. Incremental encoders are therefore particularly suitable for asynchronous servo motors.

Sin-Cos Absolute Value Encoders

Sin-Cos absolute value encoders also contain a disc that is optically scanned. The optical signals are used partly to generate incremental signals, which return Sin-Cos voltages with 512, 1024 or 2048 periods per revolution. In addition, the absolute angle is also determined and made available to the servo drive via a serial interface. The Sin-Cos signals are interpolated by calculation of the arc tangent, meaning that a very high angular resolution can be achieved. This high resolution leads to excellent smooth running performance of the motor at low speeds.

Single-turn Encoders

Single-turn encoders determine the absolute phase information within one revolution, so that all signals for operating synchronous motors are provided similar to the resolver. Multi-turn encoders often have an additional gearing and additional sensors in order to distinguish up to 4096 revolutions directly. This means that a machine can start immediately after it is switched on and does not first have to be homed.

 

Holding Brakes

Servo motors are often equipped with a brake that is used to hold the position if the power supply to the drive is switched off. Servo motors with brakes are used in particular for applications involving lifting and lowering, or for holding a position against a particular force.

Normally, braking during operation to bring the motor to a stop is performed using the servo drive and as a wear-free process. The brake is only used in emergency stop, e.g. in the case of mains failure, and for holding duties.

Two types of brakes are used:

  • Spring-applied brakes apply the braking force using spring pressure through an armature plate and onto the brake rotor.
  • Permanent magnet brakes generate the braking torque by exerting the power of the permanent magnets onto a rotating armature plate.

The advantage of permanent magnet brakes is their backlash-free properties. The braking torque is transmitted to the shaft using a metal disc with no play. This means that high positional accuracy can be achieved. On the other hand spring-applied brakes have lower inertias, and can continue to operate even after there has been extensive wear from repeated dynamic braking of the drive.

Conclusion

If you're considering servo motors as part of your portfolio of automation solutions, let one of our experienced engineers at ACD walk you through the advantages and finer details of using servo motors in your unique application.

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Topics: Motors, Motor Control

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