Day: November 13, 2019

Step motors are widely used in automation due to their high resolution, precision positioning, minimal control electronics, and low cost. As an open loop system, traditional step motors are driven without the need for sensors to feed information back to a controller; however, the open loop configuration of step motors has challenges.

Position Verification — When pushed beyond its limits, a step motor will stall before reaching the endpoint. This event typically occurs when motors are not adequately specified for high-cycle applications. An encoder can provide position feedback at the end of the motion profile, indicating if the step motor stopped before reaching the end position. The controller compares the encoder counts that define the actual motor position to the target motor position at the end of a move to determine if there is a difference. If the encoder counts don’t match to the actual motor position, a corrective move or motion profile is calculated and executed.

Stall Detection — Stall detection notifies the user/system/machine as soon as a motor stall occurs, eliminating the uncertainty of whether or not the motor reached its target position. A more advanced function than position verification, stall detection (Figure 2) enables the controller to compare the registers of the encoder counts and target motor position on a continuous basis instead of just at the end of the move.

Stall Prevention— While greatly increasing system functionality, stall detection does not inherently improve step motor performance; it still requires the operator to perform a corrective move and re-reference the axis to the home position. Stall prevention, on the other hand, dynamically and automatically adjusts the move profile to prevent a stall, enabling the motor to operate with constant torque to get into an accurate end position without stalling.

Servo Control and Increased Motor Torque — Using stepper motor encoder feedback to servo-control, a step motor increases motor torque for greater dynamic performance. With peak torques up to 50% higher than the rated holding torque of the motor, the servo-controlled step motor system can operate at higher acceleration rates and with higher throughput for faster machine cycles.

When using a stepper motor, integrating an incremental encoder is relatively straightforward. Still, there are some general guidelines to consider.

Incremental encoders (like any stepper motor encoder) all function as part of a feedback system — providing closed-loop operation. Using information from the encoder, the drive system alters motor operation.

However, incremental encoders don’t keep track of position once power is lost. They also need a reference position to return to every time upon startup.

Keep this in mind when using and driving a stepper motor, because the machine design must set to a reference position when using incremental encoders.

Incremental encoders are often useful when speed control requirements are part of a system. If there’s less concern over the position of the shaft — and more of a concern over how fast it is moving — then the fact that incremental encoders don’t track position once off is less critical. In fact, here their simple operation and low price benefit the design.

Incremental encoders keep track of speed where only the difference between two positions is necessary.

There are a few different methods to mount an encoder to a stepper motor. Each of them is useful for various situations, and the choice depends on the motion system.

Incremental encoders with shaft mounting …
A coupling connects the encoder to the shaft. This creates mechanical and electrical isolation, but also adds cost because the coupling is an extra part and because this method requires a longer motor shaft.

Incremental encoders with a hub or hollow-shaft setup …
The encoder directly mounts to the motor using a spring-loaded tether. This is a design that is easy to install and requires no alignment. The only caveat is that this geometry requires careful electrical isolation.

Incremental encoders with a bearingless or ring mount …
Here, the sensor is in the form of a ring that mounts to the motor’s surface. A wheel mounts to the motor’s shaft. There are useful for heavy-duty applications.

What are the advantages and disadvantages of the different functionalities?