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.
While excelling at applications with well-defined loads, repetitive motion, and lower speeds, step motors can stall when too much torque is demanded in an application. A stall happens when the motor stops short in successfully moving to its endpoint. Since the motor is open loop, the drive/controller powering the motor is unaware of the stall, and the system continues to operate as if the motor completed a successful move. This potential problem, and other traditional limitations of step motor systems, is avoidable by adding an encoder to the system. An encoder can make a step motor more robust and able to perform better in industrial settings.
The addition of an encoder to the step motor system (Figure 1) adds functionality to detect and even prevent stalls by providing feedback to the drive. Depending upon how an operator programs the controller, encoder feedback can verify motor position, immediately detect motor stall, prevent motor stall, and create a closed loop servo system.
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.
Position verification uses the simplest algorithm and is performed using the most basic controller or microprocessor; however, this function requires waiting for the motion profile to complete before making calculations and corrections. As a result, operators can wait a long time before executing the corrective move. Position verification is ideally suited for low-cycle and low-volume applications such as in a test or lab environment, or operations with manual processes where cycle time is not an issue.
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. The comparison runs continuously in the background. As a result, the stall condition is detected immediately without waiting for the motor to complete an empty cycle so corrective moves are executable sooner.
Upon detecting a stall, the controller also can alert a higher-level PLC, PC, or HMI to avoid disruptions in overall machine function, or request intervention by a human operator. As soon as the controller detects a problem in the move profile, it triggers corrective action. Stall detection is better for time-sensitive applications or when cycle times are important. It is the minimum level of functionality for an industrial solution.
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. The controller intuitively adjusts the motor speed where the torque is sufficient to keep the motor moving and eliminate the lag between actual encoder counts and target motor position. In this way, the motor continues to run, albeit at a reduced maximum speed.
The scale of the speed reduction is directly related to the difference in the motor's available torque and the torque demanded by the motion profile. In many cases, the change in speed will be very small or imperceptible to the user. While the motor completes a move profile and successfully reaches the target end position, the tradeoff is increased overall motion time.
Stall prevention offers advantages over stall detection should operations not go as planned due to increased friction, load variation, or other factors. In practice, you wouldn't design a system so that the drive/controller is constantly adjusting the move profile. Ultimately, the system will take longer to complete moves, which could negatively impact overall machine operation.
Servo Control and Increased Motor Torque — Using 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 working as part of a fully closed loop system, step motors run cooler, more efficiently, quieter, and with faster settling times than their open loop counterparts. Unlike the other encoder applications described here, servo control applies a peak torque that enables the motor to get past stall conditions without sacrificing speed. Some manufacturers offer motors (Figure 3) already preconfigured with a high-resolution incremental encoder and closed-loop servo control firmware.
Which Encoders are Best?
An established technology, optical encoders offer accurate and reliable performance along with a wide range of resolutions; however, they are susceptible to signal degradation and loss when exposed to dust, oil, or similar contaminants. They are best used in clean environments. Capacitive encoders utilize newer technology, offer similar benefits, and ultimately provide the same position and speed information as optical encoders. But they are immune to environmental contaminants.
This article was written by Eric Rice, Application Engineer at Applied Motion Products, Watsonville, CA. For more information, visit here.