Feedback for machine motion

Closing servo-motor loops with feedback devices improves performance, but care must be taken with design and installation. Closing the loop improves monitoring, measuring, and control—even when outside forces cause deviations to the desired setpoint. A one supplier system should be considered first, with escalation to a custom system only when off-the-shelf systems cannot meet performance demands.

04/13/2015


Figure 1: Preconfigured and tested motion control solutions, such as this Rockwell Automation Kinetix 6500 drive, provide a quick path to precision machine motion. Courtesy: Dave PerkonUnderstanding issues and basic design techniques for closing the feedback loop on servo motors can help alleviate many of these issues, especially when purchasing preconfigured systems. Specific design guidelines and installation techniques also can help close the loop and improve performance.

The primary advantage of closed-loop control for servo motor systems is a self-adjusting output that improves position, velocity, and/or torque control within automated equipment. In other words, closing the loop improves monitoring, measuring, and control—even when outside forces cause deviations to the desired setpoint.

Subtracting the actual condition (process variable) from the desired setpoint and applying the difference to the control loop forces the error to zero, enabling machines to adjust for changing conditions. While open-loop servo-motor systems save money and design effort for a variety of applications, these systems will usually be less repeatable and less accurate than closed-loop systems.

Figure 2: Available in many configurations, these Allen-Bradley absolute encoders from Rockwell Automation enable accurate feedback of rotary position for precise servo system control. Courtesy: Dave PerkonPosition feedback or closed-loop control can be implemented in many ways, but all serve the same functions. The output of a servo controller (Figure 1) drives a motor, which affects the controlled position. A digital encoder (Figure 2) or an analog feedback device reports the actual position back to the controller.

The controller compares the requested position with the actual measured position to calculate position error. The controller then adjusts the position-output command to eliminate the error, therefore closing the loop. This is in contrast to an open-loop system where the position or angle of the load is not measured, eliminating the opportunity to correct for error.

Making the right connection

Motion control systems are some of the more complex applications on automated equipment, and there are usually several options to attain the desired result. Once the general approach is selected, there's a choice to design a custom system, or to purchase a preconfigured and tested system from one supplier. Although some very complex applications may require customization using components from multiple vendors, most can be implemented using the much simpler single-supplier approach.

With a custom system, the designer must specify all the servo, controller, and feedback components. This may provide optimal performance, but can also open the door to a variety of motion feedback and control issues as listed in Table 1 and discussed below.

To start, there is a heavy design cost. Those who have done it know that combining a servo controller from vendor A with a servo motor from vendor B with hookup cables that must be designed and integrated can be a long and difficult road. Both vendors may need to work together to make the system successful, and this can cause issues. Just understanding where to install the feedback device—at the motor, the gearbox output, or the load—can be difficult.

Hooking up the encoder to the servo drive and motor can cause problems if any wiring is incorrect. If there's one connection mistake, turning the power on and enabling the drive can cause the motor to accelerate dangerously. If the motor, with coupling attached, is sitting on a table, the motor can actually jump and rotate so fast the coupling will break off and fly through the air. 

Rotary servo, absolute encoder

In another example, a rotary servo motor with hall-effect commutation from one vendor, connected to a drive controller from another vendor with encoder feedback from yet another vendor, can be a difficult integration task. After a potentially difficult process to determine the correct power wiring, the hall-effect commutation connections must be dealt with.

Not only can the hall-effect sensor feedback be commutated in three different ways, the hall-effect feedback signals to the motor must be correctly connected to provide the proper commutation sequence with the correct phase. The servo drive must then be configured to use these feedback signals to get the servo turning without faulting the drive controller.

The encoder feedback must be scaled, with the pulses converted to engineering units. With preconfigured hardware, one can just pick an appropriate level of resolution and move on to developing the application. With custom applications, chasing the proper scaling values and then having to measure and confirm via actual measurements can delay project completion.

In an absolute encoder feedback application, where position data is not lost when power is cycled, one must pay close attention to encoder resolution, output signal, output type, and voltage. Specifications such as pulses per revolution (PPR) to values of 1024 PPR or more are common, as is a supply voltage of 5 V dc, 12 V dc, or 24 V dc. Open collector, line driver, and single-ended signals can provide gray code or binary position data to a variety of controllers.

These issues can be avoided by purchasing a preconfigured and tested servo system from a single supplier, but other problems are common to both custom and preconfigured systems. 

Good design practices

Common feedback problems include missing pulses or noise in the feedback signals. If encoder counts are being lost, it's often due to mechanical coupling slippage at the servo motor, gearbox, or ball screw actuator mimicking missed signals. This can also be confused with noise.

Searching for intermittent problems, such as noise or missing pulses on a feedback signal, can be a very time-consuming process. While differential, line-driver signals can help reduce noise, good practice demands that feedback signals be kept separate from noisy power signals. That way, if there are feedback errors, effort can be focused on finding an open circuit or shorted wires in the feedback cable, instead of chasing a noise problem.

Other failures are more easily avoided with proper installation. Installing encoders with an improperly aligned coupling or too much tension on an encoder drive belt can quickly turn feedback into a troubleshooting exercise to locate a worn-out bearing.

Preconfigured advantages

Figure 3: Using a single-supplier solution in common applications, like this food packaging line, simplifies and speeds design, test, and commissioning. Courtesy: Dave PerkonMany off-the-shelf single-supplier solutions include motors matched to servo drives along with power and encoder cables of various lengths. This is a true plug-and-play solution, which greatly simplifies motor and encoder feedback hookup as the supplier has matched the servo motor to the controller. For many applications, such as the food packaging line shown in Figure 3, there's no need to look further.

With a servo motor and controller purchased from and preconfigured by one supplier, all parameters are set to commonly used default values, allowing quick operation out-of-the-box. Of course, these parameters can be readily adjusted as needed to make the servo system suitable for a very broad range of applications.

The single-supplier solution also simplifies cabling. Designing a 20-pin connector for interfacing the servo motor encoder to the controller is not a trivial task. By contrast, using a factory-made and tested cable provides easy connection to motor, brake, and encoder without any required design and testing.

A single-supplier solution alleviates many of the potential issues discussed above in design, implementation, and test. As such, these solutions should be considered first, with escalation to a custom system only when off-the-shelf systems cannot meet performance demands. 

- Dave Perkon is an independent machine automation consultant for a variety of Fortune 500 companies in the medical, semiconductor, automotive, defense, and solar industries. He has more than 28 years of experience in custom automated equipment development, system engineering, project management, sales, controls design, and programming. Edited by Eric R. Eissler, editor-in-chief, Oil & Gas Engineering, eeissler@cfemedia.com 

Key concepts

  • Feedback loop on servo motors should consider basic system design.
  • Closed-loop servo motor control improves position, velocity, and/or torque control for automated equipment.
  • Using elements from one supplier can simplify system integration.

Consider this

Have you seen an open-loop servo-motor system save in capital cost but eat up profits because it's less repeatable and accurate than a closed-loop system?

ONLINE extra

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