It pays to understand the feedback devices that are currently available for fine-tuning servo motor performance and how to choose the right type for your motion-control application.
Servo motor-powered motion control systems are expected to be fast and accurate, and when correctly specified they are. However, there are numerous factors that can cause motors to miss the target.
A servo system can perform only as accurately as the feedback device that reports the motor’s position. In addition, errors in speed or position can be introduced into the system by the less-than-perfect mechanisms that transfer motor power to the load. Environmental factors like electrical noise or temperature may also introduce positioning errors.
Sometimes these errors are acceptable. More often, they are not. Servo motors and drives are specified with the expectation that they will be the most reliable and accurate of all positioning systems.
Your choice of feedback device is critical for achieving the desired results. These devices fall into several categories, each offering unique advantages and disadvantages, both electrical and mechanical, which determine whether a given feedback technology is best suited to a particular application. Here’s a high-level view of the most important selection criteria.
Feedback device location
The optimum location of a feedback device is at the load, where the motion-control requirements apply most directly. This arrangement mitigates errors introduced by less-than-perfect transmission components that transfer the motor’s motion to the load, such as gearboxes, belts and pulleys, ball screws and so on. While feedback devices are typically mounted inside the motor, adding a feedback device at the load can significantly improve accuracy in transmission-driven systems.
Brushless motors require that position feedback be incorporated into the motor to provide immediate rotor position data for electronic commutation (discussed below). When using a motor-mounted feedback device, it is important to determine the cyclical and cumulative error associated with the transmission and feedback device to determine whether the error is acceptable.
Direct drive servo motors have the advantage that the internal feedback device is effectively connected directly to the load, thereby eliminating transmission compliance and backlash. This is in addition to the advantages of fewer components and reduced maintenance, which make direct drive motors ideal for applications requiring precise motion and high bandwidth.
Commutation
Commutation is the control of current to produce torque. In permanent magnet motors, torque is produced when the magnetic field from the windings interacts with the field from the magnet. When current is channeled to the proper winding(s), optimal torque is produced.
As the motor moves, the position of the windings relative to the magnets changes. This means that the optimal path to channel the current changes, depending on the motor position.
In a brushed motor, the path changes automatically by means of brushes and a commutator connected to the armature windings. In a brushless motor, rotor position is fed back to the drive, which then electrically switches current to the appropriate windings via transistors.
Single-turn or multi-turn feedback
Single-turn feedback sensors track position through each 360º of mechanical rotation. An example of when this might be useful is to control the speed of a conveyor belt, where velocity can be derived from the position of a roller through time, but the number of roller turns is irrelevant. However, a single-turn device would be inappropriate in an application that requires multiple rotations of the motor shaft to move a load a given distance, and precise control of that distance is critical.
For the latter type of application, a multi-turn feedback sensor tracks position through 360º of rotation and also adds the ability to track how many complete revolutions have occurred. Typically, multi-turn feedback devices range in their counting capability from 4,069 turns (12 bits, or 2^12) to 65,536 turns (16 bits, or 2^16). Multi-turn feedback can be implemented through a system of gears with individual readouts or using an electronic counter, which is a more compact solution and generally provide higher turn counts.
Absolute or incremental feedback
Feedback sensors report either absolute position or incremental relative position. A single turn absolute-position sensor has the ability to accurately report position within one mechanical cycle of the motor being powered up. By contrast, an incremental-position sensor typically provides output pulses for each increment of motion, but without reference to any particular location within the device’s range of motion.
These incremental pulses — when combined with periodic marker pulses, a machine home switch and a counter — allow load position to be known. However, should the electronic feedback circuits lose power, the system loses track of its location. For some position-critical applications using incremental encoders, the controller can connect to an uninterruptible power supply to maintain position information.
Alternatively, a multi-turn absolute encoder provides accurate positioning information, across multiple rotations, without the need to keep power applied. This can be accomplished by means of a battery backup system or using energy-harvesting technology that maintains the proper count in nonvolatile memory even when power is off, and the rotor is moved manually.
Precision and reliability
Another important consideration is the type of technology used in the device. Some sensors are extremely rugged and are targeted at the industrial machine-control industry. Others are relatively fragile and are intended for applications such as high-precision laboratory equipment. There are also applications where these requirements overlap — for example, semiconductor manufacturing, where conditions call for high accuracy in a particularly clean environment along with rapid throughput to meet high production schedules.
Motion system geometry
Motion systems are either linear, rotational or a combination of the two. Feedback sensors are specifically designed for each case. They may have different mounting features and motion directions, but the basic principle of feedback device operation typically applies to either configuration.
For linear systems such as those found in X-Y-Z axis positioning, the position data indicates the exact locations of all axes simultaneously, which can be crucial in some applications. In an E-stop (emergency-stop) situation, for example, being able to restart motion components at the point where they stopped can prevent machine jams and reduce waste.
Speed information is commonly derived from position data by taking the derivative with respect to time, making a feedback device a single purchase for most servo-based systems. However, for applications requiring precise speed information at low speeds, sometimes a device designed for that specific purpose, such as a precision analog tachometer, is preferred.
The good news: Feedback device selection can be simple
Feedback devices play a critical role in closed-loop control systems. Not long ago, choosing the right one was a daunting task, but now selection has been greatly simplified.
Many motion control manufacturers offer complete motion-control systems, with the motor, feedback device, drive and cables combined into an optimized package. Such packages handle more than 90% of today’s motion applications. For machine engineers, the benefit is that they don’t have to separately wire or mount the feedback device into the servo system, where wiring connections could be as few as four or as high as 9 or 13 wires.
In addition, some manufacturers like Kollmorgen offer “smart” feedback devices in their motors, allowing plug-and-play operation by providing the drive with an electronic motor nameplate that specifies motor parameters. These parameters configure the drive, allowing feedback configuration in seconds. Smart feedback devices can be based on any of the standard feedback types with the addition of an embedded chip containing the motor parameters.
So, what do you need to know to select the optimum feedback device for your application? First are the positioning accuracy and resolution requirements. Additionally, environmental factors such as distance between the motor and drive, electrical noise or temperature can be factors in determining the optimum device.
A wide variety of devices are available to suit nearly any feedback requirement, including Hall-effect sensors, resolvers, general-purpose encoders (of a wide variety) and sine encoders. Fortunately, many servo motor suppliers offer multiple feedback options for a given motor to accommodate a wide range of performance or environmental requirements.
Hall-effect sensors are among the simplest and least expensive feedback devices. These are digital on/off devices that detect the presence of magnetic fields. Made of semiconductor material, they are rugged, can be operated at very high frequencies (equating to tens of thousands of motor rpm), and allow for accurate commutation sequencing. Because the position information these devices provide is inexact, they are best used to support trapezoidal (six-step) control rather than sinusoidal control. Hall-effect sensors are suitable for torque control or coarse speed control, and they simplify drive electronics by directly switching the motor phase power devices.
Resolvers are rotary transformers that are well suited to harsh environments, where extreme temperatures or vibration and shock are factors. They can also handle motor speeds in excess of 10,000 rpm. These are low to moderate on a cost scale and provide moderate accuracy and resolution that is suitable for most industrial applications.
Incremental encoders come in a variety of configurations, from non-contacting optical to contacting types, in both linear and rotary versions, and with multiple line count variations. These encoders provide excellent accuracy and can be operated up to many thousands of rpm. While today’s incremental encoders are more rugged than ever before, some are not suited to extremely harsh environments.
Sine encoders offer very high-level performance. Although more expensive than resolvers or incremental encoders, they are best suited to applications requiring high accuracy coupled with high resolution.
Learn more
Be sure to read all three of our Feedback Devices blog posts Hall-effect sensors and resolvers and linear, rotary and sine encoders to become conversant in this crucial motion topic. And feel free to contact a Kollmorgen engineer to discuss your specific application and get recommendations for the best servo-loop technology to suit your needs.
