Fundamentally, a servo system can perform no more accurately than the accuracy of the feedback device controlling it. In addition, errors in speed or position can be introduced into the system by the less-than-perfect mechanisms that transfer the motor power to the load. Environmental factors like electrical noise or temperature may also introduce positioning errors. Sometimes the errors are acceptable. More frequently, however, they are not. When it comes to high-performance servo applications, feedback devices fall into several different categories. Each offers unique advantages and disadvantages, both electrical and mechanical, that make one better suited for a particular application than another.
The optimum location of a feedback device is at the load, where the controlled motion is required. This arrangement eliminates errors introduced by less-than-perfect transmissions that transfer the motor’s motion to the load. This sometimes means adding a feedback device to the system in addition to the device that is typically mounted inside the motor. Brushless motors require that position feedback be incorporated into the motor to provide immediate rotor position data for electronic commutation. 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 if the error is acceptable.
Direct drive servomotors have the advantage that their internal feedback device is effectively connected directly to the load, thereby eliminating compliance and backlash. This, in addition to the reduction of components and maintenance, makes direct drive motors an ideal solution for applications requiring precision motion and high bandwidth.
Absolute or Incremental
Feedback sensors report either absolute or relative incremental position. The former has the capability to report its position within one electrical cycle when the system is powered up. By contrast, the incremental position sensor typically provides output pulses for each increment of motion, but without reference to the particular location within the device’s range of motion. This data, in combination 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 critical applications using incremental encoders, the controller can connect to an uninterruptible power supply to maintain position information. Alternatively, a multi-turn absolute encoder will provide the same function without the need to keep power applied.
A second 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 more for precision laboratory equipment. And, of course, there are applications where the requirements overlap.
A third consideration is 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 axes positioning, position data also indicates the exact locations of all axes simultaneously, which can be crucial in some applications. In an E-stop (emergency-stop) situation, being able to restart the motion components at the point 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 these devices a “one-stop” purchase for most servo-based systems. However, for applications requiring precise speed information at low speeds, sometimes a feedback device designed for that specific purpose, such as a precision analog tachometer, is preferred.
The Good News
Feedback devices often 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 where the motor, feedback device, and drive and cables are combined into an optimized package. Such packages handle more than 90% of today’s motion applications. The benefit to the engineer is that he or she doesn’t have to separately wire or mount the feedback device into the servo system, where wiring connections can be as high as nine or 13 wires, or as few as four. In addition, some manufacturers offer “smart” feedback devices in their motors, allowing plugand- play operation by providing the drive with an electronic motor nameplate with motor parameters. These parameters configure the drive, allowing motion 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 does one need to know to select the optimum feedback device for their 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 feedback 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 servomotor suppliers offer multiple feedback options for a given motor to accommodate a wide range of performance or environmental requirements.