Digital drives and controllers have raised the bar on feedback devices, fueling the demand for faster, more precise sensors capable of performing intelligent tasks over digital links. In response, a new type of motor feedback system has emerged that builds on previous advances in encoder technology. Some of the early adopters of this new technology include Control Techniques, Delta Tau, Custom Servo Motors, Allen-Bradley, and Kollmorgen.

Since the inception of closed-loop servo control, engineers have sought better ways to measure angular position and velocity. The current crop of devices that have sprung up as a result of this search – encoders, resolvers, and tachometers – have met most of the demands over the years. But lately, with the advent of digital drives, sensing technology is emerging as the weak link in the automation chain.

What digital drives need, and what ordinary sensing techniques can't provide, is a device that offers velocity, position, and commutation feedback in a single package small enough to fit, if need be, inside a typical motor housing. Besides being rugged and inexpensive, the device must also be easy to install and connect, self-diagnosing, and able to resolve shaft movements into diminishingly small steps. For the past several years, researchers have been working toward just such a device, and what they've come up with, though it may not be the "ideal" sensor, is better than anything else currently available.

Role reversal

It used to be that controllers were the main factor in determining the performance of an electronic motion system. But now, because of digital drives, motor feedback systems are rapidly becoming the most critical component.

Consider this: To achieve their potential for dynamic acceleration and velocity, digital drives require extremely short sampling intervals. Typical intervals range from 50 to 600 μs. Of course, as sampling times decrease, the number of counts from the feedback device must increase. A drive with a sampling interval of 400 μs, for example, requires a sensor capable of measuring 1.5 million steps/rev, assuming the motor runs at 0.1 rpm.

Offering several mounting options, new motor feedback systems provide resolutions in the millions, accuracy down to ±5 arc-sec, and intelligent features such as automatic adjustment of the drive during startup. These feedback systems are absolute and provide all the signals necessary for drives, at systems costs competitive with current technologies.

In the past, it took several sensors to provide commutation, position, and velocity feedback for servoloops, involving almost 20 wires. But now, because digital drives provide a single connection point for all feedback signals, it is not only possible but advantageous to integrate all feedback functions into a single device. Unfortunately, such a device has been hard to come by.

Conventional feedback devices have their strong points, but all come up short in one way or another when it comes to digital drives. On the surface, resolvers seem to be ideal because they combine practically all necessary feedback functions in a single package. However, their accuracy and effect on the control circuit can limit drive potential. For example, some of the benefits of digital speed regulation -- synchronization, dynamic response, and load stiffness -- are next to impossible to achieve with resolvers.

Hall-effect sensors, used for years to provide commutation feedback, aren't the answer either. In fact, they are rapidly falling out of favor in all but the simplest applications because even conventional position and velocity feedback devices have the ability to provide commutation information.

Encoders, though they've become more common in the past few years, have drawbacks too. For one thing, they're limited by resolution. Another problem, particularly with incremental types, is that they require additional lines for commutation feedback.

With absolute encoders, using serial interfaces like SSI, the connections aren't as bad, but the price – absolute types are often two to three times more expensive than incremental types – can be a major obstacle. Also conventional absolutes can’t provide updates quick enough to keep up with dynamic drives.

Better than the rest

Out of the struggle to keep sensing technology on pace with digital drives, sensor designers have come up with a new type of feedback device that combines the advantages of incremental and absolute encoders.

The optical configuration is similar to that of conventional absolute encoders, but instead of the pattern on the glass code disk producing a binary value, the new device produces three pairs of sine/cosine signals with an increasing number of periods (1, 8, and 64). These signals go to an on-board chip that calculates the vectors of each sin/cos pair and chains them together to form a 9-bit signal. The signal is then interpolated another 5 bits to get a final binary value of 14 bits.

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