Multi-turn Encoders Find Home in Industrial Applications
- Sunday, 01 February 2009
Single-turn encoders have long been used in industrial automation to gauge angular displacement in a broad range of applications, ranging from steering control on a mobile equipment vehicle to boom extensions. A single-turn encoder is a transducer that precisely measures angular displacement over 360 degrees and assigns a value to each position. These values can be decimal, binary or grey code, and systems typically offer up to 17–bit resolution.
An alternative to single-turn encoders, multi-turn encoders measure shaft position and also track the number of complete revolutions the encoder shaft is rotated. The ability to keep count over multiple turns — up to 4,096 revolutions — saves users significant time and hassle.
The advent of multi-turn encoders allowed users to secure greater amounts of data at faster speeds. This advantage could be likened to a car’s odometer. If a single-turn encoder was coupled to the shaft of a car tire with a circumference of 80 inches, for example, the encoder would track one 80-inch rotation, then start again at 1 inch. While the car may have traveled miles, a single-turn encoder would only measure up to 80 inches, over and over again. Since a multi-turn encoder can track revolutions as well, it would continue accumulating the inches from each rotation until the encoder reached its physical limit—compiling the total distance traveled, with no manual computations needed.
By utilizing either electronic counters or gears, the multi-turn encoder’s shaft position can typically be monitored up to 4096 turns (12 bits). Recent technological advancements allow plastic gearing to keep track of the number of turns. Plastics make the gearing cost-effective and lightweight, which reduces inertia for improved performance — especially enhancing encoder speed ratings, which currently are up to 9000 RPM.
Multi-turn encoders utilize a glass disc that has a chrome pattern deposited on its surface. The chrome patterns divide the disc and are interpreted as grey code numbers inside the encoder. Glass is used for the disc because it is translucent, provides a stable platform for the code pattern, and is the most stable material over a wide range of temperatures. Photo receivers are employed on one side of the disc, while LEDs provide the light source on the other side. The light received through the disc is encoded and interpreted as a grey code pattern. Grey code is an ordering of 2n binary numbers and is used as only 1 bit changes at a time, leaving little room for counting errors.
Multi-turn encoders are typically larger than standard single-turn encoders because they must hold a gear train. In the past, large gear circumferences precluded the encoders from employing a through-hole design. The gears in early encoders were contructed of metal in order to stand up to high-speed operation, as plastic gears were prone to failure due to backlash.
Previously photo sensors and LEDs used in encoders were discrete items, making the high cost of the encoder— and labor required to install it — one of the cost drivers of absolute encoders. Today, the photo sensors are included in the primary ASIC (application specific integrated circuit), which provides about 95% of the encoder’s functionality, thus reducing the cost and labor to build such a device.
To track shaft position, the multi-turn encoder utilizes a trio of plastic gears driven off of the main shaft or hub. These gears utilize special coatings that allow light to either be absorbed or reflected back, activating several critically positioned photo sensors. The on/off patterns of these sensors define the multi-turn position. One sensor is required per bit of information, so a 17- bit-per-turn encoder will require 17 photo receivers to carry out the job.