Advancements in Technology for Multiturn Encoders

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 the resolution of such systems is typically to 17 bit.

Multiturn encoders employ the single turn position data, but also track the angular displacement over many turns. By utilizing either electronic counters or gears, the encoder’s shaft position can typically be monitored 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 light weight. Weight reduction offers lower inertia for improved performance – especially the speed ratings of the encoder which currently are up to 9000 RPM.

Multiturn encoders utilize a glass disc which has a chrome pattern deposited on its surface. Glass is used because it is translucent, provides a stable platform for the code pattern, and is the most stable material over a wide range of temperatures. The chrome patterns divide the disc into divisions and are interpreted as grey code numbers inside the encoder.  Photo receivers are employed on one side of the disc, while LEDs provide the light source on the other side. The reception of light 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.

As the name implies, multiturn encoders will also track the shaft position over multiple turns; for example, to 12 bits (4096 turns). A trio of plastic gears is driven off of the main shaft or hub. These gears utilize special coatings which allow light to either be absorbed or reflected back, activating several critically positioned photo sensors. The on/off patterns of these sensors define the multiturn position.  With one sensor required per bit of information, a 17 bit per turn encoder will require 17 photo receivers to carry out the job. In the past, these photo sensors and LEDs were discrete items, thus the cost of the encoder and labor to install them was 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 encoders functionality, thus reducing the cost and labor to build such a device.

Multiturn encoders utilize either an input shaft or the increasingly popular hollowshaft construction.  When connecting shaft encoders to another shaft, there are inherent challenges in trying to align the shafts. As perfect alignment is impossible, a flexible coupling must be used to connect both shafts or possible bearing overload will occur. Hollowshaft encoders allow the mating shaft to pass through them, and lock to the shafts with a collet style mechanism. A tether mechanism holds the encoder to the machine base providing two functions: the encoder’s body is prevented from rotating, and any axial and radial misalignments between the shaft and encoder are absorbed in the tether minimizing any overloading forces to the bearings. Hollowshaft encoders are typically self aligning, and the overall installed height is reduced as compared to a shaft encoder which requires a standoff flange. Reduced installation time, no coupling, and lower installed profile are just some of the benefits from hollowshaft encoders.

Optical recognition is the preferred method of sensing by many manufacturers, as it is virtually impervious to magnetic fields. Absolute encoders employing this technology are ideal candidates for use on gear motors utilizing brakes. In the past, brakes wreaked havoc on many absolute multiturn encoders. Today, we are seeing more and more successful applications of such.

With a total of 17 bits per turn and 12 bits of turn data, a total data word of 29 bits can be realized. To transmit this in parallel fashion there would need to be 29 wires, which is just not economical. This is the reason why all multiturn encoders today utilize serial transmission, which is typically carried out over two or four wires. Some popular methods are SSI, PROFIBUS, DeviceNet and CANopen. These serial types of transmission are achieving speeds up to 10MHz for near real-time position updates.  Another benefit of the serial transmission is the ability to program the encoder online, including  baud rate, end of line termination resistor, addressing, or setup with dip switches inside the encoder. Things like counting direction, home (or “0”) position, instantaneous velocity, acceleration/deceleration rate, position transmission update rate, and many more can be read or programmed to the encoder. Many new options are now available to users which were simply not before. These new options are endless and allow these multiturn encoders to be used in applications where they were never used before.