Encoder Selection

Factors to be considered for Selection of Encoders

Incremental vs. Absolute

Can you afford to lose position in case of power failure? If the answer is no, then you must use an absolute encoder. An incremental encoder simply generates pulses proportional to the position, whereas an absolute encoder generates a unique code for each position. After a power outage, with an absolute encoder the machine operation will pick up from where it had left off even if the encoder shaft had moved during power down which is very typical as the encoder shaft will coast to a stop when power is lost. In an incremental encoder the pulses generated are counted in a counter and at power loss it will lose the count and consequently you will have to home the machine before you can start the operation. Typical application examples for Incremental Encoders are “Cut to Length”, Conveyor Control, Augur Control, metering equipment, and machines that use lead screws for motion control such as a milling machine. Upon power down, you have to re-sync the controlled apparatus. Absolute Encoders are used when the machine/process has to know the true position all the time and re-sync is not allowed, such as a Press or Assembly machine or a Dam control or an Oil Valve control.

Also, an incremental encoder is generally more susceptible to electrical noise. Whereas absolute encoder may give you a false output under noisy conditions, the true position is restored when noise is gone. On the other hand, if you can false counts with noise when using an incremental encoder, the bad count would remain there until reset or re-synced. The absolute encoders are more expensive than the incremental encoders, therefore, a price/ feature trade-off may be worth considering.

Electrical ratings:

We recomend 24VDC power and 7272 outputs for highest reliabilty and imuunity to electrical noise.

Optical Encoder vs. Resolver

This decision is primarily based on the operating environment. The environmental integrity of a brushless resolver is unchallenged. Being simple rotary transformers, the resolvers can take much more abuse than optical encoders and exhibit no significant wear or aging. Especially, if the operating temperature is below freezing or above 150 °F, there is no other choice, but to go for resolvers. Operating temperature range of resolvers is typically between -67 °F to +248 °F. In extremely hostile environment such as continuous mechanical shock and vibrations, humidity, oil mist, coolants and solvents, resolver is the best choice.

Mechanical ratings

Choice of encoder size and NEMA rating also depends on the operating environment. where as size 25 is the work force of the industry due to its ideal size and mechanical strength, NEMA 4 size 15 from Autotech with 50 pound load strength would be ideal where space is a constraint. Size 40, water submersible and explosion proof encoders are self explanatory. Please also note that built-in gear trains are available in only size 40 encoders.

Single-Turn vs. Multi-Turn

In a single-turn encoder, the encoder shaft makes one revolution for one complete cycle of machine operation, where as in a multi-turn application, the encoder shaft makes more than one revolutions to complete one machine cycle. Absolute multi-turn encoders and resolvers are available with various built-in gear ratios.

Built-in vs. Remote Decoder

In resolver based encoders Autotech offers a choice of built-in decoder inside the encoder housing or remote decoder. In outdoor applications where temperature could be extreme as well as applications where the encoder is located far away(up to 2500 ft) from the control panel, remote decoder is the best choice.

How do you Determine direction of rotation and resolution in an Incremental Quadrature Encoder?

A typical Incremental Quadrature encoder, both optical and resolver, has two channels called A and B. These electrical outputs from the encoder are phase shifted to each other by 90 degrees, which allows a counter to be able to determine the direction of rotation. If A leads B (that is A transition occurs before B), the counter can determine which direction the encoder is moving, either clockwise or counterclockwise. If B begins to lead A, it means the encoder has changed direction. A third channel called Z ( for Zero) is an optional signal which provides a pulse once per revolution that can be used to synchronize zero value in the counter.

PPR is the number of pulses per revolution of a single channel, for example A output. It is also sometimes referred to as Resolution which is somewhat of a misnomer as the total resolution available from an encoder is 4 times the term PPR or Resolution. Essentially, one cycle of A and B has four signal transitions. Thus an encoder with a Resolution or PPR of 1000 will resolve the encoder shaft to 4000 counts per revolution or 360/4000= 0.09° or 5.4 minutes of rotation.