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.

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.