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ELM404

ELM404DSA

Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

Rotary Encoders

A rotary encoder (sometimes referred to as a

quadrature encoder) is a device that produces digital

(on/off) outputs in response to rotary, or circular,

motion. It is often constructed such that it looks very

much like a potentiometer, or audio volume control

(see the picture of a typical device, at the right).

As the encoder shaft is turned, internal contacts

open and close, creating two waveforms that are

ideally separated in phase by 90 degrees (ie ‘in

quadrature’). Actually, you need to provide external

‘pullup’ resistors and a power supply to create these

waveforms, as the contacts themselves can not do

this. An ideal waveform from a rotary encoder would

look like this:

Due to the 90 degree phase difference, when one

waveform changes, the other is always stable. By

noting the direction of the change and the level of the

other input at that time, you can determine the

direction of motion of the shaft.

Rotary encoders are not ideal, however. Due to

their construction, and variations in shaft speed, the

waveforms are not perfectly square with the 50% duty

cycles shown. Figure 2 shows a captured trace from a

real rotary encoder that is more representative of what

you will typically find. Note that the two ‘scope

channels (1 and 2) represent the encoder outputs A

and B, respectively. The ch 1 (A) waveform leads the

ch 2 (B) waveform, which usually means that the shaft

is turning in a clockwise direction.

The first rising edge of the channel 2 waveform

shows another problem that occurs with moving

mechanical contacts - multiple pulses due to bounce.

When two contacts meet, the moving one will tend to

bounce, like a ball does when it is dropped on the

floor. Each bounce results in an electrical connection

being made, then broken, which will look like multiple

inputs to a fast electronic circuit. Various mechanical

means are used to reduce the amount of bounce, but it

can never really be eliminated. The following section

discusses how the ELM404 uses electronic means to

remove the bounce.

Figure 2. Actual Rotary Encoder waveform

Figure 1. Quadrature Waveforms

A

B

A typical rotary encoder