Circuit Reads Encoder Direction of Movement

Posted on Feb 6, 2014

Incremental-rotation or linear encoders are very common, but normally they do not provide a direction signal. This design shows an easy way to detect forward or reverse direction. Incremental encoders normally provide two output signals, usually named Channel A and Channel B. These signals deliver both clock information, depending on resolution, a

Circuit Reads Encoder Direction of Movement
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nd rotating speed. They differ only in phase margin (for example, 90 ° for clockwise and +90 ° for counter clockwise). The circuit in Figure 1 uses these signals as inputs to a 4538 single-chip, dual monostable multivibrator. Depending on the speed required for the application, the IC could be a metal-gate device, a 74HC, or a 74HCT type. 1. The direction discriminator circuit is based on a dual monostable multivibrator, which can be a metal-gate, 74HC, or 74HCT type depending on the speed required by the application. The feedback from one of the outputs to an input is used to avoid re-triggering. This isn`t strictly necessary but helps to keep the pulse duration constant. On the other hand, an important function is to trigger forward pulses from one edge of the input signal and reverse pulses from the other edge of that signal ( Fig. 2 ). 2. This scope printout shows the behavior of the circuit when the encoder shaft is moved a little between clockwise and counter-clockwise. Channel 1 is In Channel A (U1 pin 4). Channel 2 is In Channel B (pin 13). Channel 3 is Out Forward (pin 6). Channel 4 is Out Reverse (pin 10). This is why the same mechanical position, or slot edge of the encoding wheel, delivers, for instance, a positive edge in the forward direction and a negative edge in reverse. So if a design uses the same electrical edge for triggering, the result will be a hysteresis in changing the direction of one encoder slot...

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