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Angle Encoders: How To Measure Angle Using Encoder (1)

May 09, 2017

Angle encoders measure the rotational position of a load in relation to a shaft or point. The angle encoder provides output that corresponds to displacement and the reading device (PLC, counter, etc.) processes that data into angular readings. There are three approaches  to measuring an angle with an encoder:

  • Direct angle measurement with an encoder mounted to the pivot point of the load

  • Indirect angle measurement with an encoder mounted to the motor driving rotation

  • Indirect angle measurement with a multi-turn encoder mounted along the circumference of the load

Direct Angle Measurement by Tracking the Load

To measure the angular displacement of the load directly, the encoder should be mounted on the center pivot point of the load. As the load turns, the encoder relays the signal to the receiving device. This data only consists of either displacement pulses (incremental encoder) or initial and final absolute positions (absolute encoder). The reading device, whether it is a drive, a controller, or some type of counter/display device, needs to process the raw data into useful information.
Angle Encoder Direct Tracking Diagram
In the case of an incremental encoder, the angle α can be expressed as
Incremental Angle Encoder Expression
where P equals the number of pulses and PPR equals pulses per revolution

In the case of an absolute encoder, α can be expressed as
Absolute Angle Encoder Expression
where C equals counts and CPR equals counts per revolution.

Indirect Angle Measurement by Tracking the Motor Shaft

It is also possible to measure the angular displacement of a load by monitoring the motor shaft or an Idler wheel. In this case, we get best results by turning the motor shaft a set number of times and correlating it to the displacement of the load to come up with a conversion factor.

 Angle Encoder Indirect Tracking Motor Shaft Diagram

This is the traditional use case for a rotary encoder. It has the benefit of improving motor performance through direct feedback. The drawback  is that it may not accurately track the movement of the load. Mechanical compliance introduced by couplings, gearboxes, etc. can introduce errors like backlash and hysteresis. The conversion factor theoretically takes this into account, but mechanical effects can drift over time.

Indirect Measurement by Tracking Edge of the Load

Some loads are incompatible with center-pivot encoder mounting. Satellite dishes, for example, need to be rotated to point in the direction of the signal of interest to a high degree of control. Both the mechanical design and the presence of electrical cabling passing up through the central shaft make it impossible to install an encoder at the pivot point to monitor position directly. Depending on the degree of control, an extremely high resolution encoder may also be required. The solution is to track these devices along their circumference. 
Angle Encoder Indirect Tracking Load Diagram
In the indirect circumferential method, a geared wheel attached to a multi-turn absolute encoder engages with the circumference of the load to track its angular displacement.

As the band on the load moves past, it turns the wheel and the movement is registered by the encoder. The reduction ratio of the gearbox enables it to deliver a high number of rotations for every turn of the gear wheel. This delivers a final resolution RF given by:
Angle Encoder Load Tracking Resolution Formula
where RB is the number of counts on the band and N is the reduction ratio of the gearbox on the multi-turn encoder. The contact may consist of a toothed gear wheel engaging with a toothed band on the load. Alternatively, it could be a friction contact from an encoder measuring wheel or even a belt.

Error sources include mechanical couplings, backlash from the gearbox, shaft runout and de-centering, belt slippage in the case of friction contacts, and others. The high reduction ratio of the gearbox minimizes the effect of mechanical error. The exception to this is friction in the gearing and seals of the gearbox. In this case, the impact of friction is multiplied by the number of turns.