Rotary encoders are commonly used to measure belt speed in conveyor applications including synchronizing conveyors to pick-and-place operations and synchronizing the operation of multiple conveyor belts to one another. Rotary encoders are also used to track linear distance of an item travelling on a conveyor after a checkweigher or x-ray so that it can be removed from the line by a rejecting mechanism.
There are three methods for measuring the speed of a conveyor belt:
Indirect measurement with encoder monitoring motor shaft
Indirect measurement with encoder monitoring roller shaft
Direct measurement using encoder measuring wheel
The indirect measurement method involves monitoring the shaft of the motor driving the conveyor belt. The encoder should be mounted directly to the motor shaft. It will output either a pulse stream (for an incremental encoder) or a digital word (for an absolute encoder) that corresponds to the displacement of the motor shaft.
To convert from encoder output, it is necessary to physically measure the amount of travel introduced by one rotation of the motor. For a constant travel L , the drive can calculate speed s in feet per second by:
where fp is pulse frequency in hertz.
The drawback to this approach is that it does not take into account mechanical slop that may be introduced by the coupling, gearbox, belt slip, etc. This also brings up an important point: the mechanical performance of the conveyor belt and the speed resolution of the motor are the limiting factors in maintaining consistent speed with a conveyor belt. All an encoder can do is report. The resolution needs to be high enough that it the encoder can accurately report the change in position that gets converted to speed. Specifying an encoder with resolution above that will not improve system performance and may even lead to an erroneous reading.
Depending on access to the motor, it might be easier to mount the encoder on the roller shaft.
In this case, the drive converts the encoder output to linear speed s in feet per second as follows:
where fp is pulse frequency in hertz , R is resolution in pulses per revolution. Linear speed s is given by:
where D is diameter of the roller in inches.
Although more accurate than a motor-mounted encoder, this approach still suffers from mechanical error introduced by belt slip, etc.