How to Select the Right Encoder for Your Machine?

When working on a machine automation project, you will mostly find an encoder in that. The encoder is used to measure how much a moving part has moved. This gives precise feedback on the machine’s current position and helps in accurate movements and output. Encoders are thus a very integral part of machine automation.

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Choosing an encoder is a hectic task, as it has a lot of factors to be seen. In this post, we will see the factors which need to be considered for selecting the right encoder.

What is an Encoder?

An encoder is a measuring instrument that converts mechanical motion into an electrical signal output. This output is in the form of high-speed pulses which cannot be counted manually by us. The encoder requires a PLC or other type of controller which has a high-speed input configuration for counting these pulses. Encoder has a rotary wheel on it which is connected to the moving shaft of the device to be monitored for motion (maybe a motor or gearbox shaft or crank box or screw belt).

As the wheel rotates, pulses are generated inside relative to the motion generated. These pulses are in electrical form and fed to PLC as inputs. The PLC then reads the counts and the programmer then uses formulae inside to convert the raw pulses into actual measured values.

For example, 3000 pulses received can correspond to a movement of 30 cm.

How to Select the Right Encoder?

Let’s discuss the main factors which are to be considered when selecting an encoder.

Resolution

This is the most important criterion for encoder selection. Resolution denotes the number of pulses generated in correspondence to one revolution of the encoder shaft.

For example, if the encoder is designed for 3000 PPR (pulses per revolution), it means that for every one encoder revolution, 3000 pulses will be generated. So, the higher the PPR, the higher is the resolution of the encoder. It means that if the resolution is high, then even a small change of motion will generate more pulses and you get more calculation area for defining it.

A small resolution means that the encoder will be required to move a lot to generate an equal number of pulses. So, select the encoder PPR according to your application. If it requires a very high accuracy for precise control, then the encoder with a high PPR must be selected.

Accuracy

Mechanical errors are some common issues faced in encoder outputs. It means that the pulses received by the encoder will be 2500, corresponding to a 250-degree position; but the actual position of the shaft will be at a 255-degree position. This can happen due to improper encoder circuit working, loose mechanical assembly, lack of tendency to deal with mechanical errors, etc.

In this case, accuracy is the difference that shows between the measured position and the actual position. So, choose the encoder which practically has worked in many applications under testing mechanical conditions and is still given a proper output.

Incremental / Absolute Encoders

The main difference between both these types is that absolute encoders hold their count even after the power cycle, while incremental does not. So, choose the one according to your application. If it is required that the counts resume from the last position, then go for absolute ones.

You can go for an incremental encoder too, but that requires additional programming in PLC.

Linear / Rotary Encoders

Is your application a rotary motion or linear motion? A linear encoder is mounted on a block moving in a straight line, whereas a rotary encoder is mounted on a shaft. Depending on where you want to measure motion, choose the encoder accordingly because both have different mechanical assemblies for sensing motion.

Type of signal required by PLC

If the PLC does not accept digital inputs and instead accepts communication protocols only like Modbus, CANopen, SSI, SEI, Profibus, or Profinet, then choose the encoder accordingly which provides communication outputs, and not electrical digital outputs.

Environmental factors

See the encoder ratings as to how much it is resistant for temperature, humidity, and pressure. Also, ruggedness to vibrations and mechanical shocks matter and before choosing the encoder, we must take care of what are the effects of the environment on it.

Power supply

See what power supply is available in the control panel for the encoders. Normally, it works on 24V DC. If this is the case, then ensure that SMPS is installed in the panel. Any other form of power supply must require the corresponding means for that. So, choose the encoder according to your power supply.

Speed of the machine

Every machine has its own speed of operation. The encoder shaft must rotate according to its RPM, as the encoder RPM too matters in this. Choosing encoders with inappropriate RPM matching can hamper the system’s operation.

Encoder housing

You need to check what material the encoder is housed in, is there any sealing done in the housing, how is the cover type and what mounting it supports. This housing architecture helps to choose the encoder according to current mechanical and environmental conditions.

Cable length

Choose the encoder according to the distance for the cable to be laid. Differential outputs work good for longer distances and single-ended outputs work good for shorter distances. So, select the encoder according to your distance cabling.

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