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Tachometers vs. Encoders - Whats the Difference

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As it is possible for tachometers to perform the same basic function as encoders it’s common to find that sometimes there is a bit of confusion between the terms. This can often lead to ‘over-design’ or ‘over-engineering’ because there are a wide-range of complex encoders available but a simple tachometer will often get the job done. But first let’s look at what each device does. In short both tachometers and encoders are designed to provide some information about the movement of a motor shaft.

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A tachometer will enable you to calculate the RPM of a motor. As an analogy if you can imagine a trundle wheel ignoring the fact they’re actually designed to measure distance they make an audible click once every full revolution. Count the time between clicks and you can figure out how fast the wheel is spinning - or its RPM.

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There are two main encoder types that help tell you about the position of the motor shaft. One simplification that we’ve made is that we’re only discussing single turn encoders. Multiturn encoders where we are able to determine the shaft’s position beyond a single rotation would require a much more in depth discussion about encoders in general.

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Incremental encoders are the simpler of the two types. They provide a reading every time the motor shaft rotates a certain distance. Think of a playing card clipped or glued to a bike frame with one end in the spokes of the wheel. As the wheel turns the card hits the spokes and makes a noise. If we know the angle between spokes and can count every time the card makes a noise then we can figure out how far the wheel has turned. This is very similar to the idea of an incremental encoder. Obviously with more measurement points or ‘spokes’ in our analogy we can increase the accuracy of our calculation. However note that there is no information about the actual position of the wheel - only relative to the starting point. For this reason some applications might use a start-up algorithm that takes the rotor to a known position.

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Conversely absolute encoders do provide an exact position of the motor shaft. There are various types of measurement used beyond the scope of this blog post the most common are optical and magnetic based readings. In each method the processor is able to calculate the exact position of the motor shaft with surprisingly high accuracy. The big difference is that because each angle section is uniquely coded the shaft position is not relative to any other point. So there’s no need for a starting algorithm and the position can be determined immediately with no need for movement.