Choosing the correct linear encoder feedback system for position or speed feedback in your machine application can be a daunting task. Specialist encoder supplier HEIDENHAIN explains some steps to linear encoder selection.
Several key factors need to be considered when establishing the most suitable type of linear encoder for a specific application. Here are four questions that will help you narrow down your options.
What resolution and accuracy does my application require?
With reference to linear encoders, the resolution is the smallest linear step that can be measured. For precision machines, this can typically be micrometres (µm) or even nanometres (nm). This resolution can be seen on a display or used within the system control loop – generally, the finer the measuring step, the better the closed-loop performance of the control.
The accuracy of a linear encoder is defined as how close the reported measurement is compared with a standard; in the case of linear measurement, this is the SI unit of length, the metre (m). For many machine applications, it is more desirable to make parts that are repeatable than to have ultimate accuracy, so this is a consideration.
Linear encoders can use various scanning technologies for measuring, from capacitive, inductive, magnetic through to optical.
For example, optical is the preferred technology for high precision applications as linear encoders have very fine graduations on substrates such as steel, glass or glass ceramic.
If a machine is subject to significant variations in temperature, consideration should be made regarding the substrate material used within the linear encoder. Ideally, the substrate should match the materials used in the machine to ensure they have similar thermal coefficients of expansion to achieve the highest possible accuracy.
What environment will the linear encoder be operating in?
The environment where the linear encoder will be sited is a significant consideration. The encoder position should be as close to the moving axis as possible (to reduce Abbe errors) but away from possible sources of contamination. Linear encoders using the inductive scanning principle are very tolerant of liquid or dust, whereas optical linear encoders for higher performance applications have to be of the ‘sealed’ type. In clean environments, the selection of the linear encoder is primarily based on other factors mentioned here in this article.
For vacuum applications, which are inherently clean, special ‘open’ linear encoders are required. The materials used in the manufacture of these products are specially chosen to withstand high temperatures (bake out) and prevent outgassing so as not to contaminate the vacuum environment.
What speeds will my linear axes be operating at?
The speed at which an axis will operate can have a bearing on the selection of linear encoder design. A ‘sealed’ linear encoder with an integral coupling may not be suitable for high-speed applications due to the limitations of a mechanical coupling between the reading head and scale. An ‘open’ encoder would be preferred for these dynamic applications as there is no mechanical connection between the linear scale and the reading head so that higher speeds can be attained.
The encoder’s resolution has to be considered with respect to the speed of operation; for example, a linear encoder with a very high resolution can produce very high-frequency output signals that have to be handled in the subsequent control. If maximum frequencies are exceeded, this can lead to miscounting or lost motion.
Does my machine need to power on in a known state?
Power cycling a machine can, in some circumstances, result in the axes positions not being known. Using absolute linear encoders (as opposed to incremental linear encoders) can solve this problem. An incremental linear encoder usually generates a series of pulses as a result of movement. A control system would have to initiate a referencing procedure to move to known zero point or datum. Absolute linear encoders can ascertain their position immediately on power up, so the control knows exactly where the axis is in relation to its zero point or datum. This results in the machine powering up in a known state, saving time referencing.
