The basics of encoder selection
The feedback sensor of choice for many application with small motors is the digital incremental encoder. Urs Kafader of Maxon Motor considers the encoder properties, and looks at the key criteria impacting on product selection.
Each application for encoders is different. The main task may be position control or speed control. The level of accuracy in speed or position control can be very different and should be defined before encoder selection. Speed control at low speed (below 100 rpm) needs a better feedback than speed control at high speed (1000 rpm and above).
The load may be coupled directly onto the motor or there may be a mechanical transformation system such as a gearhead. Encoders are typically mounted on the motor shaft, but can also be on the load itself. The mechanical properties of the transformation mechanism influence encoder selection, as factors such as gear reduction and mechanical play have to be taken into account.
Environmental conditions such as temperature, vibration, electromagnetic interference may also have an influence on encoder selection. Optical encoders, for example, should be protected against dust. Magnetic encoders may be sensitive to external magnetic fields – including those of the motor – and may require shielding.
The characteristic parameter of an incremental encoder is the number of rectangular pulses per motor revolution. Typically, there are two channels delivering the same pulse number. The two signals have a relative phase shift of one quarter of a pulse length. This arrangement allows the detection of the direction of motor rotation and gives four distinctive states per pulse, representing a real resolution which is four times higher than the number of pulses on one channel.
However, encoder pulse lengths may vary due to mechanical tolerances (including shaft runout, length of magnetic poles and others). The pulses in one range of motor rotation may be shorter than the pulses of other ranges. As a result the measured position deviates from the real position in a periodic way over one motor revolution.
The required positioning resolution of the application dictates the encoder resolution to be selected. A well-tuned system can maintain the position within 1 encoder state (quadcount). Hence, the encoder resolution in quadcounts (states) should at least correspond to the maximum permissible positioning error. Depending on the response time of the system, a higher encoder resolution should be chosen in order for the controller to detect deviations faster and counteract quicker.
Signal jitter – particularly if large compared to the nominal state width of the encoder – reduces the accuracy in terms of repeatability that can be achieved. In this respect, direct sensing optical encoders have advantages over interpolated magnetic encoders. Direct sensing larger optical encoders also have advantages concerning the absolute accuracy. Their integrated non-linearity (INL) is very small.
A very high accuracy in positioning is difficult to achieve with mechanical transformation and the associated play. Therefore, high resolution encoders only make sense on direct drive applications. Very often, high precision positioning not only requires a high number of states but also a high absolute accuracy. Optical encoders have here advantages, both due to a high resolution and a low INL.
Drive systems with mechanical transformations such as gearheads or lead and ball screws do not require a high encoder resolution. The resolution of the encoder mounted on the motor will be multiplied by the gear reduction. Similarly, on a screw with 5mm pitch a moderate encoder resolution of 512 quadcounts (128 cpt) will result in a theoretical position accuracy of the nut of about 10 microns. That’s often much less than the mechanical play in the coupling and the nut and the accuracy of the screw thread.
Incremental encoders just give position changes. For absolute positioning, a reference or home position must first be established. This is achieved by moving the mechanism to an external reference; this could be a mechanical end stop or a limit switch.
Some encoders feature a third channel with one pulse per turn. The edges of this index channel give absolute position references within one turn. The limited accuracy of external references can be improved by an additional move to one of the index channel edges. However, observe that the index channel is not a prerequisite for positioning. In fact, machine builders try to avoid using the index for referencing because it requires new calibration if a motor-encoder unit has to be replaced.
Furthermore, homing procedures are typically performed at low speeds, taking time that is not available in some applications. In multi-axis systems – for example in kinematically complex robotic applications with mechanically interdependent axes – homing could cause collisions and damage. In such cases, absolute encoders can be used as an alternative to incremental models. After being switched on absolute encoders provide the actual position directly (without a homing procedure) within one motor turn (single turn) or multiple turns (multi turn).
In industrial applications, absolute encoders with a serial interface are often used, transmitting the actual position as a bit-stream. A total of only six lines is sufficient for the supply voltage, data transmission and synchronisation of the transmission timing.
For single-turn absolute encoders, one axis revolution is coded in N steps. The coding repeats when rotating more than 360°. Typical resolutions are 12-bit (4096 positions) and more per revolution. In multi-turn absolute encoders, the numbers of revolutions are additionally coded and stored in the same bit stream. Multi-turn encoders are required when the number of measurement steps of a single-turn encoder is not sufficient.
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