Page 139 - Electric Drives and Electromechanical Systems

132 Electric Drives and Electromechanical Systems

In a linear optical encoder, the movement of the sensor must be parallel to the
encoded track, if not the reading could degrade or in the case of a housed encoder,
damaged. In some cases, manufacturers will provide a template or specialist tools
to assist with alignment.
All encoders are subject to various types of acceleration during operation and
mounting, again these will be detailed in the manufacturers’ specifications. Due to
presence of a glass code wheel or linear scale, encoders are considered fragile,
particularly during the assembly of a system.
Both linear and rotary encoders have internal friction, particularly if the design
includes a seal, this is normally specified as a torque or force in the specifications.
All types of encoders should not be subjected to excessive vibration during opera-
tion: this can have a significant impact on a poorly long linear encoder. To func-
tion properly, the more solid the mounting surface the better. It is recommended
that linear encoders should not be mounted on hollow parts.
As discussed in Section 2.7.5, protecting the encoder from the environment is a
major factor when selecting components. For example, linear encoders are nor-
mally protected to IP 53 (see Table 2.3) provided that they are mounted with the
linear seal is facing away from possible sources of contamination. If the encoder
will be exposed to heavy concentrations of coolant and lubricant mist, the scale
housing can be fed with filtered compressed air to raise the internal air pressure,
allowing the IP rating of the encoder to IP 64, by preventing entry of any
contamination.

4.4.2 Electrical interconnection
The wiring and connectors between the transducer and the processing electronics are
critical to the operation of a system. If they are not satisfactory, in the case of a digital
encoder, any electrical noise which is introduced will probably result in additional pulses
being counted and hence in an increasing positional error. In analogue systems,
electrical noise resulting from poor connections will result in a poor signal-to-noise ratio
and hence in a degraded performance. These problems can be reduced by the use of
twisted screened cables and high-quality connectors throughout the system. As shown
Fig. 4.19, at high speeds the encoder output frequency can exceed 100 kHz, and therefore
the wiring and associated electronics must be designed to accommodate signals of this
frequency; in particular stray capacitance must be minimised.

4.4.3 Determination of datum position
When a rotary incremental encoder is used in linear applications one of the design
requirements is to accurately and repeatably determine the datum position. This is the
point to where all measurements on a particular axis are referenced. As shown in
Fig. 4.17B, it is common practice for an incremental to have a track that provides one