To function effectively in the world, robotic systems must accurately and reliably measure the positions of their actuators, joints, and other moving parts. As in humans, this “self-sensing” is referred to as proprioception, and it is a fundamental feature of virtually all robotic systems.1,2
Sensing the position of moving parts within an automated system is typically carried out using encoders: devices capable of measuring movement in a given direction (linear encoders) or rotation about an axis (rotary encoders).
An encoder consists of two parts: a sensor and a scale. The role of the sensor is to accurately measure the position of the scale, enabling precise measurement of linear or rotational motion. The role of the scale, then, is to provide an accurate and well-defined periodic pattern that the sensor can detect.
While there are many types of encoder technologies, optical encoders are preferred for precision motion applications. Optical encoders use light sources and photosensitive detectors to measure the change in position between the sensor and scale.
There are three main types of optical encoders:
In transmissive optical encoders, the scale consists of alternating transparent and opaque lines arranged in a 50/50 duty cycle.3 The scale is illuminated on one side by an LED light source, and photosensitive detectors are positioned on the other side. As the scale moves relative to the sensors, the sensors measure a sinusoidally varying light intensity pattern. Using two detectors enables the optical encoder to determine the direction of motion by comparing the lag between the two signals. Comparing the ratios of the two signals also increases the resolution of the optical encoder. Because the light source and sensor are on two different sides of the scale, the overall size of the transmissive optical encoder is greater than that of reflective and interferential optical encoders.
Reflective optical encoders work on a similar principle to transmissive optical encoders, except the scale consists of alternating reflective and non-reflective lines. This means that in a reflective optical encoder, the photodetector is positioned on the same side of the scale as the light source, reducing the overall size of the optical encoder. The accuracy and resolution of the reflective optical encoder inferior to transmissive and interferential optical encoders.
Interferential optical encoders use a slightly different approach. In this type of optical encoder, the scale features alternating reflective and non-reflective lines created using either chrome deposition on a glass scale or laser-etched lines on a metal tape. The scale is illuminated by a laser, which produces a reflected interference pattern. This interference pattern is picked up by a carefully positioned photosensor, enabling extremely accurate position measurement.4 Interferential optical encoders are the most precise type of optical encoder, offering accurate measurement of motion down to the nanometer level.
All optical encoders use a scale made from either metal or glass. While metal scales, also referred to as tape scales, are lower in price, glass scales provide superior accuracy. This is due to a few different reasons. Firstly, the production of glass scales lends itself to more uniformity (also referred to as scale linearity) than tape scales. In addition, glass has a lower coefficient of thermal expansion than metal: this means that glass scales are more reliable in thermally variable environments, especially in compact robot joints where there are several sources of heat (e.g., drives, brakes and motors) enclosed in a small area.
Celera Motion provides a full range of complete optical encoder systems and optical encoder scales for all applications. Are you interested in learning more about optical encoders? Please read our guide to optical encoder technologies here, or get in touch with us today to find out more about our range of position encoding technologies.
References and Further Reading