Technical Blog

Ultra-slow motion control with Zettlex position encoders

Position encoders play a crucial part in many motion control applications including: - Industrial Test and Measurement - Automation - Security Systems - Surveillance Systems

The term “motion control” often gives the impression of a high-speed systems with rapid changes of direction and speed. High speed and dynamic motion control applications endure technical challenges however, this also applies to slow speeds.

It is quite common in modern-day applications for high-zoom optical systems to focus on a target over 20km away. When a target moves, the field of view is limited, so if the target is not tracked smoothly it will disappear from view. For example, if the target is moving at 20km/hr at a range of 20km then this totals a rotational speed of 0.05rpm which is an ultra-slow speed.

To ensure the target is within the field of view, a slow speed needs to be establised and maintained as the target’s speed and direction changes. To manage a central position in the field of view (equivalent to 0.3m at a range of 20km) then >200k enocoded points are required - or 18 bits of encoded position.

Conventional high-resolution position encoders

A conventional technique is to use a position encoder on a system’s motor and to increase the counts per rev through a reduction gearbox connected to the motor. This also means a greater amount of backlash and a less responsive servo-system. A system of this type would have a limited dynamic range and would be unable to track a fast moving object at a distance of a kilometer or less, as it would lack the performance required at faster rotational speeds.

Multi-speed resolvers, precision optical or capacitive encoders have traditionally provided high-resolution (>18 bits) position sensing. Precision resolvers are expensive and often present packaging problems due to their bulk, weight, and precision installation tolerances.

Similarly, high-resolution optical ring or capacitive encoders are also expensive and require precise mechanical installation. Unlike resolvers, which are usually very robust, optical encoders use glass scales and so offer limited resilience to shock or vibration. Plus, their operating temperatures are limited. Optical and capacitive devices suffer from reliability problems from foreign matter, such as dust or condensation. High-precision optical devices are particularly susceptible to foreign matter because they use fine optical gratings to attain the high number of counts per revolution.

Next-generation inductive position encoders

The modern approach is to use a high-resoltution position encoder on the output shaft of the gearbox. This helps to avoid backlash effects whilst maximising the servo’s dynamic performance. This type of approach has only just been more recently used because high-resolution encoders used to be more expensive.

Most traditional high-resolution position sensing techniques have limitations, so design engineers are turning to a new type of sensor. The sensors may be thought of as a new generation of inductive encoder that offer high-resolution measurement of up to 4 million counts per rev (22 bits). Zettlex inductive encoders use the same basic physics as resolvers, and this allows them to offer high resolution, non-contact measurement in tough operating environments. Operating in dirty or wet conditions allows a design engineer to eradicate the seals, bushes, or O-rings required to protect optical or capacitive encoders.

Rather than the traditional wire wound transformer constructions found in resolvers, encoders use printed circuits as their principle components. This provides a further advantage in form factor, low axial height, and a large bore. The design makes it easy for cables, shafts, and pipes to pass through the middle of the sensor. Position encoders offer a simple electrical interface featuring a dc supply and an absolute, digital output.

Take a look at the Zettlex Encoders available on our website.

For more information on Zettlex visit their website

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Article published on: 02/11/2016