Precision control for slow motion applications

Paul Boughton

As the market demands ever more precise position and speed control, some traditional electrical control, actuation and sensing methods are being discarded and new techniques are emerging. Mark Howard highlights some of the problems with traditional approaches and explains the new technique.  
An ideal motion control system is capable of precision control over a high dynamic range – in other words, at high speed as well as low speed. There are a myriad of techniques for the control of electric motors and actuators at either high speed or low speed but problems arise when there is a large difference between the two. This is especially true when the low speed is almost but not quite stationary.
For example, in a camera system, tracking a target moving at 4km/hr at a range of 5km equates to a rotational speed of 0.01O/second, whereas tracking an object moving at 120km/hr at a distance of 100m equates to a rotational speed of 18o/second. This equates to a dynamic range of about 2000. At the slow speed, even a modest discrepancy between the actual and required speed control will mean a large relative error. For example, in a camera system the target object would be lost because a tracking error of 0,01 degrees would mean that the target object might appear to jump or disappear outside the field of view. Such problems are also present in a wide range of applications such as gunnery control systems, radar, location devices, laser systems and scanners.[Page Break]
Various technical problems can arise when there is a high dynamic range for the actuation system. At low rotational speeds, traditional motors are either hugely inefficient or do not receive sufficient counts per second from their corresponding motor encoder for reasonable speed control. In some cases, motors are very inefficient and can overheat at low speed because the cooling usually provided by shaft or winding rotation no longer provides any cooling effect. To ensure more reasonable performance at low speed, a gearbox with a very large reduction ratio can be used so that a reasonable motor speed can be maintained and hence provide stable control at the output shaft. Unfortunately, this then leads to problems at high speeds, because the motor is simply unable to reach the required speed for rapid actuation of the output shaft.
One solution is to provide the motor with a feedback or servo in the form of sufficient rate of ‘ticks’ even at low speed that it can maintain its low speed. This requires a high resolution encoder to feedback position and/or speed information from the output shaft. Typically, a high resolution, absolute encoder is required. Traditionally, such functionality has been provided by multi-speed resolvers or precision optical encoders (usually referred to as ring encoders). Precision resolvers are notoriously expensive and can present engineers with packaging problems due to their bulk, weight and precision installation tolerances. Similarly, optical ring encoders are expensive and also require precision mechanical installation. Unlike resolvers, which are usually very robust, optical devices in such applications offer limited resilience to shock or vibration (due to their use of glass scales) and their operating temperatures are limited. They also suffer reliability problems from foreign matter. 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. Their absolute position output can be thrown by particulates such as dust or sand whose dimensions approximate to the features of the optical grating.[Page Break]
However, a new generation of sensor has entered the market in recent years and has a growing reputation not only in the traditional high-end aerospace & defence markets, but also in the more mainstream industrial and domestic sectors. This new generation of sensor uses the same basic physics as traditional resolvers and linear transformers and so offers comparable levels of robustness. This new generation of sensor uses printed circuit boards and modern digital processors as opposed to the traditional bulky transformer designs and analogue electronics. The approach is elegant and also opens up a range of applications for inductive sensors as high precision, high resolution angle encoders.
Zettlex technology is the forerunner of this new generation inductive technique and has grown over recent years thanks to some high profile design wins.  The use of printed circuits enables sensors to be printed onto thin flexible substrates, which can also eradicate the need for traditional cables and connectors. The flexibility of this approach – both physically and from the ability to readily provide customised designs for OEMs – is a major advantage. As with traditional inductive techniques, the approach offers reliable, precision measurement in harsh environments but without the drawbacks of weight, bulk, cost and precision installation tolerances. Such devices can also be encapsulated to enable operation in submerged or chemically aggressive environments. There are also some important advantages:
* High precision from high resolution (up to 16M counts per rev), high repeatability and zero hysteresis
* No need for bearings, seals, bushes, service or maintenance
* Naturally suited to large through bore design – allowing through shafts, slip rings, hydraulics and rotary joint mechanics to pass through
* Simplification of the electrical interface – typically a DC supply and absolute, digital signal.
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