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Low-power position sensing in extreme environments

03 Jul 2015  | Mark Hoferitza

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For a variety of reasons, the demand for low-power position sensing is increasing in many product categories. With appliances and consumer goods, government regulations and environmental programs often are motivating factors in the drive for higher efficiency.

Low power consumption also is required in an entirely different category of products, for a different reason: in remotely installed instruments, or in subterranean or submarine sensors, low power is a purely financial and operational requirement. In many of these applications, low power consumption must be combined with extreme robustness: operating conditions might be harsh because of wide temperature ranges, high humidity, high pressure, or the presence of contaminants.

This article explores the latest options for position-sensing systems that combine low power consumption with tolerance of extreme conditions.

The applications of position sensing
Accurate rotary and linear position sensors are found in both simple and complex applications. At its simplest, a rotary position sensor might measure the angular displacement of a rotary input device such as a knob or dial as part of a human-machine interface. Here, the requirement for sampling speed, resolution and accuracy typically is moderate, but reliability and ease of assembly are of high importance.

In contrast, extremely complex systems in fields such as robotics and motor control place severe demands on the speed, accuracy and precision of the position sensor.

Typically today, simpler applications tend to use a classic potentiometer as the position sensor, and more demanding applications will often use an optical encoder. When robustness and tolerance of harsh conditions are added to the list of requirements, however, both of these device types fall short. Potentiometers are prone to a variety of electrical and mechanical faults which can lead to early failure. Optical encoders as well as potentiometers are readily impaired by contaminants such as dust, dirt, grease and liquids. Neither device type maintains accurate measurement outputs under the influence of extreme shock or vibration.

Some protection from environmental contaminants may be gained by sealing the sensor, but the process is expensive and may compromise either the assembly or mechanical design of the host product. Moreover, any wear or degradation of the sealing tends to shorten the operating life of the sensor system.

Robust contactless sensing
Moving beyond optical encoders, magnetic position sensing offers another means of implementing contactless sensing. In the past, designers have shied away from magnetic position sensors in low-power applications because they assume they consume too much power. While not an unreasonable assumption given the mode of operation of magnetic position sensors, that thinking is outdated.

First, let's look at the principles of magnetic sensing to see why power is a matter of concern to users. The Hall Effect, discovered by Edwin Hall, explains how a magnetic field affects current flowing through a conductor. Simply stated, Hall Effect magnetic sensors work on the principle that a magnetic field (B) applied perpendicularly to a conductor affects current flow (I) through that conductor (figure 1). The rotation of a two-pole magnet will cause the strength of the magnetic field experienced by a conductor in proximity to it to change as the polarity swings from maximum north to maximum south and back again. This change in the magnetic field will proportionately affect the current flowing through the conductor. And this changing current develops a voltage (VH) across the conductor that can readily be measured.

Figure 1: The Hall effect describes how a magnetic field affects the flow of current through a conductor.

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