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Low-power position sensing in extreme environments

03 Jul 2015  | Mark Hoferitza

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In magnetic position sensors (MPOS), this voltage signal may be converted mathematically into a measurement of angular or linear displacement. But for the sensor to work, a current must flow through the conductor to activate the Hall element. Because a Hall sensor draws current in this way, it typically uses more power than other technologies such as potentiometers and optical encoders. Modern implementations of Hall sensing use silicon to develop the voltage, and include signal-processing and computation functions in a single integrated circuit. But a current on which the magnetic field can impinge is an inherent requirement of the circuit.

Low-power variants of the MPOS
The MPOS, however, has an advantage over the potentiometer that helps to mitigate this power problem. As a passive electro-mechanical device, the potentiometer offers no means for the design engineer to configure its power profile. It is always on, always drawing the same current.

An MPOS, by contrast, is a smart device with in-built control and configuration circuitry, and this allows for the implementation of power profiles.

The most dramatic way for an MPOS to conserve power is power cycling, which offers the benefit of reducing power consumption to 0W when in Stop mode. It does, though, require more current at power-up than in steady-state operation, and also needs settling time at power-up. The resulting latency between the time a movement began and the time the MPOS supplies a measurement of displacement might be unacceptable in some applications.

Low-power modes provide an alternative means of reducing power use. This involves powering down non-critical elements in the sensor and only energizing them when a measurement is needed. The more circuit elements are powered down, the longer it takes to capture a sample. In other words, the lower the power usage, the higher the sampling time.

To help the designer achieve the right balance between speed and power in any given application, ams recently introduced a series of rotary MPOS, the AS5050A and AS5055A which offers the user a choice of deep and shallow power-down modes. Two low-power states and a Normal (always on) mode each support a different sampling rate. This allows the designer to select a sampling rate to suit the overall system power requirement.

In deep power-down, the sensor draws just 3µA (9.9µW maximum power consumption). The other low-power mode keeps the Power-On Reset circuit activated; in this mode, the IC draws 33µA (108.9µW maximum power consumption). In Normal mode, maximum power consumption is 28mW.

The time required for the device to power up is rated at 580µs, and the latency between capturing a sample and producing an Interrupt output to a microcontroller is 500µs.

The very low current requirement in these low-power modes makes it feasible, for the first time, for harsh and remote applications to use a Hall Effect sensor in a system with a tight power budget.

Practical example of hardening for an underwater application
It's clear that in theory, MPOS can work in harsh environments. But how reliably do they work in practice?

To answer this question, the author devised a simple underwater motor-drive application using a standard MPOS from ams. This device, the AS5055A, produces a digital output which, while not readily seen on an oscilloscope, can easily be displayed in a graphical user interface (GUI).

Figure 2: Conformally coated AS5055A position sensor adapter board.

The MPOS was mounted on a standard adapter board from ams, with its power and output pins connected to wires. The entire board was then coated with an off-the-shelf rubberized coating, of a type available in DIY stores (figure 2). The board was immersed in the coating compound and left to cure. After curing for a few hours, the conformally coated board was now ready for testing. A simple DC motor was then attached to the fixture in such a way that a 6mm diametric magnet attached to the end of the rotor was centred over the adapter board's MPOS chip (figure 3).

Figure 3: A magnet mounted on the rotor is centred over the AS5055A magnetic position sensor.

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