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issue: May 2009 APPLIANCE Magazine

Appliance Engineer - Electronics Report
Magnetless Sensing


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An ultrathin inductive position sensor that requires no magnet to operate offers robustness, EMI immunity, and a small footprint.

 

POSIC S.A. (www.posic.com), a manufacturer of position sensing products based in Neuchâtel, Switzerland, introduced a new type of inductive position sensor for measuring the speed and position of gears, disks, and linear bands. With an ultrathin profile and requiring no magnet to operate, the sensor offers a robust and compact alternative to optical or magnetic sensors typically used in home appliance and consumer electronics, says Sietse Wouters, marketing and sales manager.

“The performance of optical sensors is affected by contaminations like dust, particles, fluids, or condensation. The magnetic sensors do not have this problem, but they are sensitive to magnetic fields in the environment,” Wouters explains. “POSIC’s integrated microcoil inductive sensor has high tolerance for adverse environmental conditions such as water, oils, and metal filings, and is insensitive to magnetic fields.” This suits the sensor to applications in which harsh environments, electromagnetic pollution, and size are major concerns.

The small form factor comes from a technology that integrates a differential microcoil transformer onto a silicon chip with the associated electronics. “Typical inductive sensors have coils that are wire-wound or integrated on a multilayer PCB. To our knowledge, POSIC is the only company that has pushed the miniaturization down to the chip level,” says Wouters. “As you can imagine, all kinds of coupling and crosstalk may occur if this cointegration is not perfectly controlled. It required a lot of time and good engineering skills to get to the performance that we achieve today,” he tells APPLIANCE. With the silicon chip containing the sensor elements and the electronics thinner than 0.5 mm, the total profile of the sensor and target is generally between 2 and 3 mm or less.

Wouters says two sensing techniques are instrumental in enhancing the sensor’s robustness. “The differential measurement principle is applied, so any perturbation that acts on both sides of the differential coil-pair is eliminated from the differential signal,” he explains. “The principle of frequency modulation is also applied. The primary coil of the transformer structure generates an ac magnetic field with a frequency of 1 MHz. This field is coupled into the secondary coils and the coupling is amplitude-modulated by the target, for example, steel gear. The 1-MHz am-signal is synchronously demodulated, which means that any interference, such as a 5–10-kHz PWM signal to drive an electric motor, is filtered out.” Those factors make the sensor especially robust against the magnetic fields and electromagnetic interference, Wouters adds.

Another advantage of the sensor is its ease of adoption. “Many semiconductor position sensors are supplied in the form of an integrated circuit. The engineer has to design a PCB that holds the sensor into a mechanical environment,” Wouters says. “The POSIC sensor is ready for cable soldering and can be glued, screwed, or clipped directly into a mechanical application.” The firm also offers evaluation kits with its sensors, allowing an engineer to start off within a few minutes.

The sensor comes in standard as well as custom versions. “The sensor must be positioned with a certain accuracy into a 3-D mechanical system to accurately measure the position and speed of an object,” Wouters says. “Our experience is that no two mechanical systems are the same, and therefore it is impossible to define a ‘standard’ mechanical form that fits in all systems.” In order to overcome this problem, the firm offers a limited number of standard sensors and the possibility to adapt the mechanical interface.

As motors used in appliances and CE become smaller while delivering increased torque, Wouters says, the electromagnetic pollution will increase. Those applications will benefit from sensors that are insensitive to the magnetic fields created by their magnets and windings and the EMI created by brushes and PWM drive signals.

The sensors have an operating temperature range of –40° to 125°C and work with gears and disks made of ferromagnetic materials such as iron and nickel, as well as electrically conducting materials such as aluminum, copper, and brass.

POSIC integrates a differential microcoil transformer onto a silicon chip together with the associated electronics, making the sensor both compact and robust.

 

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