Good positioning

Mercedes-Benz has a well-deserved reputation as a luxury brand with a ride as smooth as silk. Interestingly, one of the secrets behind the superlative handling of Mercedes-Benz vehicles is a linear position sensor that costs well under $50.

The industry's first ultra low-cost magnetostrictive position sensor has been providing suspension feedback on the struts of production Mercedes for the last four years. Relatively few engineers know about it, however, because it is strictly a high-volume automotive product. But the core technologies and processes developed for the Mercedes device have paved the way for inexpensive position sensors targeting the industrial masses.

As a quick review, a magnetostrictive position sensor consists of a permanent magnet that slides over a waveguide, a protective housing, and combo sensing/output conditioning electronics. The electronics produces a current or interrogation pulse that's applied to the waveguide. The pulse generates an electromagnetic field that interacts with the field generated by the permanent magnet. The interaction of the two fields produces a strain pulse, which travels at sonic speed along the waveguide until it is detected at the head of the sensor. Electronics determines the position of the magnet with high precision by measuring the time elapsed between the application of the interrogation pulse and the arrival of the resulting strain pulse.

Manufacturers have augmented basic magnetostrictive sensor functions over the years with extra capabilities. For example, inclusion of device-level intelligence lets these sensors report numerous details back to supervisory controllers. Of course, more sophisticated operations carry an extra cost. Though there has been a trend towards distributed intelligent systems, there are still many applications where a bare-bones but reliable sensor would do.

One such area is packaging machines where designs have traditionally used multiple proximity or limit switches for feedback. Newer machines often employ servos rather than bang-bang pneumatic controls, making linear or curvilinear position feedback more desirable. Inexpensive position sensors like linear potentiometers are a possibility for this role, but they have a severely limited life.

Such applications may benefit from work done with automotive manufacturers toward inexpensive position-feedback devices. These are what might be called fourth-generation sensors. They are magnetostrictive devices priced under $50 and produced at a rate of up to 250,000/yr. Since their introduction in 1998, none of the 750,000+ sensors in automotive use have failed.

Spin-offs of sensors for suspension, steering, and similar areas are finding their way into medical, mobile hydraulic, and other new markets. Low cost position sensors targeting automation and off-highway uses have come out of these efforts as well.

Technological beginnings

Magnetostrictive position sensors have evolved in two distinct directions. The first is toward more capable smart sensors in smaller packages. The second is toward inexpensive sensors designed for specific applications or industries.

It is interesting to see how magnetostrictive position sensors reached their present state of development. The underlying (and often conflicting) themes include modularity, better accuracy and response, more functions packed into smaller packages, and of course emphasis on lower prices.

First-generation magnetostrictive sensors were known for being reliable and precise with a stroke range far exceeding that of traditionally used LVDTs. Introduced in 1975, they were designed specifically for hydraulic cylinders up to 300 in. long with static pressures to 5,000 psi. The initial design, called the Temposonics I, gave designers a means to produce true servohydraulic performance with longer-stroke hydraulic cylinders. This first design was distinguished by an external driver/conditioner electronics box of substantial size which was necessary to interrogate the sensor and generate the desired output signal.

The Temposonics II came along in 1988. It displayed slightly better nonlinearity than the T1, but its main claim to fame was in shrinking the performance of the earlier device into a more integrated, compact design. The smaller packaging came from use of surface-mount electronics and custom ASICs designed specifically for magnetostrictive position sensing. This compact sensor could fit in numerous uses where space limitations were an issue.

The Temposonics II also was unique in that different "personality modules" let it produce the desired output signals. For example, three different modules produced voltage, digital TTL timing pulses (start/stop), and pulse-width-modulation outputs. The personality module fastened by two screws so it was easy to swap out to get a different kind of output.

The success of the T2 led in 1996 to a new modular design. (It also attracted competition after the original patents expired in 1990.) The T3 or R Series is comprised of four distinct elements that define all possible configurations for a magnetostrictive sensor. Each sensor consists of a core sensing element, a particular style of application housing, an electronics module, and an interconnection assembly for hooking up to the outside world. It can resolve distances down to 0.00008 in. (2 microns) thanks to a 4-GHz clock, a factor of 50 better than earlier devices. Better waveguide homogeneity and other tweaks have also reduced nonlinearity from 0.02 to 0.01% of full stroke.

As with all magnetostrictive position sensors, an external position magnet mounts to the moving member. The application housing provides mechanical guidance for the position magnet as well as protection for the sensing element and electronics module. The interconnect assembly also protects against reverse polarity and overvoltage due to miswiring.

The modular approach makes it possible to optimize sensors for different applications. For example, packaging has expanded beyond the original hydraulic (rod) housing style to include profile (extrusion), detached electronics, and flex. Extrusion style housings double as a travel surface for the movable magnet. The magnet attaches to the positioning device via a linkage. Housings with detached electronics handle situations where there is no room for an electronics module on the head of the hydraulic cylinder, or where the sensor itself must operate at temperatures that would cook circuitry. The flex style has a sensing element that bends. It is often the choice for sensing over superlong distances (exceeding 200 in.) to avoid the need for equally long shipping containers.

4G sensors

Fourth-generation magnetostrictive sensors are a radical departure from the first three generations in that they are developed to meet specific price, performance, and reliability constraints of a particular industry. Two examples are the MH and EP2 sensors. Both have resolution and nonlinearity specs comparable to standard industrial products. But their price is attractive because they only incorporate functions a specific market requires. A combination of automated production and a modular design approach makes this approach possible.

As an example, the MH sensor was designed for the mobile hydraulics market. Its extended temperature range and immunity to shock, vibration, and high levels of EMI are critical to this industry. On the other hand, by limiting the stroke range, power supply, output, and packaging options, it can sell at a price that makes this industry take notice.

The EP2 is similar, utilizing the same basic modular design platform but with a single power supply, output, and application housing style that easily adapts to external mounting on specialty applications. The only option is stroke length. Unlike standard industrial products that offer an unlimited number of lengths, the EP comes in just 14. Limiting these options keeps tooling costs down. And keeping options to a minimum reduces product complexity. For example, offering only a three-wire output option (power, signal, and ground) rather than the standard five or six wires cuts the chances of miswiring.

All in all, the evolution of magnetostrictive sensing is not unlike the computer industry. Every year brings more performance in a smaller package for less. Sensor manufacturers hope, for their sake, progress doesn't happen at the same rate!