One-coil displacement sensor

Conventional inductive displacement sensors usually consist of a ferro-magnetic plunger which moves between two coils wired with two resistors to form a Wheatstone bridge circuit. As the plunger moves from the centerpoint, coil impedance changes, forcing a corresponding change in voltage.

Advantage: The output voltage has a linear relationship to the distance moved and, assuming temperature changes affect both coils equally, the configuration provides temperature stability. Disadvantage: Producing two identical coils is expensive, while the in-line configuration results in a long sensor package.

A new design from Hottinger Baldwin Messtechnik (HBM), makes good with just one specially wound coil. To cancel out eddy current effects as the plunger moves into the coil--the main cause of non-linearity--the coil features a non-linear distribution of measurement windings. The plunger, therefore, meets an increasing concentration of windings as it moves into the coil.

A temperature-compensating resistive winding, wound with the main measurement winding, replaces the second coil in the bridge circuit. To eliminate parasitic inductances affecting linearity, a bifilar winding configuration is used. Since current in the second half of the resistor winding retraces the path it took in the first half, the inductive effect is exactly canceled.

The result: A shorter, more compact displacement sensor. Rainer Schicker, product manager at HBM, says, "hydraulics suppliers are demanding more compact models for mounting inside cylinders," and he gives an example:

A conventional displacement sensor with a range of plus or minus 250 mm measures about 1m in length. HBM's WA500 single-coil sensor has the same range of 500 mm, but is only 600 mm in length. Despite its different configuration, the design is electrically interchangeable with conventional models.

F. Rene Mitchell, Marketing Manager,
Spectris Technologies Inc., HBM Div.,
4021 La Colina Rd., Santa Barbara, CA 93110;
(805) 898-1099

One-chip Hall sensors

Hall-effect technology, widely used in sensors, is based on a voltage that is generated in one direction when a current and a magnetic field pass through semiconductor material in the other two perpendicular directions. A separate circuit, which may be on another chip or formed by other components on a circuit board, normally amplifies the output signal.

Austria Mikro Systeme (AMS) has now succeeded in integrating the Hall sensor onto an application specific integrated circuit (ASIC), so no further circuitry is necessary to obtain a digital output signal. Paul Paddan, AMS design & applications manager in the UK, regards this stage as a next logical step. "Putting the Hall sensor onto a CMOS chip, together with the signal processing needed, opens up a whole new range of applications for the Hall sensor."

One such application is fuel-level measurement in a car petrol tank, where a magnet mounted on a float-arm transfers the level signal to an encapsulated circuit. Conventional Hall sensors with their separate signal conditioning circuit were regarded as too ex-pensive for this high-volume market. Instead, cur-rent designs use magneto-resistive sensors. These, however, require extra circuitry for offset and calibration. Manufacturers also find good reproducibility difficult to maintain. In contrast, the new integrated Hall sensors are autocalibrating, require less cabling, and are inexpensive.

In addition, the on-chip sensor array can be designed to suit the application. AMS is building up a library of magnetic functions which can be implemented as required in a modular manner similar to that employed for other CMOS functions.

Melexis Microelectronic Integrated Systems also fabricates Hall sensors as an integrated circuit using silicon CMOS technology; circuitry on board the chip enables programming through an input lead.

One of the pins on the four-pin device doubles as both an output and an input which accepts programming pulses. The pin goes from the output mode to the programming mode when the supply voltage to the chip is raised from 5 to 13V. PC software controls the programming, downloading device parameters onto the chip.

Parameters programmed include the characteristic slope, offset, and also temperature compensation for the magnetic material used. For example, temperature compensation can be programmed in the range 100 to 1,000 ppm/degree C.

Once the values have been checked in the chip's RAM, they are "zapped" into the one-time programmable memory (OTPROM) by raising the sup-ply voltage from 13 to 18V. Chad Pepin, applications engineer at Melexis, highlights just one of the advantages of programming: "The properties of magnets may vary from one sample to the next. Programming matches the Hall sensor to the magnet to improve product reproducibility." He quotes scales for weighing as a typical application for matched Hall sensors.

Bob Monroe,
Austria Mikro Systeme Int'l.,
Suite 460, 20863 Stevens Creek Blvd., Cupertino, CA 95014;
TEL: (408) 865-1217;
FAX: (408) 865-1219

Melexis,
41 Locke Rd., Concord, NH 03301;
TEL: (603) 223-2362;
FAX: (603) 223-9614

Fingertip security

From keys and magnetic strips to voice and face recognition, restricting access to possessions, money, and information has preoccupied man throughout the ages. Siemens AG has now added its contribution to this field with the development of a semiconductor chip that recognizes fingerprint patterns.

Developed by a multi-disciplinary team at Siemens, the new chip consists of an array of more than 65,000 tiny sensor elements which measure the exact distance of the skin to the surface of the chip. The operating principle of the novel sensor is based on the variation in capacitance between the fingerprint ridges and the chip. The CMOS chip passes the fingerprint image as a digital data record to a processor which then extracts between one and two dozen characteristic points and compares them with the original stored data.

The sensor is highly integrated with on-chip digitized data being passed to a computer via a standard interface. Since it is manufactured in CMOS technology, the same technology that is used for memory chips, the price can be kept low, increasing the number of mass product applications. These include cell phones, smart cards, and car ignition keys.

Thomas Scheiter, who headed the development team at Siemens AG, looks to the future with image processing and data comparison being carried out in a signal processor on the card. "The implications are this would mean that all sensitive data could be stored on the card itself with only confirmation of the user's identity being transmitted to external devices."

Christofer Hierold,
Siemens AG, Corporate Technology,
Otto-Hahn--Ring 6 D-81739, Munich, Germany;
Tel: +49 89 636 46749;
Fax: +49 89 636 47069

Distance sensor

Just out from Sick Optic-Electronic is a new sensor that makes sensing objects at long distances possible without some of the drawbacks of triangulation systems or ultrasonic switches. A Class 1 infrared laser functions as the light source. Possible applications include edge positioning, detecting and counting vehicles, and determining the presence of bolts or cavities .

Known as the DS 60, this class of sensors uses the principle of light transit-time to sense the distance to the object in question. A pulsed laser beam is emitted. If it meets an object, it is reflected back and evaluated by the scanner's receiver. The time between transmission and reception of the pulse is directly proportional to the distance between the scanner and the object. The object's contour is determined from the sequence of received pulses.

A sensing range of up to 20m on reflective tape and 6m diffuse is possible. Almost any object in the sensing range, say developers, can be detected, even in the presence of a reflective background object. (Background suppression up to 100m against shiny objects is available.) Compared to typical triangulation systems, there is no specific direction of movement of the objects being scanned, and highly fluctuating remission does not affect the output.

David Lagerstrom,
Sick Optic-Electronic,
6900 West 110th St., Bloomington, MN 55438;
TEL: (612) 941-6780;
FAX: (612) 941-9287

One-piece pressure transducers

Microfused, micromachined, silicon strain-gauge technology gives this panel-mount pressure transducer the ability to function in the harshest of environments. Offered in pressure ranges up to 340 bar, the MSP-200 from Measurement Specialties Inc. completely eliminates the need for adapters, preventing leaks in applications associated with intrinsically safe control panels, explosive proof boxes, and ammonia-based refrigeration systems.

Measuring 25.4 x 34.3 mm, and weighing 120g, the MSP-200 provides 0-100 mV full scale output (FSO) using a supply of 5 Vdc. Accuracy (combined linearity, hysteresis, repeatability) is 0.5% FSO (maximum) with zero offset at plus or minus 3.0% FSO. The proprietary microfused strain-gauge technology provides a long-term stability of plus or minus 0.25% FSO while both zero and span thermal errors are less than plus or minus 2.0% FSO over the compensated temperature range of 0 to 70C.

For more traditional applications, Measurement Specialties' MSP400 incorporates the same microfused micromachined silicon strain gauge technology in a single piece of stainless steel to guarantee leak-proof operation. In fact, package design and electronic circuits eliminate RFI, EMI, and electrical transient problems.

Available in pressure ranges to 700 bar, the MSP400 offers: output signal levels of 0.5-4.5V dc (ratiometric), 1.0-5.0V dc (fixed), and 4 - 20 mA (two-wire); plus or minus 0.5% FSO accuracy maximum; plus or minus 0.25% FSO long-term stability; small size--6.35 mm long x 2.22 mm diameter; and 112g.

Mark Cappiello,
Measurement Specialties Inc.,
80 Little Falls Rd., Fairfield, NJ 07004;
TEL: (201) 808-1819;
FAX: (201) 808-1787

Magnets for sensors

Hall sensors can only operate in a magnetic field and this is usually provided by a permanent magnet. The German company WIDIA Magnettech-nik supplies plastic-bonded NdFeB (KOERDYM P) and hard ferrite magnets (KOEROX P) in a wide range of shapes and sizes and with various magnetic characteristics to meet needs of a multitude of applications.

Automatic processes are used to produce injection molded NdFB, hard ferrite, and resin-bonded neodymium in the shape required. The wide range of materials can also be designed for other characteristics such as temperature range. In many cases automotive designers need sensors with a temperature range from -40 to 200C, for example, for ABS and transmission systems, as well as other applications. This extended temperature range can be provided with suitable binders and well-adapted magnet powders.

In addition, hard ferrite and NdFeB powders can be mixed to produce so-called hybrid magnets that feature both high magnetic and thermal stability.

International Magnaproducts Inc. (IMI),
3103 Cascade Drive, Valparaiso, IN 46383;
TEL: (219) 465-1998;
FAX: (219) 462-5146,

Integrated sensors enhance machine performance

Mold-tool pressure sensor

The measurement of injection-mold tool cavity pressure is an exacting sensor application in terms of an operating environment. This type of pressure sensor uses the variation in capacitance produced when a diaphragm deflects under pressure. The diaphragm must be thin enough to give an adequate deflection for the pressure range to be measured, but robust enough to meet the wear and tear of an injection-molding application.

Previous designs of mold-tool sensors incorporated a metal cylinder between the sensing element and the thermoplastic medium in the mold tool. This method works fine for thermoplastics which solidify on making contact with the mold tool walls. A problem arises though with low-viscosity melts which penetrate and solidify in the mounting hole, restricting cylinder motion and sensor accuracy.

Now Kistler Instruments, with its model type 6167A sensor, has refined the diaphragm design using techniques such as finite element analysis. The diaphragm responds to forces along its axis and response is not impaired when the sensor is held in the transverse direction by solidified thermoplastic material. Siemens cooperated in trials of the new sensor, using it on machines producing molded integrated-circuit components for mobile and cordless telephones.

Richard Cadile, Kistler Instrument Corp.,
75 John Glenn Drive, Amherst, NY 14228-2171;
TEL: (716) 691-5100;
FAX: (716) 691-5226

Valve monitor

Sometimes electrical interlocks and safety equipment need an independent electrical signal to monitor the state of a hydraulic-actuator system. But because the actuator may be positioned at a point inhospitable to electrical sensors such as presses or injection-molding machinery, equipment designers are reluctant to mount sensors on the actuator itself.

One way around this problem is to fit sensors in the hydraulic valve controlling the actuator. This is a technique adopted by Vickers with its range of DG4V -3/5-HS7 and HS8 series of hydraulic actuators. Both single- and double-solenoid models are fitted with either one or two integral inductive proximity switches. This permits multi-position sensing rather than a simple indication of when the valve spool has reached a spring-returned position.

Vickers Product Manager Adrian Higginbottom feels that there are substantial advantages in this type of valve design: "Fitting the sensors remote from the exposed operating environment improves reliability and reduces maintenance. Also, fitting sensors directly to the valves keeps the wiring simple and reduces installation costs."

Vickers Inc.,
5445 Corporate Drive, Box 302, Troy, MI 48007;
TEL: (248) 641-4200
FAX: (248) 641-4927