LVDTs Go the Distance

Distance-measuring sensors range from precise laserinterferometers to simple potentiometers, but for linear distance measurementsin industrial equipment and testing apparatus, a linear variable differentialtransformer (LVDT) proves difficult to beat. This versatile sensor measuresdistances in valves, robotic actuators, dispensing equipment,dimensional-measuring instruments, production testers and so on.

LVDTs can measure distances on the order of fractions ofmillimeters up to several feet with a linearity of about A plus or minus 0.25 percent over thespecified measurement range and with a repeatability error of about 0.01percent of full-scale output (FSO). And unlike some sensors, an LVDT providesabsolute-position information. So, when you start equipment you know the core'sposition and don't have to return an actuator to a known reference point.

A typical LVDT relies on three coils wound coaxially butseparately around a hollow cylinder. A piece of highly permeable metal servesas a core that moves through the hollow cylinder (see figure). External electronicspass an ac sine-wave signal through the center, or primary, coil. The moveablecore magnetically couples to the two outside sensing, or secondary, coils. Theposition of the core determines how much of the ac signal couples into eachsecondary coil. Connecting these two coils in series but with opposite polaritycreates an ac output that varies in amplitude as the core position changes. Thephase of the ac output signal - often around 3 kHz - shifts by about 180degrees as the core moves through a "null point" at which the output drops to aminimum.

LVDT manufacturers produce cores threadedat each end for attachment of a mechanical link. Although an LVDT offers goodlinearity, if a core moves beyond its specified limit, the output begins toflatten, so response looks like a "bow tie." Engineers might use a look-uptable or a linearization algorithm to obtain results in this extended region,but in my opinion you're better off finding a more suitable LVDT.

Equipment designers can buy LVDT signal conditioners thatcreate a dc voltage proportional to the position of an LVDT core. Or, they candesign their own circuit around an IC such as the AD598 from Analog Devices.John Matlack, global business-development manager at Macro Sensors, an LVDTsupplier, says if someone needs fewer than 10 or so sensors, they usually buyoff-the-shelf electronics or LVDTs with internal electronics. If they use moreLVDTs, they often design their own signal-conditioning circuits.

The use of internal electronics limits the temperature rangean LVDT can experience to the range tolerated by the electronic components,about -40 to 85C. And according to Matlack, if an LVDT fails, it's usually theelectronics that have died, not the sensor.