Output lag times are inherent in some position transducers because signal-processing take time. In high-speed applications, this can mean that the controls only know where components have been recently, not where they are right now. If absolute linear-positioning data is important to an application, the TDP family of transducers — from Transducers Direct — incorporates a hybrid technology that offers a reliable approach to position sensing data.

TDP-100 transducers are immune to stray magnetic fields, which make them well suited for arc and ultrasonic welding.

In the TDP-100 version — configured to mount inside cylinders — a contact slides inside a tubular track. The track consists of a high-resistivity plastic film bonded to a wire-wound element. The special conductive plastic film is wiped by the self-cleaning, precious-metal contact.

This unit is essentially a modern linear resistive transducer. It acts as a voltage divider. If supply is a 10-Vdc signal, the output will be between 0 and 10-Vdc. If supply is 12-Vdc, the output will be between 0 and 12-Vdc. Most users choose 10-Vdc because most controllers have a 0 to 10-Vdc input module. (A magnetostrictive transducer typically would need a 24-Vdc supply. Its integral circuit board calculates the position, then converts this value in a digital-to-analog converter to produce the widely accepted 0 to 10-Vdc output signal.)

No such calculations or conversions are necessary with the TDP-100, so it requires no electronic boards and produces real-time outputs. The design needs no regular maintenance or re-calibration of the control system. The transducer also is very easy to troubleshoot; all you need to do is check the supply voltage against the output voltage.

Keeping current
Most applications work fine with the 0- to 10-Vdc output. However, for an electrically noisy environment (or if the user requests a different output), Transducers Direct offers a signal-conditioning module that converts 0- to 10-Vdc into a 4- to 20-mA or ±5-Vdc output for valve controller cards. Two advantages of current signals are that they are more immune to noise and can handle longer cable runs.

TDP-100 linear displacement transducers — configured to mount inside gun-drilled piston rods — are available in lengths as long as 43 in. The picture shows the sensor inside a pneumatic cylinder. It also shows displays that interface with the cylinder in stand-alone applications where only local indication is required.

The hybrid technology provides infinite resolution and long life — because most of the current still flows in the wire, the conductive film’s carbon content is low, and the film is very hard. Linearity is 0.15%; the temperature coefficient of resistance is low (and predictable); and resistance stability under humid conditions is excellent. Most applications are repeatability dependent, so the TDP is well suited for cylinder- position feedback.

The TDP 100 can be supplied with a choice of in-cylinder mounting styles. The internal flange style is more suited to clevis style actuators; the pressure flange is suited to tie-rod style actuators. It fits through the cylinder end cap via an industry-standard ¾ - 16 threaded port.
Other versions of the same technology are designed with protective housings and IP-66 sealing for external-mounting applications, where gun-drilling the piston rod is not possible or feedback is needed from an existing cylinder.

Contact Transducers Direct, Cincinnati, at (513) 583-9491 or visit www.transducersdirect.com.