TechArchive

Original article date: March 1999

While the trend towards digitalisation in automation continues there remain many applications for which analogue signals are still required – or preferred. ROY MOFFATT Managing Director of Variohm Components sets out the ground rules.

Most people associate the word potentiometer with the low cost unreliable noisy units used on radio and TV receivers. But recent developments have resulted in conductive plastic units which offer long life reliability and repeat accuracy. Provided that a few basic rules are observed when applying this type of transducer they will provide both position and reliability even in harsh or hazardous environments over a long period of time at a surprisingly low cost.

A typical example is for position measurement in position control loops of the type used in plastic injection moulding machines hydraulic presses machine tools automatic handling tools and sizing machines. But conductive plastic potentiometers have proved themselves reliable and cost-effective enough to be used by the automotive industry in engine management ride height control and drive-by-wire steering applications. However this is only possible if the units are installed and maintained correctly.

Conductive potentiometers are “absolute” transducers meaning that they provide an output proportional to their position. Even if power has been removed then restored no re-calibration or referencing is required. Their analogue signal output can be converted into an industry standard 4-20mA current signal or it can be digitised for processing in a PC or PLC. Conductive plastic potentiometers work in real time with no phase delays even at high speed.

There are only two rules which must be remembered:

• They must always be used as three-terminal voltage dividers never as variable resistors. When used in this way all the effects of temperature and humidity are cancelled out sot the resulting outputs are ultra-stable even though the working environment is changing.
• Wiper current must be kept to an absolute minimum – the wiper must be connected to an operational amplifier or other device with an equally high input impedance. By limiting the wiper current the effects of the wiper contact resistance are overcome and the full benefits of linearity resolution and repeatability will be obtained.

The temperature coefficient of conductive plastic transducers is very high – somewhere in the region of 220ppm. Simply to use series resistors for zero and span adjustment isn’t possible as differential temperature effects will occur which will destroy the stability of the system. It is far better to use some form of impedance matching circuit which incorporates zero and span adjustment or merely to connect the potentiometer directly across a stable power supply and scale it mathematically using a microprocessor.

Non-parallel movement is the biggest enemy of linear conductive plastic position transducers. Using spherical rod end bearings offsets of up to 30deg are permissible. Field results indicate that when used on applications in the manufacturing engineering and process industries provided they have been mounted correctly the linear units will work at speeds of up to 10m/s and still deliver linearity better than 0.05% and resolution of 0.01

Eccentricity which has a major influence on linearity is as harmful to rotary potentiometers as non-parallel movement is to linear potentiometers. The smaller the track length of the potentiometer in comparison to the offset the greater will be the error. A shaft offset of 0.1mm will produce a linearity error of 0.2% in a 50mm diameter unit. This is nearly three times greater than the unit’s inherent non-linearity. The best method of mounting rotary units is to use a rigid shaft coupling and allow the potentiometer body to move or float.

• Variohm Components
• Contact: Roy Moffatt
• Tel: 01296 770341

March 1999