There are a number of different options for precisely measuring linear distances or movements. Depending on the requirements concerning the measuring range and accuracy, measuring tapes, calipers or micrometer screws are used, for instance. All of these measurements are based on comparison with a measuring tool which is used as a reference. The most well-known reference for length measurements is the original metre standard in Paris. This was used to compare and calibrate all metric length measuring systems for almost 2 centuries. In the course of the international harmonisation of dimensions and weights, the INCH is now also based on the metre.
Of course, only measuring systems with an electronic interface are used for automated measurement. These are also based on a reference as a material measure. The range of electronic measuring system designs is also very broad, as there are a wide variety of measuring ranges, required accuracies and necessary interfaces in this case as well. The intention here is to introduce a new electronic measuring system and compare it against familiar systems.
The simplest and most well-known electrical length measuring system is the potentiometer. It consists of a resistance track which is scanned by a slider. This resistance track consists of a wire coil or, in the case of more modern potentiometers, of conductive plastic. The conductive plastic track can be reworked with laser trimming to increase its accuracy. In this laser trimming process, the conductive plastic track is compared with a high-precision reference encoder and trimmed by a laser beam.
A voltage UB is applied to the resistance track. This voltage is distributed evenly over the entire length of the resistance track. This results in a voltage between 0 V and UB on the slider depending on its position. The advantage of this measuring system is the simplicity of its design. It can be produced relatively inexpensively. The disadvantage is resistance track wear caused by the slider. Modern potentiometers enable 50 -100 million slider cycles. The resistance track is then worn to such an extent that precise measurement is no longer possible. The number of permissible slider cycles appears to be high - but this is only true at first glance. It is irrelevant to the wear whether a measurement is genuinely carried out or whether vibrations are the cause of the slider's movements. A system that vibrates at 3 Hz has generated 1/2 million slider cycles after 24 h in continuous operation. The potentiometer is then unusable after less than one year.
In a double coil, a plunger core made of nickel iron (shown in green) causes a change in the inductivities. The coil has a centre tap and therefore consists of two parts. The nickel iron core is located centrally in the coil, with the result that displacement of the plunger changes the inductivity of both coil parts in the opposite direction. A downstream electronic system measures the alternating current resistance of both coil halves and generates an output signal corresponding to the displacement of the plunger. The coil with the centre tap operates according to the principle of a so-called half-bridge. The half-bridge principle is very widespread in measurement technology. It provides reliable results over a wide range of applications and enables the compensation of external influences such as e.g. temperature changes and ageing effects. A microprocessor in the electronics permits various adaptations. The characteristic curve can be linearised or adapted to special customer requirements, for instance. The output transformer can convert the signal into a transmission protocol or simply amplify it.
The sensor system operates contactlessly internally. The plunger rod runs in the housing; no contact is required. Lateral forces can lead to plunger contact but do not cause wear that affects the measurement. The measuring principle enables the sensor to be cast in order to improve its resistance to vibrations and shocks. Planar coils are printed coils. They are applied onto a printed circuit board as a conductor track with several windings. This replaces a wound copper wire coil. Planar coils have lower inductivity than wound coils. They can be manufactured inexpensively.
The linear transducer registers the absolute position of a plunger contactlessly and without wear using an inductive resonator. The system consists of an excitation coil which excites a resonator circuit attached to the moving plunger to generate electrical vibrations. These vibrations are transferred to the printed coils. The printed coils are immovably fixed in the housing. Internal electronics evaluate the vibrations according to amplitude and phase position. As the phase position and amplitude of the vibrations in the printed coils change according to the distance from the resonator coil, the position of the resonator coil can be determined precisely and converted into a signal proportional to the distance. The planar coil measuring system is similar to the good old potentiometer and the inductive system in equal measure.
Due to the good reproducibility of printed coils, the precision of measuring systems with planar coils comes close to that of high-precision potentiometers. Their freedom from wear results in a very long service life as in the case of inductive systems. Measuring systems with planar coils therefore fill the gap between potentiometers and inductive position measuring devices. Incidentally, this also applies to manufacturing costs.
