Non-contacting potentiometer opens new perspectives
Monday, 05 December, 2005
While the degree of precision and reproduction accuracy are unable to match those of costly digital systems, there are many applications that do not require this performance level.
In fact the reduction in useful life caused by mechanical wear and accompanying symptoms such as poor contacting and long-term drift create additional system inaccuracies.
Using ASICs in a cost-effective design a solution to this dilemma is in sight.
The daily routine on the modern factory floor is characterised by ever increasing speeds and cycle rates.
The development towards high speed is turning wearing parts into an increasing burden. The potentiometer enjoys widespread use as a cost-effective method of absolute position transducering.
One indication of the popularity of this method is the fact that users are even willing to accept the need for regular exchange after only a few months of use due to wear of the mechanical components.
Cycle times of under one second - in CD production, for example - are a common feature in modern manufacturing processes.
This makes the search for alternatives to the tried and tested mechanical potentiometer a priority issue.
The dream of a 'non-contacting' potentiometer is not a new one: there have been many attempts in the past to develop a system combining the positive characteristics of the potentiometer such as high resolution, good linearity, a wide temperature range and good temperature and humidity coefficients while eliminating its drawbacks.
Experiments using magnetoresistive, inductive, Hall, magnetostrictive and other sensor technologies in a bid to replace the potentiometer have so far failed to come to grips with the excessive secondary input involved or were not feasible for reasons of cost.
Figure 1: Alternating field over the voltage divider.
Novotechnik has now come up with a promising new development in this field: a resistance track is supplied with an alternating voltage (Figure 1). A moving probe capacitatively picks off a displacement current whose level depends on the respective position of the probe in the direction of displacement.
To avoid the need to trail a contact cable, the displacement current is transmitted to a collector track (Figure 2) running parallel to the resistance track.
The displacement current is not evaluated until transmitted to the collector track.
Figure 3 indicates the equivalent circuit diagram for this configuration. The capacity values Cm and Ck are formed by the surface of the probe and the surface of the resistance/collector track below it.
The stray capacitance of the probe against ground is taken into account with Csm.
A downstream integrator with an input voltage of 0 cumulates the displacement currents, whereby the input resistance Riv and the input capacity CSV are negligible.
The charge transfers cumulated within a defined time frame now form a measure for the position of the probe.
For the output voltage Ua, the following formula applies depending on the path x or the rotary angle:
Fp is the potentiometer function which depends on the position of the probe, and Up is the applied voltage at the probe.
Earlier solutions developed along the same lines failed to take into account the fact that the capacitances Cm, Ck and Csm can certainly not be considered constant values. The fact that these values are subject to change over the linear and angular range must be taken into account in the signal evaluation process.
It is only possible to offer an unambiguous statement for Ua if the entire right-hand fraction can be considered constant. The potentiometer function can then be clearly mapped on the output voltage, simplifying the relation to.
To hold the capacitive voltage divider at a constant level, Novotechnik uses the following procedure:
The potentiometer voltage Up is adapted by a control circuit in such a way that the overall expression remains constant.
In practical terms, this is implemented using one of two similar methods, either the 'reference pulse control' or the 'summation control' method.
Using the reference pulse control method, initially the operating point of the first amplifier stage is set.
The subsequent reference pulse phase is decisive in eliminating the variability of the stray and coupling capacitance levels:
An alternating voltage with adjustable amplitude is applied simultaneously at both ends of the resistance track. This produces a position-independent output signal which depends solely on the signal amplitude and the measuring probe coupling.
An integral calculator compares the result to a highly stable reference signal and adjusts the difference by means of the amplitude.
The determined amplitude can subsequently be used to perform one or more measurements. The output signal is processed in a downstream amplifier stage.
Figure 4: Block diagram, reference pulse control.
During the summation control process (Figure 4), two measurements are carried out, one with reverse polarity of the potentiometer.
As both partial paths together add up to the same overall path, the sum of the results must always be equal, provided the outcome is not influenced by error.
To eliminate the possibility of error, the values are added using a cumulating integral controller and compared to a constant reference signal. The sum of both signals is thus held constant through the control system at every possible measuring point.
Laboratory testing performed using this non-contacting potentiometer produced some very promising results.
A comparison of the linearity curve progression clearly indicates the direct relationship of the development to the classical potentiometer in terms of its macrostructure (screen printing technique) (Figure 5a).
In both cases, independent linearities of 0.1% are achieved. A positive aspect in the non-contacting potentiometer is the considerably finer microlinearity.
Figure 5a: Linearity progression of a non-trimmed potentiometer (wiper system).
This is due to the integration effect produced by the probe length (Figure 5b), which serves to substantially increase the resolution of this type of system. It is possible to show the same effect using relative gradient variation (RGV).
While RGV depicts the microstructure (RGV <= 1%), the macrostructure reflects the track linearity error. The RGV can be easily influenced by the length of the probe, and using a typical probe length of 5 mm is some 20 times lower than when using the wiper system.
Values on a comparable scale can also be achieved for the temperature and moisture coefficient, provided the drift of the electronic evaluating circuit is restricted by means of temperature compensation.
The mechanical components such as the guide and bearing do not differ from those used in conventional products. This means that from the process engineering point of view, the company was able to make use of tried and tested components to the benefit of system reliability.
The demands made on the quality of the resistance track are minimal in view of the elimination of any mechanical stress effects.
As the system allows for complex electronic solutions to be provided in the form of less costly user-specific components, (ASICs), the costs involved should not differ considerably from those involved in conventional systems.
Facility for servicing and diagnostic functions, as well as standardised current and voltage interfaces (0)-20 mA, 0-10 V) are ready-integrated in the electronic circuitry. Plausibility studies are simple to execute using these circuit variations.
The opportunities for application of a system enjoying the benefits of the potentiometer but without its restricted service life and contacting drawbacks are practically unlimited.
Because resistance and collector tracks can be enclosed, oil or water-related applications are conceivable, for example inside hydraulic or pneumatic cylinders (smart cylinders).
In many cases, they can even replace markedly more costly LVDTs which are frequently prey to temperature sensitivity problems.
The use of non-contacting potentiometers generally makes sense in applications calling for transducers with a long service life, even under adverse operating conditions.
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