DE3522775A1 - Device for determining the contact resistance on the slider of a potentiometer - Google Patents

Abstract

This potentiometer is connected, on the one hand, to a supply voltage source, and, on the other hand, to earth, and its slider is connected to an evaluating unit via an electrical line. A time-dependent test signal is coupled into the line via a test resistor and the contact resistance is determined from the voltages then present at the evaluating unit and from the individual resistances.

Description

The invention relates to a device for determination the contact resistance on the wiper of a potentiometer according to the preamble of claim 1.

When recording mechanical quantities such as paths and angles of rotation etc., in devices such as a motor vehicle, such as injection system, electronic accelerator pedal, Fuel sender etc., the use of a potentiometer is often a technically satisfactory and inexpensive solution. According to conventional technology, there is one used here Voltage divider from a series connection of a first Resistor, the potentiometer and a second resistor, the first resistor being connected to a supply voltage source and the second resistor with ground, whereas the Grinder of the potentiometer over an electrical line are connected to an evaluation unit; are in principle the first and second resistance not necessary to serve them however, cable breaks in the power supply and Ground connection and short circuits can be recognized.  

However, the measured value can be on the wiper of the potentiometer tapped voltage can be falsified - what special cause problems with safety-sensitive devices can - because the contact resistance on the grinder is age-dependent and contact disorders can occur.

The object of the invention is therefore a device to create with which the contact resistance on Grinder of a potentiometer can be determined.

This object is achieved with the characteristic Features of claim 1 solved. With the device can increase the contact resistance are recognized long before the measuring function fails, wherein according to a feature of a further claim then generates a signal from the device if the amount of the determined contact resistance exceeds a predetermined amount.

An embodiment of the invention is in the Drawing shown and is described in more detail below.

In a voltage divider 1, the resistance R _1, the potentiometer R _p, and the resistance R ₂ in a series circuit lying, wherein the resistance R ₁ with a source of supply voltage U _o and the resistance R ₂ is connected to ground. The potentiometer R _p has a wiper 2 , which is connected to an evaluation unit 4 via an electrical line 3 for measuring the divider voltage U _x on the voltage divider 1 between the wiper 2 and ground. The grinder 2 itself is connected via a mechanical operative connection 2.1 to a device whose mechanical size is to be detected. If R _{x is} the set resistance on the potentiometer R _p , the output voltage is (divider voltage)

However, if the contact resistance R _s on the potentiometer wiper 2 becomes large due to aging, the measured value is falsified depending on the input resistance of the evaluation unit. To determine the contact resistance R _s , the electrical line 3 is therefore also connected via a test resistor R _T to a test voltage source U _T , from which a time-dependent test signal U _T ( t ) is transmitted via the test resistor R _T - at least two different values with the difference Δ U _T = U _T ( t ₁ ) - U _T ( t ₂ ) are used - is coupled. Correspondingly, the voltages U _m ( t ) are then present at the input of the evaluation unit 4 , from which the difference Δ U _m = U _m ( t ₁ ) −U _m ( t ₂ ) is formed.

The contact resistance R _s is then approximately determined from these values using the following relationship: in which is, that is
Total resistance of the voltage divider resistors R ₁ + ( R _p - R _x ) and ( R ₂ + R _x ) connected in parallel between the grinder 2 and the AC voltage connected to ground.

Is determined in the design of the voltage divider R 1 ₁ = R ₂ = R _p / 2, then R _K varies between 0.75 R _p and R _p and preferably may be assumed to be approximately constant with 0,875 R _p.

Now exceeds the transition resistance detected R _s a predetermined depending on the requirement amount, for example, the 0.2 - to 1 times the amount of the potentiometer R _p (R 1 R _s ~0,2 ÷ _p), then by the evaluation unit 4 a signal generates, for example as an optical display, which signals that the measuring voltage U _m due to the contact resistance R _s on the grinder 2 no longer corresponds to the ideal divider voltage U _x , so that a falsified measured value is present at the evaluation unit 4 .

Although the device can work with both digital and analog circuitry, an embodiment is preferred in which an already used microprocessor takes over the function of the evaluation unit 4 (including the test signal generator), which has the essential advantage that only the test resistor R _T as an additional one Component is needed.

The function f ( t ) of the test signal is arbitrary and can be generated both via an oscillator circuit and digitally; nor does it need to be periodic. It is only essential that at least two different values are generated with a difference Δ U _T.

With analog circuit technology, the useful signal and test signal are separated by suitable filters. The error case ( R _S ≦ λτ R _S crit ) is when Δ U _{m is} the value exceeds. Approximately also applies here

Claims (5)

1.Device for determining the contact resistance ( R _s ) on the wiper of a potentiometer, in particular a potentiometer in a series circuit with further resistors as a voltage divider for detecting mechanical quantities, which is connected to a supply voltage source ( U _o ) on the one hand and to ground on the other and via whose grinder and an electrical line the divider voltage ( U _x ) is applied to the voltage divider between the grinder and ground at an evaluation unit, characterized in that the electrical line ( 3 ) is further connected via a test resistor ( R _T ) to a test voltage source ( U _T ) of which a time-dependent test signal U _T ( t ), of which at least two different values with the difference Δ U _{T are} used, for the instantaneous determination of the contact resistance ( R _s ) on the wiper ( 2 ) of the potentiometer ( R _p ) via the test resistor be coupled and from which then on the Auswe Voltage unit ( 4 ) applied voltages U _m ( t ), the difference Δ U _m and then the product of the quotient Δ U _m / Δ U _T and the amount of the test resistance ( R _T ), the product and the total resistance ( R _K ) of the difference formed between the grinder ( 2 ) and the AC voltage-connected parallel voltage divider resistors approximately determines the contact resistance ( R _s ). 2. Device according to claim 1, characterized in that the amount of the test resistor ( R ) is a multiple of the amount of the voltage divider resistors ( R _p ; R ₁ , R ₂ ), preferably 10 times. 3. Apparatus according to claim 1, characterized in that the amounts of the voltage divider resistors ( R ₁ , R ₂ ) have the same order of magnitude as the amount of the pontiometer ( R _p ), preferably 0.5 times. 4. The device according to claim 1, characterized in that a signal is generated by the device when the amount of the determined contact resistance ( R _s ) a predetermined amount, preferably 0.2 to 1 times the amount of the potentiometer ( R _p ) , exceeds. 5. The device according to claim 1, characterized in that the evaluation unit ( 4 ) is a microprocessor ( 4.1 ), of which an output via the test resistor ( R _T ) with the electrical line ( 3 ) for coupling the test signal ( U _T ( t )) connected is.