## Resistive sensors potentiometers Updated by: Scarlett

On: 18 Jun, 2019

Viewed:357 times - 6 hour, 10 minute, 4 second ago

Category:

Rating:
3.2
48 out of 100 based on 417 user ratings

Sensors based on the variation of the electric resistance of an element are probably the most common. That is because there are many physical quantities able to affect the electric resistance of a material. Thus resistive sensors are used to solve...

Sensors based on the variation of the electric resistance of an element are probably the most common. That is because there are many physical quantities able to affect the electric resistance of a material. Thus resistive sensors are used to solve many measurement problems. In the case of temperature dependent resistors, they also offer a means for thermal compensation that can be applied to systems measuring other quantities.
In this chapter we describe the more common sensors based on a variation in resistance. we will describe their fundamentals, technology, equivalent electric circuits and applications. We will use sensors, models, and definitions given in Chapter I when dealing with some of the applications.
chapter 3 will describe the circuits to obtain a useful electric signal. The different resistive sensors will be classified by the physical quantity being measured as mechanical, thermal, magnetic, optical, and chemical variables.

POTENTIOMETERS

A potentiometer is a resistive device with a linear or rotary sliding contact (Figure 2.1). The resistance between that contact and the bottom terminal is
given by  where x is the distance traveled from the top terminal and a is the corresponding length fraction. From a dynamic point of view, it is in principle a zero-order system, although it can be itself a component of a nonzero-order sensor, for example, a mass-spring system.
The behavior described by (2.1), means that the resistance is proportional to the travel of the wiper. This implies the acceptance of several simplifications that are worth explaining because they can not always be taken for granted.

First, we assume that the resistance is uniform along the length /. But the resistance is not perfectly uniform, which limits the linearity of the potentiometer. Second, we assume that the sliding contact gives a smooth resistance variation, not a stepped one, and therefore that the resolution is infinite. But that is not true for all resistive elements.

For the model described by (2.1) to be valid, if the potentiometer is supplied by an alternating voltage, its inductance and capacitance should be insignificant. For low values of R., the inductance may be significant, particularly in models with wound resistive elements. For high values of Rn, the
parasitic capacitance may be important.

Another factor to be taken into account is that resistors drift with temperature. Therefore the model is valid only if the temperature does not change.
Temperature changes can be produced not only by fluctuations in ambient temperatirre but also by self-heating due to the finite power that the potenti ometer dissipates. The rms value of the voltage V must be where P is the power
If the measr."-"n, circuit has a low input impedance, it will load the potentiometer, and may cause excessive heating in part of the potentiometer
(see Problem 2.1).
Another factor limiting the validity of the model is the friction and inertia of the wiper. These should be insignificant but at the same time ensure a good contact. As a compromise the force required to displace the wiper is from 4 to 15 g. For variable movements the starting torque is approximately twice the dynamic torque, and this is reduced by lubrication. For rapid movements there is a risk of losing contact during vibrations. Thus some units have two wipers of different arm lengths, and therefore different resonant frequencies.

Finally, if the desired resolution is high, then the noise associated with the contact resistance must be considered. Its value can increase because of dust, humidity, oxidation, and wear. When contact resistance changes with movements from one position to another, current circulating through it produces changes in the output voltage. These fluctuations may be significant for the attached measuring device.

Most of these limitations are outweighed by the advantages of this device. It is simple and robust and yields a high accuracy relative to its cost. Models available accept linear and rotary movements (one or more turns in helical units). In some models the output is deliberately nonlinear with respect to the displacement . In other models the output is a sinusoidal function of the angle turned by the sliding contact. If, for example, a wire is wound or a conductor is deposited on a triangular mandrel (Figure 2.2), the resistance between the sliding contact and the left end is where / is the length of the mandrel, r is the distance to the right end, A is the wire's cross section, D its diameter, and p its resistivity. A nonlinear relationship can also be obtained by using a nonuniform spacing for the wire or by varying its size along its length. When the measuring circuit loads the potentiometer a nonlinear characteristic is also obtained (Section 3.2.1). A computation method to generate a resistor geometry with a prescribed potential drop along the wiper path is described in .

To obtain a useful device, a single wire is not used. Even if the wire were very thin (while retaining enough strength), it would be impossible to obtain
a high enough resistance value. The usual configuration has beqr a wire around a (ceramic) insulating form. Some of the materials used are nickel-chrome, nickel-copper, and precious metal alloys. But then the inductance is high and the resolution low. Advantages are a low temperature coefflcient
and a high power dissipation capability.

High resolution and long life at a moderate cost are obtained with potentiometers based on a carbon film, sometimes mixed with plastic and deposited  on a form. Their temperature coefficient is high. For high power dissipation and high resolution, there are models in which the resistive element is based
on particles of precious metals fused in a ceramic base. Table 2.1 gives some specifications of commercially available models. The Thevenin equivaldnt circuit for a potentiometer shows that its output impedance depends on the wiper position. If a direct voltage is supplied, the output resistance Ro is the parallel combination of R.(1 - a) and Rna, Potentiometers find application in the measurement of linear or rotary displacements exceeding full-scale values of I cm or 10o. Displacements of
this magnitude can be found in position feedback systems and also in some sensors-for example, in pressure sensors based on Bourdon tubes, bellows, or capsules (Section 1.7.3), Figure 2.3, and in float-based level sensors.

## Shunt Calibration of a Strain Gauge Sensor Definition - Shunt calibration is the known, electrical unbalancing of a strain gage bridge, by means of a fixed resistor that is placed, or “shunted”, across a leg of ...

## Sensor Bridge Calibration Equation (3.31) shows that the sensitivity of a sensor bridge depends on the supply voltage V, on the sensor's resistance under no load condit-:ns Rs,
and on the ratio k between the resistance...

## Analog linearization of resistance temperature detectors Resistance temperature detectors (RTDs) are commonly used in industrial and scien-tific temperature measurements. The most common types are pure platinum (Pt) formed into wire or evaporated in a th...

## LINEARIZATION OF WHEATSTONE-BRIDGE Wheatstone bridge circuits have been in the field for a very long time and still are among the first choices for front-end sensors. Whether the bridges are symmetric or asymmetric, balanced or unba...

## Analog Linearization of Resistive Sensor Bridges To obtain a voltage proportional to any size changes in one ofthe resistances in a Wheatstone bridge, we can either modify the structure of the bridge or
perform analog processing of the outpu...

## Sensitivity and Linearity of wheatstone bridge A common way of obtaining an electric signal in a Wheatstone-bridge-based measurement is with the deflection method. Instead of measuring the action
needed to restore balance in the bridge, th...

#### Get daily update

As the fastest growing demand of circuit and wiring diagram for automotive and electronics on internet based on different uses such as electronic hobbyists, students, technicians and engineers than we decided to provide free circuit and wiring diagram base on your needed.

To find circuit and wiring diagram now a day its easy. E-learning through internet as a right place to search an exact circuit and wiring diagram of your choice and it's much fun and knowledgable. On internet you will find thousands of electronic circuit diagrams some are very good designed and some are not. So you have to modify them to make them according to your needs but some circuits are ready to make and require no changes.

There are many categories of circuit and wiring diagrams like automotive, audio circuits, radio & RF circuits, power supply circuits, light circuits, telephone circuits, timer circuits, battery charger circuits etc. There are many types of circuit and wiring diagrams some are very easy to build and some are very complicated, some are so small and some contain huge list of parts.

We provides free best quality and good designed schematic diagrams our diagrams are free to use for all electronic hobbyists, students, technicians and engineers. We also provides a full educational system to students new to electronics. If you are new to electronics you are a student or a electronic hobbyist and want to increase your knowledge in electronics or want to understand electronics in a very easy way so this is the right place for you we provide electronics beginner guide tutorials to easily understand complicated electronic theory. Our mission is to help students and professionals in their field.