Linear Potentiometer

9 Key excerpts on "Linear Potentiometer"

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  eBook - ePub

  Understanding Position Sensors

  • David Nyce(Author)
  • 2023(Publication Date)
  • CRC Press

    (Publisher)

  4 Resistive/Potentiometric Sensing

  DOI: 10.1201/9781003368991-4

  4.1 Resistive Position Sensors

  Resistive linear and angular position sensors are very popular, relatively inexpensive, and are also the most easily understood type of position sensor. They are normally three-wire devices, and are also called potentiometric position sensors, Linear Potentiometers, potentiometers, or pots. So here, the terms resistive position sensor and potentiometric position sensor are considered to be interchangeable. A linear or angular potentiometer can be called a position transducer, since it controls an electrical output directly from the physical variable input. But, since the sensing element also provides a usable electrical signal with no additional signal conditioning, it is also a position sensor.

  The basic concept is the same as that used in non-digital versions of the volume and tone controls on many radios and other electronic devices having an audio output. A voltage is applied across a resistive element, and an electrically conductive wiper slides along the resistive element, the wiper making electrical contact with the resistive element. This allows a voltage potential to be read from the wiper, with respect to one end of the resistive element. As the wiper voltage varies, it indicates the position of the wiper along the resistive element.

  In an audio circuit, the voltage across the resistive element of a potentiometer is usually AC (alternating current), having amplitude and frequency variations indicative of the audio signal. But in a resistive position sensor, the voltage applied across the resistive element is normally DC. The wiper voltage is approximately determined by the linear or angular distance of the wiper from one end of the resistive element, divided by the total length or arc of the element, and multiplied by the total voltage across the ends of the element. In electronics engineering terms, this kind of function is called a voltage divider, and is shown in Figure 4.1

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  Innovative Instrument Design and Applications

  • Lazo M. Manojlovic(Author)
  • 2019(Publication Date)
  • Arcler Press

    (Publisher)

  The potentiometer resistance is proportional to its length. In order to convert the actual position of a movable contact of the potentiometer into a corresponding electrical counterpart, a constant voltage is applied across the potentiometer fixed contacts by using an external voltage supply. The potentiometer output signal is the voltage between the movable contact, i.e., the wiper arm, which slides along the coil (or film), and the reference voltage terminal of the coil, as presented schematically in Figure 4.1. The potentiometer output voltage v o is proportional to the movable contact displacement x as: , (4.1) where k is the proportionality factor. Figure 4.1. Schematic diagram of a potentiometer. Analog Sensors and Transducers 71 The relationship between the output voltage of the potentiometer and the wiper arm displacement, given by the equation 4.1., didn’t take into con-sideration that the output terminals of the potentiometer can be loaded thus providing a zero output current. However, in the real-life situations, there is a load at the potentiometer output that has finite impedance . Typically, at the potentiometer output terminals, there is the circuitry into which the potentiometer signal is fed, such as conditioning, interfacing, processing, and control circuitry, as schematically presented in Figure 4.2. Due to the leakage of the output current, there is the drop of the output voltage to v’ o even if the reference voltage V R remains constant under load variations. The corresponding effect is known as the electrical loading of the transducer. Therefore, the linear relationship, given by equation (4.1,) cannot be valid anymore thus causing an error in the displacement measurement. Loading effect can influence the displacement measurement in two ways. Due to the finite internal resistance of the reference voltage source, the load can affect the reference voltage, i.e., by loading the voltage source.

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  Sensor Systems

  Fundamentals and Applications

  • Clarence W. de Silva(Author)
  • 2016(Publication Date)
  • CRC Press

    (Publisher)

  The incrementing can be programmed linearly, logarithmically, etc., using a microcontroller or other digital device, depending on the application. It is clear that a digital pot is not a displacement sensor but rather a resistance splitter or voltage splitter. It is mentioned here to avoid any misconception. The potentiometer has disadvantages such as loading problems (both mechanical and electrical), limited speed of operation, considerable time constants, wear, noise, and ther-mal effects. Many of these problems arise from the fact that it is a contact device where its slider has to be in intimate contact with the resistance element of the pot and also has to be an integral part of the moving object whose displacements need to be measured. Next, we consider several noncontact motion sensors that do not have these shortcomings. 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.1 1– x x L L + 1 + 0.1 +1 = 1 R c R c v o v ref R L R L Normalized displacement x / L 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1. 0 R c / R p = 0.1 R c / R L = 0.01 R c / R L = 0.1 R c / R L = 1.0 Normalized output voltage v o / v ref FIGURE 8.7 Behavior of the optical potentiometer for R c / R p = 0.1. 366 Sensor Systems 8.4 Variable-Inductance Transducers Motion transducers that employ the principle of electromagnetic induction are termed variable-inductance transducers . When the flux linkage (defined as magnetic flux density times the number of turns in the conductor) through an electrical conductor changes, a voltage in proportion to the rate of change of flux is induced in the conductor. This is the basis of electromagnetic induction . This voltage is called the “electromotive force” (emf), which in turn generates a magnetic field that opposes the original (primary) field. Hence, a mechanical force is necessary to sustain the change of flux linkage. The rate of change in magnetic flux that “induces” the voltage in the conductor can be caused in two principal ways: 1.

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  Sensors and Actuators

  Engineering System Instrumentation, Second Edition

  • Clarence W. de Silva(Author)
  • 2015(Publication Date)
  • CRC Press

    (Publisher)

  This is the voltage that the insulation between the resistance element and the outside (housing and shaft) of the pot can safely withstand (say, 2.5 kV). Other precautions include using a nonmetal (say, plastic) slider arm (for linear pot) or shaft (for rotary pot) and proper grounding. 5.3.2.2 Resolution A coil-type pot has a finite resolution. When a coil is used as the resistance element of a pot, the slider contact jumps from one turn to the next. Accordingly, the resolution of a coil-type potentiometer is determined by the number of turns in the coil. For a coil that has N turns, the resolution r , expressed as a percentage of the output range, is given by r N = 100 % (5.6) TABLE 5.2 Loading Nonlinearity Error in a Potentiometer Load Resistance Ratio R L /R C Loading Nonlinearity Error ( e ) at θ/θ max = 0.5 (%) 0.1 −71.4 1.0 −20 10.0 −2.4 311 Analog Sensors and Transducers Resolutions better (smaller) than 0.1% (i.e., 1000 turns) are available with coil potentiometers. Virtually infinitesimal (incorrectly termed infinite) resolutions are possible with today’s high-quality resistive film potentiometers, which use conductive plastics or cermet. Then, the resolution is limited by other factors such as mechanical limitations and SNR. Nevertheless, resolutions in the order of 0.01 mm are possible with good rectiLinear Potentiometers. In selecting a potentiometer for a specific application, several factors have to be considered. As noted earlier, they include element resistance, power consumption, loading, resolution, and size. 5.3.2.3 Sensitivity The sensitivity of a potentiometer represents the change ( Δ v o ) in the output signal associated with a given small change ( Δθ ) in the measurand (the displacement). The sensitivity is usually nondimensionalized, using the actual value of the output signal ( v o ) and the actual value of the displacement ( θ ).

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  eBook - PDF

  Introduction to Control System Performance Measurements

  The Commonwealth and International Library: Automatic Control Division

  • K. C. Garner, W. Fishwick(Authors)
  • 2014(Publication Date)
  • Pergamon

    (Publisher)

  However, should there be linearity errors present, these will certainly introduce signals at spurious frequencies (ref. 2.4), the significance of which will be determined by the type of measuring equipment being used with the potentiometer. RESOLUTION. AS the wiper moves, in a wire-wound potentio-meter, from one turn to another, the voltage varies in steps instead of continuously, because of the finite increment of resistance of each turn of wire. For a wiper that leaves one wire turn exactly at the instant that it connects with the next, the resolution is given 34 CONTROL SYSTEM PERFORMANCE MEASUREMENTS as the reciprocal of the total number of turns. If the wiper tem-porarily bridges two adjacent turns, the voltage increments are halved, so that the resolution doubles. This resolution is the ulti-mate limit on potentiometer accuracy, i.e. if there are a total of n turns the theoretical minimum error is given by 100/2« %. Film tracks are not perfectly continuous in general, and their resolution can be safely considered to be some fifty times better than the best wire-wound potentiometers. A great deal more can be said on the subject of potentiometers, but the discussion here is restricted only to those features which are concerned with the measurement of system response. The range of useful application, without the use of levers or gearing, can be taken to be between 0-5 in. to several feet as a rectiUnear displacement device, and from 5° to 5400° for angular displace-ment. Sliding friction between the wiper and the resistance track can be minimized to give very low resultant torques down to 0-003 oz in., which enables the potentiometer to become the output pick-off of a variety of other mechanical transducers as will be seen later. Potentiometers may be operated from both a.c. and d.c.

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  Linear Position Sensors

  Theory and Application

  • David S. Nyce(Author)
  • 2004(Publication Date)
  • Wiley-Interscience

    (Publisher)

  Capacitive transducers require a set of driving and conditioning electron- ics, and therefore are inherently more complex than a contact resistive trans- ducer. A typical capacitive linear position transducer comprises a variable capacitance sensing element, electronics, and suitable mechanical components to house them. The housing provides the means to maintain alignment of the movable elements while receiving the mechanical input of the measurand. Contrary to the capability of a Linear Potentiometer, a capacitive sensing element does not produce a directly usable output. The basic concept is that the electronic circuit drives the sensing element with an alternating current, the sensing element changes capacitance due to changes in the measurand, and the resulting signal is demodulated by the electronic circuit. In addition, the circuit conditions the power supply and converts the demodulated signal Linear Position Sensors: Theory and Application, by David S. Nyce ISBN 0-471-23326-9 Copyright © 2004 John Wiley & Sons, Inc. 62 CAPACITANCE 63 into the desired output. A housing is usually designed to contain all the components, including the sensing element, electronics, cable strain relief or connector, actuator assembly, and mounting features. 4.2 CAPACITANCE To understand and design capacitive sensors, it is helpful to be familiar with the nature of the electrical property of capacitance. The capacitance of a system is a measure of its capability to store electrostatic energy. An analogy can be drawn between an electrical circuit with capacitance and a water system with a storage tank. In this analogy, a tank with a large diameter (capacitance value) can hold a lot of water for a given height (voltage). Voltage acts like water pressure, and water pressure is often measured by its height (e.g., meters of water head, or pressure).

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  Instrumentation in Process Control

  • E. J. Wightman(Author)
  • 2017(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  analogue voltages. The ultimate choice of transducer is often finally made on the basis of the 'devil that is known'. Many vehement arguments have taken place over the choice of displacement transducer; exponents of the simple d.c. potentiometer will point to successful aeroplane autopilots that for years have relied on d.c. potentiometers, whereas the more industrially minded exponents of the present day solid state era will probably shudder at the thought of potentiometers and prefer inductive pick-offs or optical encoders which are basi-cally non-friction devices that with suitably designed associated electronics can result in less noise in the signal circuit. In recent years the development of miniature photo-transistors has stimulated the application of moire fringes to precision digital displacement measurement systems, particularly in the machine tool field. The operation of this form of transducer will be ex-amined in detail. Costs of displacement transducers vary widely. Undoubtedly the d.c. potentiometer provides the cheapest solution in the long run because although an inductive or photo-electric transducer can be made relatively cheaply, the low signal level obtainable from such devices requires amplification by drift-free electronics to bring the output up to a useful signal level and this attendant cost should be taken into account. 6.2 Potentiometric Displacement Transducers The potentiometer is one of the foremost components to be used in electrical engineering and its use in electronic equipment and control systems is still very much a way of life; consequently it is not proposed to dwell on this form of displacement transducer but to highlight typical characteristics and features obtainable. In general the wire-wound resistive element is still a preferred form of construction because the linearity can be controlled to 0.1%. Developments in metal film and conducting plastic tracks 8 - IPC

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  Resistive, Capacitive, Inductive, and Magnetic Sensor Technologies

  • Winncy Y. Du(Author)
  • 2014(Publication Date)
  • CRC Press

    (Publisher)

  Apply Equation 2.5 and ignore the sign (for easy explanation): V R R V R R S out = = = 2 1 2 2 2000 10 0 005 ( ) . As R 2 varies from 0 to 2 k Ω , the output voltage V out will change linearly from 0 to 10 V. 2. When V out is 7.5 V, R 2 is R V 2 0 005 7 5 0 005 1 5 = = = out . . . . k Ω Since the water level x is proportional to the resistance change R 2 , therefore x X X X R R R R x R x --= --⇒ --= --⇒ = min max min min max min 2 2 0 80 0 0 2000 0 0 04 . R 2 0 04 1500 60 = × = . mm 2.2.3 P OTENTIOMETRIC S ENSOR D ESIGN 2.2.3.1 Linear Potentiometers Major components of a Linear Potentiometer include three terminals (power input, ground, and sensor signal output), a fixed resistor, a wiper, a sliding track, and the housing. The terminals are often gold plated to prevent corrosion or tarnishing. The resistor is usually made of high-quality conductive materials (e.g., copper) with a lower temperature coefficient for stability and long life. The wiper uses high-quality metal or alloy (e.g., a platinum alloy) for long life and low noise. Some wipers even have a multifinger shape to prevent intermittence when used in high shock or vibra-tion environments. The wiper is connected to the moving object being measured through threads, chamfers, or spring returns. Some potentiometric sensors have other components, such as guide rails (to enhance stability during wiper sliding) and wave washers for antibacklash control. The housing holds the components in place and protects the sensor from harsh environments. The major considerations in a linear potentiometric sensor design include • The length of the stroke to be measured • Power rating and resistance value • The space limitations 30 Resistive, Capacitive, Inductive, and Magnetic Sensor Technologies • The quality of the conductive element • The means of connecting the sensor to the moving device being measured 2.2.3.2 Rotary Potentiometers There are two different designs of rotary potentiometers: single-turn and multiturn .

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  Programming the PIC Microcontroller with MBASIC

  • Jack Smith(Author)
  • 2005(Publication Date)
  • Newnes

    (Publisher)

  487 Digital Potentiometers and Controllable Filter C H A P T E R 21 Digital potentiometers (usually called a “pot” in electronics speak) are a solid-state replacement for me-chanically adjustable variable resistors historically used for volume, balance and tone controls in consumer electronics, among many other applications. Instead of responding to shaft rotation, digital pots change their value—in microseconds—based upon a received command message. Digital pots are offered in a variety of control interfaces, including 1-wire, contact closure (up/down steps), parallel, increment/decrement, SPI (3-wire) and others. Some even include nonvolatile memory to retain their last setting when power is removed. We’ll look at Microchip’s MCP41xxx/42xxx family of digital pots, with an SPI interface. Other manufacturers, such as Maxim (including its Dallas Semiconductor division), have wider product ranges, and you should review their offerings if you think digital potentiometers may be useful in your particular project. First, let’s clear up the terminology. The device we are talking about is known as a potentiometer, a variable resistor, a rheostat or a volume control, among many other terms. In reality, these terms can be boiled down into two possible connection arrangements, as shown in Figure 21-1. The potentiometer arrangement uses all three connections and is usually configured as a variable voltage divider. The variable resistor or rheostat arrangement uses two connections (the wiper—the variable connecting piece—is connected to one end of the fixed winding, but to the external circuit only two connections are seen) and operates as a variable resistance. We’ll use the term “pots” as the generic name for these devices, even though they may be connected, in certain applications, as variable resistors. One major difference between mechanical and digital pots is resolution.

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