output (and noise) is produced but more power is dissipated, leading to greater thermal effects. In general, wirewound and cermet pots are better able to dissipate heat, and thus have the highest power ratings.
Temperature Coefficient
As temperature increases, pot resistance also increases. However, a pot connected as shown in Figure 6.2 will divide the voltage equally well, regardless of its total resistance. Thus, temperature effects are not usually a major concern as long as the changes in resistance are uniform and the pot operates within its ratings. However, an increase in pot resistance also increases loading nonlinearities. Therefore, temperature coefficients can become an important consideration. The temperature coefficient, typically specified in ppm ˚C–1, can be expressed as α = ( RP /RP)/ t, where t is the change in temperature and RP is the corresponding change in total resistance. In general, wirewound pots possess the lowest temperature coefficients. Temperature-compensating signal-conditioning circuitry can also be used.
Resistance
Since a pot divides voltage equally well regardless of its total resistance, resistance tolerance is not usually a major concern. However, total resistance can have a great impact on loading effects. If resistance is large, less current flows through the pot, thus reducing temperature effects, but also increasing loading.
AC Excitation
Pots can operate using either a dc or an ac voltage source. However, wirewound pots are susceptible to capacitive and inductive effects that can be substantial at moderate to high frequencies.
Mechanical Characteristics
The following mechanical characteristics influence measurement quality and system reliability, and thus should be considered when selecting a pot.
Mechanical Loading
A pot adds inertia and friction to the moving parts of the system that it is measuring. As a result, it increases the force required to move these parts. This effect is referred to as mechanical loading. To quantify mechanical loading, rotary pot manufacturers commonly list three values: the equivalent mass moment of inertia of the pot’s rotating parts, the dynamic (or running) torque required to maintain rotation in a pot shaft, and the starting torque required to initiate shaft rotation. For linear-motion pots, the three analogous loading terms are mass, starting force, and dynamic (or running) force.
In extreme cases, mechanical loading can adversely affect the operating characteristics of a system. When including a pot in a design, ensure that the inertia added to the system is insignificant or that the inertia is considered when analyzing the data from the pot. The starting and running force or torque values might also be considered, although they are generally small due to the use of bearings and lowfriction resistive elements.
Mechanical Travel
Distinguished from electrical travel, mechanical travel is the wiper’s total motion range. A mechanical stop delimits mechanical travel at each end of the wiper’s range of motion. Stops can withstand small loads only and therefore should not be used as mechanical limits for the system. Manufacturers list maximum loads as the static stopping strength (for static loads) and the dynamic stopping strength (for moving loads).
Rotary pots are also available without mechanical stops. The shaft of such an “unlimited travel” pot can be rotated continuously in either direction; however, electrical travel is always less than 360° due to the discontinuity or “dead-zone” where the resistive element begins and ends. (See Figure 6.1.) Multiple revolutions can be measured with an unlimited travel pot in conjunction with a counter: the counter maintains the number of full revolutions while the pot measures subrevolution angular displacement.
Operating Temperature
When operated within its specified temperature range, a pot maintains good electrical linearity and mechanical integrity. Depending on construction, pots can operate at temperatures from as low as –65˚C
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to as high as 150˚C. Operating outside specified limits can cause material failure, either directly from temperature or from thermally induced misalignment.
Vibration, Shock, and Acceleration
Vibration, shock, and acceleration are all potential sources of contact discontinuities between the wiper and the resistive element. In general, a contact failure is considered to be a discontinuity equal to or greater than 0.1 ms [2]. The values quoted in specification sheets are in gs and depend greatly on the particular laboratory test. Some characterization tests use sinusoidal vibration, random vibration, sinusoidal shock, sawtooth shock, or acceleration to excite the pot. Manufacturers use mechanical design strategies to eliminate weaknesses in a pot’s dynamic response. For example, one technique minimizes vibration-induced contact discontinuities using multiple wipers of differing resonant frequencies.
Speed
Exceeding a pot’s specified maximum speed can cause premature wear or discontinuous values through effects such as wiper bounce. As a general rule, the slower the shaft motion, the longer the unit will last (in total number of cycles). Speed limitations depend on the materials involved. For rotary pots, wirewound models have preferred maximum speeds on the order of 100 rpm, while conductive plastic models have allowable speeds as high as 2000 rpm. Linear-motion pots have preferred maximum velocities up to 10 m s–1.
Life
Despite constant mechanical wear, a pot’s expected lifetime is on the order of a million cycles when used under proper conditions. A quality film pot can last into the hundreds of millions of cycles. Of wirewound, hybrid, and conductive plastic pots, the uneven surface of a wirewound resistive element inherently experiences the most wear and thus has the shortest expected operating life. Hybrids improve on this by using a wirewound construction in combination with a smooth conductive film coating. Conductive plastic pots generally have the longest life expectancy due to the smooth surface of their resistive element.
Contamination and Seals
Foreign material contaminating pots can promote wear and increase friction between the wiper and the resistive element. Consequences range from increased mechanical loading to outright failure (e.g., seizing, contact discontinuity). Fortunately, sealed pots are available from most manufacturers for industrial applications where dirt and liquids are often unavoidable. To aid selection, specifications often include the type of case sealing (i.e., mechanisms and materials) and the seal resistance to cleaning solvents and other commonly encountered fluids.
Misalignment
Shaft misalignment in a pot can prematurely wear its bearing surfaces and increase its mechanical loading effects. A good design minimizes misalignment. (See Implementation, below.) Manufacturers list a number of alignment tolerances. In linear-motion pots, shaft misalignment is the maximum amount a shaft can deviate from its axis. The degree to which a shaft can rotate around its axis is listed under shaft rotation. In rotary pots, shaft end play and shaft radial play both describe the amount of shaft deflection due to a radial load. Shaft runout denotes the shaft diameter eccentricity when a shaft is rotated under a radial load.
Mechanical Mounting Methods
Hardware features on a pot’s housing determine the mounting method. Options vary with manufacturer, and among rotary, linear-motion, and string pots. Offerings include custom bases, holes, tabs, flanges, and brackets — all of which secure with machine screws — and threaded studs, which secure with nuts. Linear-motion pots are available with rod or slider actuation, some with internal or external return springs. Mounting is typically accomplished by movable clamps, often supplied by the pot manufacturer. Other linear-motion pots mount via a threaded housing. For rotary pots, the two most popular mounting methods are the bushing mount and the servo mount. See Figure 6.5.
© 1999 by CRC Press LLC
FIGURE 6.5 The two most common rotary pot mounts are the bushing mount (a), and the servo mount (b).
TABLE 6.4 Sources of Small Mechanical Components
PIC Design
86 Benson Road, P.O. Box 1004 Middlebury, CT 06762-1004
Tel: (800) 243-6125, (203) 758-8272; Fax: (203) 758-8271 www.penton.com/md/mfg/pic/
Stock Drive Products/Sterling Instrument 2101 Jericho Turnpike, Box 5416
New Hyde Park, NY 11042-5416
Tel: (516) 328-3300; Fax: (800) 737-7436, (516) 326-8827 www.sdp-si.com
W.M. Berg, Inc. 499 Ocean Ave.
East Rockaway, NY 11518
Tel: (800) 232-2374, (516) 599-5010; Fax: (800) 455-2374, (516) 599-3274 www.wmberg.com
Bushing mount
The pot provides a shaft-concentric, threaded sleeve that invades a hole in a mounting fixture and secures with a nut and lock-washer. An off-axis tab or pin prevents housing rotation. Implementing a bushing mount requires little more than drilling a hole; however, limited rotational freedom and considerable play before tightening complicate precise setup.
Servo mount
The pot provides a flanged, shaft-concentric, precision-machined rim that slips into a precision-bored hole in a mounting fixture. The flange secures with symmetrically arranged, quick-releasing servo mount clamps, available from Timber-Top, Inc. [9] and also from the sources listed in Table 6.4. (These clamps are also called synchro mount clamps and motor mount cleats, since servo-mounting synchros and stepper motors are also available.) Servo mounts are precise and easy to adjust, but entail the expense of precision machining.
Measurement Techniques
To measure displacement, a pot must attach to mechanical fixtures and components. The housing typically mounts to a stationary reference frame, while the shaft couples to a moving element. The input motion (i.e., the motion of interest) can couple directly or indirectly to the pot’s shaft. A direct connection, although straightforward, carries certain limitations:
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FIGURE 6.6 Mechanisms that extend a precision potentiometer’s capabilities include belts and pulleys (a), rack- and-pinions (b), lead-screws (c), cabled drums (d), cams (e), bevel gears (f), and spur gears (g).
•The input motion maps 1:1 to the shaft motion
•The input motion cannot exceed the pot’s mechanical travel limits
•Angle measurement requires a rotary pot; position measurement requires a linear-motion pot
•The pot must mount close to the motion source
•The input motion must be near-perfectly collinear or coaxial with the shaft axis
Figure 6.6 shows ways to overcome these limitations. Mechanisms with a mechanical advantage scale motion and adjust travel limits. Mechanisms that convert between linear and rotary motion enable any type of pot to measure any kind of motion. Transmission mechanisms distance a pot from the measured motion. Compliant mechanisms compensate for misalignment. Examples and more details follow. Most of the described mechanisms can be realized with components available from the sources in Table 6.4.
Gears scale the mapping between input and pot shaft motions according to gear ratio. They also displace rotation axes to a parallel or perpendicular plane according to type of gear (e.g., spur vs. bevel). Gears introduce backlash. Friction rollers are a variation on the gear theme, immune to backlash but prone to slippage. The ratio of roller diameters scales the mapping between input and pot shaft motions.
© 1999 by CRC Press LLC