Control Engineering Online Update for July 8, 2005


Control Engineering Online Update - Technology

July 8, 2005


Sponsored by
Control Engineering

This review of the advantages and disadvantages of string potentiometers and string encoders, also known as cable position transducers (CPT), will help you better understand the strengths and weaknesses of CPT technology for determining the optimal displacement measurement solution for your application.

Cable Position Transducers for Control and Monitoring

Cable position transducers (CPTs), or string potentiometers/encoders, were first developed in the mid-1960s in concert with the growth of the aerospace and aircraft industries. The first applications involved the monitoring of aircraft flight control mechanisms during flight testing.

While the technology is now proven and mature, it is certainly not dated. A range of high-performance and cost-conscious applications use CPTs as the basis for key control and monitoring operations, such as:

  • Delta IV missile thrust vectoring system;

  • Military fighter level sensor;

  • Diesel engine fuel index measurement;

  • International Space Station environmental control systems;

  • Commercial and military aircraft flight data recorder input sensors;

  • Medical table actuation feedback system; and

  • Logistics sorting and positioning equipment.

The technology is so widely used that it is referred to by a variety of names, such as: cable-actuated position sensor, cable extension transducer, cable sensor, cable-actuated sensor, CET, CPT, draw wire encoder/transducer, wire rope transducer, wire sensor, wire-actuated transducer, and yo-yo potentiometer.

These names all refer to devices that measure displacement via a flexible displacement cable that extracts from and retracts to a spring-loaded drum. This drum is attached to a rotary sensor (see Figure 1).

Figure 1: How CPTs operate

CPT Advantages
Multi-axis capability —CPTs can be used to track linear, rotary, 2-dimensional, and 3-dimensional displacements. This capability makes CPTs a good choice for test engineering as well as OEM applications where size and mounting restrictions eliminate other choices.

Flexible mounting —The flexible displacement cable inherent in CPT can be attached to the application in a number of ways, and can include use of magnets, eyebolts, or other threaded fasteners. The cable can also be routed around barriers using pulleys and flexible conduits. In addition, transducer mounting bases and cable exit options provide additional mounting flexibility, eliminating the expense associated with special fixturing and adapters.

Fast installation —Flexible mounting features combined with the broad tolerance for displacement cable misalignment provide for fast installation, often in less than 2 minutes. This reduces installation costs and can be particularly valuable in test and R&D applications.

Small size —The world's smallest CPT measures a 1.5 inches (38.1 mm) displacement with a size of only 0.75 in. sq x 0.38 in. (19 mm x 19 mm x 10 mm). As the measurement range increases, the CPT's relative small size advantage becomes more obvious as shown in Figure 2 below.

Figure 2: Size comparison of CPTs to rod-and-cylinder devices such as LVDTs and linear potentiometers

Light weight —CPTs measure displacement with a low-mass, stainless steel or high-strength fabric-based cable. This feature, along with generally anodized aluminum components, results in a product that has a low weight-to-range ratio. Low mass can also increase survivability in high shock and vibration environments encountered in industrial machinery and equipment applications. The table below shows a comparison of weight to range for various displacement measurement sensors.

CPT weight-to-range compared to other sensor types
(mass shown in oz (g))

Range in inches (mm)



linear encoder


2 (51)

1 (28)

13 (369)

20 (567)

15 (425)

10 (254)

2 (57)

41 (1162)

25 (709)

29 (822)

40 (1016)

8 (227)

67 (1900)

44 (1247)

51 (1446)


Rugged —Properly designed and manufactured CPTs have performed reliably for more than 30 years in harsh industrial, aerospace, testing, and outdoor environments. The design of CPTs can be mechanically and electrically simple, resulting in high reliability, low maintenance, and years of service. CPT environmental testing has been conducted demonstrating the effective operation of CPTs in environments containing high shock, vibration, humidity, corrosion, moisture, and other parameters.

Electrical outputs —Because CPTs can incorporate a broad range of rotary sensors and related signal conditioning, the electrical output of your choice is typically available, including 4-20 mA, 0 to 5 V dc, 0 to 10 V dc,

Signal conditioning —CPTs, particularly analog potentiometer types, generally have low power and simple signal conditioning requirements. 5 V dc power or less is suitable and no special signal conditioning is required for the majority of applications.

Operating temperature —Analog-output CPTs can operate in temperate ranges from -65° to +125°C while digital-output CPTs can range from -40° to +85° or -20° to +100°C. Broader operating ranges can be attained with the use of custom sensors.

Accuracy —Using non-backlash connections and threaded drums, analog-ouptut CPTs can offer linearity-compensated accuracy exceeding

Measurement range —CPTs measurement ranges are broad: 1.5 inches to over 2000 inches (38 mm to over 50 m).

There is no perfect sensing technology, and CPTs are no exception. When considering a CPT for your application, keep the following items in mind.

Frequency response —CPTs are used extensively in vehicle impact testing where accelerations exceed 50 Gs. CPTs have also been used in other applications where accelerations approach 100 Gs. However, applications involving extreme acceleration can exceed the frequency response capabilities of CPTs. Examples where CPT frequency response can be an issue include applications that require cable accelerations in excess of 100 Gs and environments using analog-output CPTs with very high frequency, low displacement motion that tends to cause dithering wear of the potentiometric element. During extreme high accelerations, it is possible the cable reel's inertia and the power spring's insufficient torque will cause the reel to rotate to a point where the displacement cable goes slack.

Lifetime —CPTs with optical encoder or conductive plastic sensing technologies can operate in excess of 100 million shaft revolutions. However, longer range analog-output CPTs require multi-turn potentiometers with lifetimes of less than 10 million shaft revolutions. While long-displacement, high-cycle applications are not common, you should do a thorough cost and reliability analysis before specifying a CPT for this type of use.

Cable tension —Non-contact sensing devices such as ultrasonic, Hall effect, or laser do not mechanically affect the application. The CPT's cable tension imparts a load on the application. While this load can be minimized to as little as 1 oz, it cannot be eliminated. Hence, for applications sensitive to external loads, other technologies should be considered.

Accuracy —CPTs that use no-backlash mechanical connections and threaded drums deliver excellent accuracy. Nevertheless, this is sometimes insufficient for some applications. For those situations, you should consider using LVDTs, laser-based devices, or other higher-accuracy technologies. See if your application does not require raw accuracy but rather good linearity, resolution, repeatability, or hysteresis.

Catenary curve error —A catenary curve describes the shape the displacement cable takes when subjected to a uniform force such as gravity. Because the mass of the cable per unit length is so small and the cable tension is relatively high, cable sag does not produce any significant error unless the cable length is exceptionally long (over 60 feet (18 meters)). Though the cable sag error is minor compared to other error sources (generally less than

Tom Anderson is application development manager at SpaceAge Control,

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