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).

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