A String Potentiometer is a transducer used to detect and measure linear position and velocity using a flexible cable and a spring-loaded spool. Other common names include “string pot”, “cable-extension transducer”, “draw wire sensor”, and “yo-yo sensor”.
String potentiometers are composed of four main parts: a measuring cable, spool, spring, and rotational sensor. Inside the transducer’s housing, a stainless steel cable is wound onto a precision machined constant diameter cylindrical spool that turns as the measuring cable reels and unreels.
To maintain cable tension, a torsion spring is attached to the spool. The spool is connected to the shaft of a rotational sensor (a potentiometer or rotary encoder ). As the transducer’s cable extends along the movable object, it causes the spool and sensor shaft to rotate. The rotating shaft creates an electrical signal proportional to the linear expansion or velocity of the cable.
String potentiometer is used to measure the position of a moving object. The measurement cable can be connected directly to the moving part, providing a continuous measurement of its linear position. This simple type of measuring instrument has been in use by engineers and designers for nearly 40 years. [ citation needed ] String potentiometers are generally durable, easy to use, and inexpensive. The original application for string pots in the 1960s was aerospace cyclic fatigue testing . [ citation needed ] Engineers originally designed and built these units to measure the speed of airplane parts as they were cycled during testing. [ citation needed ] Today, the string pot is used both for testing and as a component of equipment. Common applications include:
- hydraulic cylinder position measurement,
- automotive and aerospace testing,
- factory automation,
- medical devices,
- oil and chemicals,
- structural test,
- industrial equipment,
Hydraulic cylinders are used in many industries such as forklifts , cranes and aerials, material handling, die-casting , oil and gas, robotics and automation. Measurement of cylinder expansion requires knowledge of its current position, and often a string potentiometer is used.
String potentiometers may be connected as a three-wire tapped resistor (voltage divider) to control circuits, or may be packaged with electronics to generate a measurement signal in a useful form, such as a variable voltage 0- 10VDC , variable current 4-20mA , pulse encoder, BUS ( DeviceNet and CANbus ) and RS232 communication.
Measurement ranges vary from about 1 inch to over 100 feet and are available in a number of suitable package sizes.
Limitations of Law
Since the measuring cable may be loosened or deflected by wind or gravity, the overall accuracy of a string potentiometer measurement is limited. The cable mechanism limits the speed at which the measured object can move. The changing temperature affects both the length of the cable and the resistance value of the potentiometer. Where multiple objects, such as articles on the assembly line, or objects that are coated with hot or wet paint, are to be measured, a non-contact method is required.
Other linear position measurement methods include LVDTs , capacitive and inductive sensors, and rack-and-pinion transducers that convert linear motion into rotary motion. Optical (in flight), ultrasonic and radar transducers exist and find special applications.