String potentiometer
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A string potentiometer is a transducer used to detect and measure linear position and velocity using a flexible cable and 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:
1. measuring cable
2. spool
3. spring
4. rotational sensor
Inside the transducer's housing, a stainless steel cable is wound on a precisely machined constant diameter cylindrical spool that turns as the measuring cable reels and unreels.
To maintain cable tension, a torsion spring is coupled to the spool. The spool is coupled to the shaft of a rotational sensor (a potentiometer orencoder). As the transducer's cable extends along with the movable object, it causes the spool and sensor shafts to rotate. The rotating shaft creates an electrical signal proportional to the cable's linear extension or velocity.
Applications
The original application for string pots back in the 1960’s was aerospace cyclic fatigue testing. Engineers designed and built these units originally to measure movement of airplane parts as they were cycled during testing. Today, the string pot is used in just about every industry in some capacity, for both testing and OEM usage.
Common applications include : Hydraulic cylinder measurement, automotive testing, aerospace testing, factory automation, medical devices, oil and chemical, structural testing, and various OEM machines. (The hydraulic cylinder category entails many industries such as forklifts, cranes and aerials, material handling, die-casting, oil and gas, robotics and automation, and OEM machines.)
When would one use a String Pot ?
String Potentiometers are useful sensors when you want to precisely measure a moving object or part. You can actually attach the measurement cable directly to the moving part, insuring you get a “real” measurement at the point of interest. This simple type of measurement device has been in use by engineers and designers for about 40 years. String pots are generally durable, simple to use, and inexpensive.
All industry-standard signal types can be found, including potentiometer (voltage divider), 0-10 VDC, 4-20mA, encoder, Bus (DeviceNet and Canbus) and RS232 communications.
What advantages do String Pots have?
Measuring the movement and displacement of objects is easy with string pots. They can usually be installed quickly, can fit into precarious or tight areas and they do not require perfectly parallel alignment. They also offer great flexibility, a small size-to-measurement ratio and they usually cost less than rod or wand-type measurement devices.
What measurement ranges are available with String Pots?
Measurement ranges go from about 1 inch all the way up to over 100 feet. There are many package sizes available that cover this wide range of lengths.