String potentiometer
Encyclopedia
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".
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
or encoder
). 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.
The original application for string pots in the 1960s 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 both for testing and as a component of equipment. Common applications include:
Hydraulic cylinders are used in many industries such as forklifts, cranes and aerials, material handling, die-casting, oil and gas, robotics and automation. Measurement or control of the extension of the cylinder requires a position transducer, and often a string potentiometer is used.
The string potentiometer may be connected as a three-wire tapped resistor
(voltage divider), in a control circuit, or may be packaged with electronics to produce a measurement signal in a useful form, such as a variable voltage 0-10 VDC, variable current 4-20mA, pulse encoder, Bus (DeviceNet
and Canbus) and RS232 communications.
Measurement ranges vary from about 1 inch to over 100 feet and are available in many appropriate package sizes.
Other linear position measurements methods include LVDTs, capacitive and inductive sensors, and rack-and-pinion transducers that convert linear motion into rotary motion. Optical (time-of-flight), ultrasonic, and radar transducers exist and find specialized applications.
Transducer
A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...
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".
Components
String potentiometers are composed of four main parts: a measuring cable, spool, string, and rotational sensor. Inside the transducer's housing, a stainless steelStainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
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
Torsion spring
A torsion spring is a spring that works by torsion or twisting; that is, a flexible elastic object that stores mechanical energy when it is twisted. The amount of force it exerts is proportional to the amount it is twisted. There are two types...
is coupled to the spool. The spool is coupled to the shaft of a rotational sensor (a potentiometer
Potentiometer
A potentiometer , informally, a pot, is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. If only two terminals are used , it acts as a variable resistor or rheostat. Potentiometers are commonly used to control electrical devices such as volume controls on...
or encoder
Encoder
An encoder is a device, circuit, transducer, software program, algorithm or person that converts information from one format or code to another, for the purposes of standardization, speed, secrecy, security, or saving space by shrinking size.-Media:...
). 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
String potentiometers are used to measure length of a moving object. The measurement cable can be connected directly to the moving part, to give a direct measurement of the point of interest. This simple type of measurement device has been in use by engineers and designers for about 40 years. String potentiometers are generally durable, simple to use, and inexpensive.The original application for string pots in the 1960s 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 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 chemical,
- structural testing,
- industrial machinery,
- robotics.
Hydraulic cylinders are used in many industries such as forklifts, cranes and aerials, material handling, die-casting, oil and gas, robotics and automation. Measurement or control of the extension of the cylinder requires a position transducer, and often a string potentiometer is used.
The string potentiometer may be connected as a three-wire tapped resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
(voltage divider), in a control circuit, or may be packaged with electronics to produce a measurement signal in a useful form, such as a variable voltage 0-10 VDC, variable current 4-20mA, pulse encoder, Bus (DeviceNet
DeviceNet
DeviceNet is a network system used in the automation industry to interconnect control devices for data exchange. It uses Controller Area Network as the backbone technology and defines an application layer to cover a range of device profiles...
and Canbus) and RS232 communications.
Measurement ranges vary from about 1 inch to over 100 feet and are available in many appropriate package sizes.
Limitations of the method
Since the measuring cable may sag or be deflected by wind or gravity, the overall precision of a string potentiometer measurement is limited. The cable mechanism limits the speed at which the measured object can move. Changing temperatures affect both the length of the cable and the resistance value of the potentiometer. Where multiple objects, such as articles on an assembly line, or objects that are hot or coated with wet paint are to be measured, a non-contacting method is required.Other linear position measurements methods include LVDTs, capacitive and inductive sensors, and rack-and-pinion transducers that convert linear motion into rotary motion. Optical (time-of-flight), ultrasonic, and radar transducers exist and find specialized applications.