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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: a measuring cable, spool, spring, and 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 or rotary 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.
String potentiometers are used to measure the position of a moving object. The measurement cable can be connected directly to the moving part, giving a constant measurement of its linear position. This simple type of measurement device has been in use by engineers and designers for about 40 years.[ citation needed ] String potentiometers are generally durable, simple to use, and inexpensive.
The original application for string pots in the 1960s was aerospace cyclic fatigue testing.[ citation needed ] Engineers designed and built these units originally to measure movement 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.[ citation needed ] 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.[ citation needed ] Measurement of the extension of the cylinder requires knowledge of its current position, 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 RS-232 communications.
Measurement ranges vary from about 1 inch to over 100 feet and are available in many appropriate package sizes.
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.
A potentiometer is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat.
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A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to analog or digital output signals.
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A rheometer is a laboratory device used to measure the way in which a viscous fluid flows in response to applied forces. It is used for those fluids which cannot be defined by a single value of viscosity and therefore require more parameters to be set and measured than is the case for a viscometer. It measures the rheology of the fluid.
A coordinate measuring machine (CMM) is a device that measures the geometry of physical objects by sensing discrete points on the surface of the object with a probe. Various types of probes are used in CMMs, the most common being mechanical and laser sensors, though optical and white light sensor do exist. Depending on the machine, the probe position may be manually controlled by an operator or it may be computer controlled. CMMs typically specify a probe's position in terms of its displacement from a reference position in a three-dimensional Cartesian coordinate system. In addition to moving the probe along the X, Y, and Z axes, many machines also allow the probe angle to be controlled to allow measurement of surfaces that would otherwise be unreachable.
A servomotor is a rotary actuator or linear actuator that allows for precise control of angular or linear position, velocity, and acceleration. It consists of a suitable motor coupled to a sensor for position feedback. It also requires a relatively sophisticated controller, often a dedicated module designed specifically for use with servomotors.
A linear encoder is a sensor, transducer or readhead paired with a scale that encodes position. The sensor reads the scale in order to convert the encoded position into an analog or digital signal, which can then be decoded into position by a digital readout (DRO) or motion controller.
A variable reluctance sensor is a transducer that measures changes in magnetic reluctance. When combined with basic electronic circuitry, the sensor detects the change in presence or proximity of ferrous objects.
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Ultrasonic transducers and ultrasonic sensors are devices that generate or sense ultrasound energy. They can be divided into three broad categories: transmitters, receivers and transceivers. Transmitters convert electrical signals into ultrasound, receivers convert ultrasound into electrical signals, and transceivers can both transmit and receive ultrasound.
SpaceAge Control is a design, manufacturing, and service company focused on 3D displacement sensing and measurement.
An incremental encoder is a linear or rotary electromechanical device that has two output signals, A and B, which issue pulses when the device is moved. Together, the A and B signals indicate both the occurrence of and direction of movement. Many incremental encoders have an additional output signal, typically designated index or Z, which indicates the encoder is located at a particular reference position. Also, some encoders provide a status output that indicates internal fault conditions such as a bearing failure or sensor malfunction.
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