Linear velocity sensors measure the linear velocity of an object using either contact or noncontact techniques. Important performance specifications to consider when searching for linear velocity sensors include maximum velocity, maximum measurement range, accuracy, and operating temperature. The maximum velocity represents the maximum linear velocity that can be measured. The maximum measurement range is the maximum range or distance at which the sensor can measure speed. The measurement accuracy of the device, measured in ±% full scale. Accuracy can have somewhat varying definitions, and can include nonlinearity, hysteresis, and measurement resolution. The operating temperature is the range of temperatures over which the device must function.

Applications

Technology choices for linear velocity sensors include:

• cable extension
• magnetic induction
• microwave, optical, or laser
• piezoelectric
• radar or radio frequency
• strain gauge
• ultrasonic

For cable-extension linear velocity sensors, the moving object is attached to a cable, which is typically connected to a potentiometer. As the object moves, the potentiometer's resistance value changes. Magnetic induction sensors are non-contact linear velocity sensors that use an induced current from a magnetic field to measure the linear velocity. Microwave sensors use microwave technology to determine speed, whereas fiber optic sensors use fiber optics or laser technology to determine speed. With piezoelectric linear velocity sensors, a piezoelectric material is compressed and generates a charge, which is measured by a charge amplifier. Often, piezoelectric sensors are used in vibration velocity measurement applications. 

Radar-based linear velocity sensors emit a radar wave, which is reflected by surface and returned to sensor. Speed is determined by measuring frequency difference (Doppler Effect). For strain gauge devices, strain-sensitive variable resistors are bonded to parts of the structure, which deforms when making the measurement. Typically, strain gauges are used as elements in a Wheatstone bridge circuit, which is used to make the measurement. Ultrasonic linear velocity sensors emit an ultrasonic pulse, which is reflected by surface and returned to sensor. Speed is determined by measuring frequency difference.

Types

Electrical outputs common to linear velocity sensors include:

• voltage
• current
• radio frequency (RF)
• switch or alarm
• serial
• parallel
• DeviceNet
• Ethernet
• SERCOS
• SSI and TTL-compatible.

Linear velocity sensors can be many different types of devices, including sensor element or chip, sensor or transducer, instrument or meter, gauge or indicator, and recorder and totalizers. A sensor element or chip denotes a "raw" device such as a strain gauge, or one with no integral signal conditioning or packaging. A sensor or transducer is a more complex device with packaging and/or signal conditioning that is powered and provides an output such as a DC voltage, a 4–20mA current loop, etc. An instrument or meter is a self-contained unit that provides an output, such as a display, locally at or near the device. These linear velocity sensors may also include signal processing and/or conditioning. A gauge or indicator is a device that has a (usually analog) display and no electronic output such as a tension gauge. A recorder or totalizer is an instrument that records, totalizes, or tracks force measurement over time. 

Features

Common features for linear velocity sensors include multi-axis measurement, vehicular or transportation applications, intrinsically safe (IS) construction. Linear velocity sensors may also provide additional measurements of diameter, direction, distance or displacement, flow, length, thickness, tilt or angle, and vibration. 

Related Information

Engineering360—Velocity Sensing

Engineering360—Linear Position Sensors Information

Engineering360—Variable Reluctance Linear Position Sensors and Switches Information

Image credit:

Trans-Tek, Inc.