Speed Sensor Types

We’re here to help you select the right speed sensing solution.

Motion Sensors offers a full range of standard speed solutions. Our standard speed sensors — and application experts who understand your requirements — make Motion Sensors your go-to source for your speed sensing needs.

speed sensor types
Output Type

The initial consideration is whether a digital or analog signal is required. The basic variable reluctance (VR) speed sensor provides an analog sine wave. The frequency of the signal will increase as speed increases. If a digital output is desired, options would include amplified versions of VR or RF speed sensors, hall effect sensors, or coupling of a preamplifier with a standard VR or RF sensor. There are a variety of output options, including 0-5VDC, 0-10 VDC, Open Collector and Output tied to Supply.

Minimum Output Requirements, Target Detail, and Air Gap

The output of a speed sensor is highly dependent upon application details such as the size, shape and material of the target. Target types vary significantly, for example spinning gears, turbine blades, a screw embedded in a turning shaft, or a ferrous metal notch on a conveyor belt. Targets can also take the form of a moving magnetic field such as center pole magnets or magnets embedded in the outer rim of the rotating device. In most cases, a ferrous metal target is required, however we can also offer modulated carrier (RF) sensors that are capable of sensing certain non-ferrous metals like aluminum and non-magnetic stainless steel.

In many cases, if the sensor output will be interfaced with a PLC or other electronic device there will be a requirement for the sensor to provide a minimum output signal. Once the target details are known, it will also be important to determine the airgap and the min/max RPM of the application.

Air gap, defined as the distance between the sensor tip and the target, will determine the strength of the signal provided by the sensor. The closer the sensor is mounted to the gear, the stronger the signal, however care must be taken to ensure that gear run-out (wobble) does not damage the sensor front. As the sensor is moved further away from the target the signal will lessen, until the distance becomes so great that the sensor is no longer able to accurately sense the rotation of the target.

Output Type Selection Guide

Fixed Voltage (TTL/CMOS)No additional components.
Known output voltage.
Signal degradation with difficult loads.
More susceptible to line noise.
Supply TrackingNo additional components. Supply voltage can be tracked. Low cost.Signal degradation with difficult loads. More susceptible to line noise. Output varies with supply voltage.
Open CollectorDifferent supply and output voltages. Optimize drive current by pull up resistor selection.Requires additional pull up resistor. See Open Collector Pullup Resistor Calculator
Single Ended Push-Pull Fixed VoltageNo additional components. Known output voltage. Improved signal integrity with difficult loads. Thermal and show circuit protection.Moderate line noise immunity.
Single Ended Push-Pull Supply TrackingNo additional components. Supply voltage can be tracked. Improved signal integrity with difficult loads.Moderate line noise immunity. Output varies with supply voltage. Thermal and short circuit protection.
Differential Push-Pull, Fixed or Supply TrackingHigh noise tolerance when paired with and impedance matched differential receiver.Requires two connections for each output.
Higher cost.
Extreme Applications

Motion Sensors speed sensors are specifically designed for use in extreme applications. Our sensors are selected for their proven ability to operate reliably in these harsh environments and are designed with these particular environmental pressures in mind. From aerospace to rail and transit, the ability to withstand temperature changes, high shock and vibration, and atmospheric factors are essential to the design of a successful speed sensor solution.

High Temperatures

Our sensors are designed to operate reliably in extremely high temperatures. High temperature Variable Reluctance (VR) sensors are designed for use in environments that see 454C continuous temperatures with up to 538C intermittently and have been used in applications ranging from steam turbines to missiles. While the VR sensors offer the highest temperatures, all of our sensors are designed to maximize temperature capabilities. We offer high temperature RF models with capability up to 400C, and all of our electronic products are offered in models that operate up to 125C.


Motion Sensors’ hermetically sealed VR sensors are ideally suited for use in cryogenic applications. The standard operating temperature range is -267C to +232C. Our sensors have been successfully used for many years in applications including space shuttle on-board equipment, liquid nitrogen flowmeters, and cryogenic rotators.

Thermal Cycling

Many of our speed sensors are encapsulated with a special potting material that provides excellent compensation for temperature extremes and thermal cycling, ensuring that the sensor internals are not damaged due to stresses from expanding and contracting components. This is especially important in applications that will see the most extreme temperature highs and lows on a recurring basis.

High Vibration

Our sensors have been tested for high vibration environments. Test requirements are dependent upon application specifications and include DO160 and military aircraft specifications, testing for on-board space shuttle use, and IEC 60068 for Rail and Transit applications. All of our standard product lines have been tested to MIL-STD-202G Method 214A, Test Condition H. Motion Sensors has in-house capability to perform vibration testing and is able to validate speed sensor vibration capabilities to specific application requirements.

Hermetic Sealing

Motion Sensors’ original speed sensor designs were all hermetically sealed, with a welded seam at the tip of the sensor and the connector. The connector incorporates a glass to metal seal, ensuring that the sensor internals are protected against moisture intrusion. We still use this design today in a large number of applications where environmental conditions make a completely sealed solution essential. Applications range from installations requiring sanitary wash-downs to aerospace fuel control sensors.

Stainless Steel Construction

In addition to hermetic sealing, Motion Sensors also constructs 99% of our speed sensors from stainless steel, providing superior protection against rust and corrosion. While this does add additional cost versus a high volume molded or plastic solution, it provides the reliability and quality necessary for the extreme applications where our speed sensors are installed.

Strain Relief

Over the years, we have worked with a number of customers to provide solutions for applications where the existing sensors failed prematurely due to a lack of strain relief. Our designs incorporate provision for strain relief throughout sensor construction, from attachment of the winding leads to PCB interconnection and connector solder joints. We also have the capability to incorporate additional external strain relief into applications with wiring and cabling requirements that will see extreme shock, vibration, and abrasion, such as under the chassis on a locomotive. This is accomplished through the use of options such as heat shrink, over-molding, hose and conduit.

Operational Temperatures

The VR speed sensor has the widest operating temperature range. Motion Sensors has VR models that range from -267C to 538C. Amplified models that include electronics have a more limited temperature range, most from -40C to 85C, with high temperature options up to 125C. Both price and sensor performance can be impacted by the temperature range due to material selection and design constraints, so it is important to understand the application temperature range in order to select the appropriate sensor.

Typical ranges are as follows:

Variable Reluctance Sensors

-55C to 120C
–267C to 232C
-267C to 538C

RF Speed Sensors

-74 to 204C
-74 to 400C

Electronics, Amplified and Hall Effect Sensors

-40C to 85C
-40C to 125C

General Considerations

Zero Speed

Hall Effect speed sensors offer true zero speed sensing capability, while a variable reluctance type sensor will require a certain amount of motion in order to sense target movement. RF speed sensors offer a “near-zero” speed response and are used in place of hall effect devices in applications where zero speed is desired but air gap, drag, or temperature limitations make the use of hall effect technology impractical.


In some applications, the gauss strength of the sensor is critical. For example, many turbine flow metering applications experience “low-flow” conditions where the movement of the turbine blade could be impeded by the magnetic field of the sensor. In these types of applications, it is imperative that the gauss strength is set to ensure that target rotation is not impacted, or that a solution such as an RF speed sensor, which has virtually no magnetic field, is used. Applications with shafts or gear teeth that are run by a mechanical source such as a motor or pulley would most likely not experience these types of concerns.

Intrinsic Safety

If the sensor application is located in a hazardous location, then an intrinsically safe certified device may be required. Motion Sensors can provide versions of our Variable Reluctance, RF, Amplified, Hall Effect, and Magneto Resistive sensors in intrinsically safe versions which are certified to both ATEX and CSA. We also offer certified versions of our stand-alone preamplifiers. There are special installation considerations for these products and limitations on the ambient temperature ranges of the applications.

Our ATEX and CSA certifications for intrinsic safety offer the ability to customize a sensor to suit a particular application or customer requirement under the existing certifications.

Benefits include:

  • full range of speed sensor customization options, including thread size/length, interconnection (connector and pigtails), NPT threading, and functional options (resistance, gauss strength, etc.)
  • ability to provide certified version of "legacy" speed sensor design to eliminate time and cost of full qualification (sensor characteristics equivalent to the legacy sensor)
  • shortened lead-time for development of custom ATEX or CSA certified speed sensors - typically 1-2 weeks (versus months if a new certification was required)
  • ability to provide custom intrinsically safe certified sensors in small quantities without costly fees of a revised certification

Principles of Operation


Motion Sensors (MSI) Hall Effect (HE) sensors are true zero speed sensing devices that utilize a special solid state transducer chip that produces an output voltage when subjected to magnetic field changes. Hall Effect transducers are produced in many different sensitivities, configurations and output types; analog or digital. MSI standard HE sensors fall into two general categories; oriented - requires orientation to the direction of travel of the target and non-oriented – which does not, but generally has poorer performance when used with finer pitch targets. Standard series are Oriented: H and HD and Non-oriented: HN.

Hall Effect sensors have limitations; "drag" when used with internal magnetics (zero-drag designs are possible employing external magnetic excitation), limited maximum sensing distance (compared to other technologies such as RF and maximum operating temperature compared to variable reluctance (VR)

Variable Reluctance (VR) sensors offer a unique solution to many system measurement problems by providing an uncomplicated, accurate, reliable, versatile counting device.

A variable reluctance sensor is composed of a winding wound around a cylindrical magnetic material, typically made of some type of ferrous material that is referred to as a pole piece. A magnet is attached behind the pole piece, creating a magnetic field through the pole piece and winding. This magnetic field projects out from the pole piece front, also known as the sensor tip. When ferrous material passes through and disrupts this magnetic field, electricity (a sine wave) is generated. They are passive devices and do not require any external source of power to generate a signal.

Therefore, when the tip of the sensor is placed near a moving or rotating device made of ferrous metal such as a gear or rotor, a simple technique for measuring rotational speed is created. The frequency of the signal is directly proportional to the speed of rotation. The amplitude of the signal is affected by the speed of rotation, the material being sensed and the distance, known as the "air gap", between the sensor tip and the rotating object.

Most manufacturers of magnetic sensors base their design on having very small air gaps of .005" to .015". Increasing the size of this gap produces a fall-off of signal voltage. In practical application, such small gaps could be hazardous, especially if the object being sensed is not concentric or tends to have a loss of concentricity with increased speed or motion. MSI's Premium line of VR sensors employ a special "focusing tip" which enables the sensor to operate at larger gaps in excess of .060". The closer the sensor is to the object being sensed, the greater the amplitude. MSI sensors, however, allow the sensor to be farther from the target without rapid signal deterioration as in other sensors.

Motion Sensors also manufactures a variation of variable reluctance sensors known as inductive magnetic sensors. These differ from standard VR sensors in that the magnet is not in the unit, but in the object being sensed. They are used when the rotating device sensed cannot be made of ferrous materials. For instance, if the target to be sensed is made of nonmagnetic stainless steel, brass, aluminum or plastic, a centerpole magnet may be placed in the center of the rotating device. Alternatively, magnets can be imbedded (equally spaced) into the target's outer rim or diameter. Inductive sensors will sense the magnetic fields and produce a signal. They are also designed to provide this capability at air gap distances substantially greater than standard VR sensors, as would be required for centerpole magnet locations.

In summary, variable reluctance sensors convert mechanical motion to electric energy without direct contact when positioned near a turning rotor, gear, shaft or other regularly moving device. The output signal can be fed into a digital counter, totalizer, analog converter, tachometer, overspeed or underspeed control, oscilloscope, or other monitoring and control device. The sensor provides a simple, reliable, inexpensive transducer for highly sophisticated control systems.

Advantages of Motion Sensors (MSI) RF speed sensors are: near zero speed response, no magnetic drag, large air gaps and the ability to sense non-ferrous metals like aluminum and nonmagnetic stainless steel, as well as ferrous metals. RF sensors use a modulated carrier signal to sense rotating metals. Unlike their Variable Reluctance (VR) counterparts, RF sensors require an internal or external preamplifier/signal conditioner to operate. When a target is sensed, eddy currents are formed which decrease the signal amplitude. The preamplifier/signal conditioner demodulates the signal, detects the voltage drop-offs and produces a square pulse wave (digital) output with a frequency directly proportional to the number of turbine blades, gear teeth, etc. that have been sensed.

Amplified (active) speed sensors combine high sensitivity amplifiers and signal conditioners with either variable reluctance or RF transducers. Unique features include near zero velocity (2 Hz) speed sensing, large air gap capability and several choices of pulse output.

Motion Sensors has developed a standard line of preamplifiers and signal conditioners for installation with our speed sensors. Our offerings include preamplifiers in several different sizes and configurations, as well as Intrinsically Safe options certified to ATEX and CSA. We also offer a digital to analog converter.

ATEX, IECEx, & CSA Certified

Intrinsically safe sensors are certified for use in both North America and Europe. Motion Sensors intrinsically safe speed sensors are CSA (w/ NRTL/C), ATEX (2014/34/EU), and IECEx certified to the most stringent requirements.

Our explosion proof sensors are Ex d certified to ATEX and IECEx and comply with the essential health and safety requirements relating to the design and construction of equipment and protective systems intended for use in potentially explosive environments.

If you have a sensor selection question you would like to discuss, please contact us at 252-331-2080 or eng@motionsensors.com. We will review the specifications available and work closely with you to identify any additional requirements and ensure that our solution meets your needs.