Speed Sensor Types

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

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.

Cryogenics

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.

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

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.

Drag

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