Type of Output
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 dependant 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.
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
RF Speed Sensors
Electronics, Amplified and Hall Effect Sensors
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 flowmetering 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.
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.