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Replacing A Speed Sensor Isn't Easy:
A guide to finding replacement magnetic field sensors in the Sensor Solutions Catalog.
Magnetic field sensors are commonly used for non-contact speed measurement. Determining a replacement that will work in a specific application can be difficult. Sensor Solutions magnetic field sensors are designed using a modular manufacturing concept. This allows us to put the same sensor functions into a wide variety of packages with the shortest lead-times in the industry. Currently over 6,000 unique sensors are available in our Catalog. This Application note specifically discusses Hall Effect and other sensors used to measure speed and provides guidance in selecting the appropriate options for your application when searching for a replacement speed sensor.
If you would like assistance in selecting a sensor, you can contact our Engineering or Sales Departments via email. Or you can call us at 970-879-9900.
Determining the Sensor Type and Characteristics
To determine the type of sensor you need, first you need to know what type of target the sensor is detecting. Most sensors produce one pulse per every tooth of a gear, hole in a tone ring, or magnetic pole that passes the sensor face. The target will define the type and characteristics of the sensor required.
Ferrous Targets: Gear teeth, slots in ferrous wheels, ferrous bolt heads, etc.
This sensor type detects any ferrous discontinuity, teeth, holes, bolt heads, etc without the sensor requiring power. They typically have 2 wires (or a 2 pin connector) and may have a pole piece sticking out of the face.
This sensor type is the best choice for detecting standard, evenly spaced gear teeth, and are Active true zero Hall Effect speed sensors.
This sensor type is the best choice for detecting unevenly spaced teeth on gears along with other ferrous discontinuities like bolt-heads or widely spaced holes.
Magnetic Targets: One or more rare earth magnets or magnetic fields.
Magnets have at least 1 North and 1 South Pole. There are multiple types of Hall Switch options. South Pole detection is the most common; Either Pole and North Pole only options are also available. Some magnets produce very little field, so they need a very sensitive Hall (or MR) Switch. Some magnets produce a large field, and will triple-trigger a sensor that is too sensitive. Some magnets have multiple-poles, so there are multiple output pulses from one magnet. A 16 Pole magnet might have 8 North Poles and 8 South poles; you need to know if the controller is looking for 8 pulses or 16 pulses, then use a S pole Hall switch or an Either pole Hall switch as required.
These sensors are active Hall Effect sensors whose output switches when one magnetic pole is introduced, but holds that output until the opposite pole is introduced. These sensors would produce 8 pulses on a 16 pole magnet.
Reed switches are passive sensors that will open or close a relay each time a magnetic pole is introduced. As mechanical switches, their life is defined by the amount of power passing through them. Therefore they are not recommended for high frequency applications.
Resolution/Scaling: A replacement sensor needs to produce the same number of pulses per revolution as the existing sensor.
If the old sensor provided 8 pulses per rotation from a 16 pole magnet wheel and you replace it with an Either Pole Hall switch that produces 16 pulses, your reading will be twice what is expected. If you are changing your target or sensor type, be sure to either match the number of targets as the original or change the scaling in the controller.
Selecting Sensor Attributes
Once you have determined the type of sensor, you can narrow down the product line by selecting attributes shown on the left side of the page. Below we have listed the attribute types and what they mean. If you do not find the selection you want from the attributes, contact our Engineering Department to discuss alternative options to those listed in our catalog. In many cases we can provide a new solution with existing materials.
When narrowing down sensor selections start with the attributes that are most important to your application. If you no longer see options that meet your requirements it may be that an attribute selected eliminated those sensors. The box can be unchecked to bring options back.
Functional Features will narrow down sensors within a category by specific features, regarding speed selection they may indicate whether the sensor is providing digital pulses versus a sine wave output, or if sensors are better for coarse vs. fine target pitches (gear tooth sensors), sensitivity (Hall Switches), or other features that will be specific to the type of sensor.
Electrical Features are related to the power supply or outputs, common electrical features are detailed below
- Internal Pull Up or Pull Down Resistors:
- Sensor Solutions magnetic speed sensors are commonly offered with 5K internal pull ups to be compatible with 5-24V power sources, most controllers work fine with a wide range of different pull up resistors. Sensors can be built with larger or smaller pull up values if required, contact our Engineering department to discuss requirements for your specific application.
- Open Collector Output:
- These sensors expect a pull up resistor to be built into the controller that is reading the sensor. Sometimes even if the controller is expecting an open collector and you use a sensor with an internal pull up, the pull up values are in parallel and the signal is the same. Other times, the sensor is powered with 12 volts and the internal pull up resistor is pulled up to 5 volts; adding an external pull up resistor to the sensor may blow up the controller.
- EMI Filters and Load Dump Protection:
- These options include additional protective circuitry for applications where EMI interference, or power stability issues may be a concern such as automotive and equipment applications powered by 12V and 24V batteries. These options can be applied to sensors with and without internal pull up resistors.
Combined with producing the correct number of pulses, a replacement sensor has to work with the same supply voltage and produce the same output as the existing sensor. Dependent on the sensor type required there will be multiple options. A few sensors are offered with multiple output types, the secondary output type will be listed under Output 2 in the attributes
These current sinking sensors provide square wave outputs that switch between 0V and the supply voltage. These sensors typically have the output pulled to the high state with no target present, and then output switches low when the target is present. The output is either pulled to the high state by a resistor within the sensor, or the pull up may be in the controller.
These current sourcing sensors provide square wave outputs that switch between 0V and the supply voltage. These sensors typically have the output pulled down to the low state with no target present, and then output switches high when the target is present. The output is either pulled to the low state by a resistor within the sensor, or the pull down may be in the controller.
Sine Wave Output
VR sensors produce a sine wave for every ferrous tooth that passes the sensor face. These sensors do not require power. The sine wave is very low amplitude peak to peak at slow speeds. This amplitude increases with speed. There are hundreds of different electrical variations of VR sensors, some designed for fine teeth and some designed for larger teeth. In general, if you get enough amplitude, you will trigger the controller that reads the signal. Since the amplitude increases with speed, you need to make sure your maximum signal peak to peak does not blow up the controller that reads the signal.
Reed Switch Sensors and some Hall Switches provide an output that opens or closes a relay dependent on the presence or absence of a target magnetic field. The relay switching can be counted by some controllers for resolving speed.
TTL Type outputs will provde 0-5V digital square waves regardless of if the sensor is powered with 5V, 12V, or 24V.
When narrowing options by the Supply Voltage to be provided to the sensor, be sure to check all boxes where your supply voltage fits in that range. Upper and lower limits of supply voltage are dependent on the sensing element as well as circuit protection and regulators that may be found within the sensor.
Sensor Solutions offers sensors primarily in Stainless Steel, Anodized Aluminum, and molded plastics. If your application has a specific material requirement or grade of steel contact our Engineering department to discuss requirements for your specific application.
Sensors are available in threaded packages with lock nuts for mounting into brackets or threaded ports. These are referred to as "Threaded Barrel" housings. They are also available in packages where one or more bolts mount into a flange to position the sensor and in smooth cylinders (refered to as smooth barrel)
For applications where the diameter or specific pitch of threads is critical, Sensor Solutions offers threaded barrel housings in a wide variety of pitches. You can select one or more thread pitches to narrow down selections for your application.
Housing Length In Inches
In this attribute you can either select specific housing lengths, or use the min and or max fields to define upper or lower limits to the length of the housing. This length is the distance from the sensing face to where the connection exits the sensor. Integral connectors do increase the length of the sensor.
Sensor Solutions offers the following 6 categories of Connection Types
Free End Wires
Free end wire connections will have individual insulated wires exiting the sensor. These are typically low cost connections and they also allow power and ground to be wired to a power supply and the output(s) to be wired to a meter or controller without jumper wires.
Ribbon cables are bonded free end wires. They are also a low cost solution for a connection type, but remain together unless pulled apart during or after installation. Sensor Solutions currently only offers a 3 conductor ribbon cable option.
Jacketed cable connections will have all the sensor wires enclosed in an outer jacket. The sensors will be provided with a couple inches of this jacket removed at the end of the cable.
Shielded cable connections are like jacketed cables, but also have a shield around the wiring to protect the signals from external interference. These cables will either have a foil shield or a braided shield along with a drain wire. Note: Sensor Solutions does not tie the shield to the sensor housing during assembly.
Sensor Solutions offers many sensor types with options to have M12 circular connectors, MS style connectors, and other options integrated directly into the back of the sensor housing. The advantage of integral connections is that it allows the sensor (or wiring harness) to be indivually replaced instead of needing to re-run the cable if the sensor is damaged or replace the sensor if the cable only is damaged.
Pigtail connectors are available with a wide variety of options including Deutsch DT Series Connectors, Paccard Connectors, Amphenol Connectors, MS Style Connectors, M12 circular connectors, and other specialty options. Standard pigtails have defined lengths and wire types. Custom pigtails can also be designed to meet the specific requirements of the application.
If the application requires a specific size of wire, it can be selected here. NOTE: Most magnetic field sensors use very little power and are running on low voltage systems. 26AWG wires are more than adequate to handle power consumption of most sensors.
Connection lengths are sorted in categories. If the connection length required for your application is not available contact our Engineering department to discuss the specific requirements of your application.
The insulation material typically determines the temperature range and durability of the cable. For jacketed and shielded cables the listed insulation material is for the outer jacket. The inner conductors may have the same or different insulation materials.
If you have questions about selecting sensors or are not finding what you need in our catalog, give us a call at 970-879-9900 or contact our Engineering department through the linked form with the requirements for your speed sensing application and we can assist in determining the best sensor for your application.
AN102 Rev B. Written By Dave Lorenzen 08/29/13, last updated by Casimir Bruski 05/19/17