1. How do Hall effect proximity sensors work?
Hall effect proximity sensors are non-contact sensors that use the Hall effect to detect the proximity of magnetic or ferrous targets to the sensors face. The Hall effect is a phenomenon whereby a magnetic field and electrical current applied to an electrical conductor perpendicular to one another, interact to form a voltage difference perpendicular to both the electrical current and magnetic field (illustrated below). The voltage difference is known as the Hall voltage and its polarity depends on whether the magnetic pole facing the electrical conductor is a north or south pole. For a fixed electrical current through the conductor, the Hall voltage is proportional to the magnetic flux density. Since the magnetic flux density increases as the magnet approaches the electrical conductor, the Hall effect can be used to measure the proximity of the magnet to the conductor.
There are two common configurations for Hall effect sensors, one for detecting magnetic targets and the other for detecting non-magnetized, ferrous targets (illustrated below). The configuration for detecting magnetic targets consists of an electrical conductor (typically a semi-conductor) and electronics which supply the electrical current and measure the Hall voltage. As the magnetic target is brought into proximity with the sensor, a Hall voltage is generated is proportion to the distance of the target from the sensor.
The configuration for detecting ferrous targets includes the same electrical conductor and built-in electronics, with the addition of a permanent magnet placed behind the conductor. The presence of the permanent magnet results a Hall voltage across the conductor. As a ferrous (e.g. steel) target approaches the sensor, it attracts the magnetic field such that the flux density becomes more concentrated (and therefore stronger) in the region between the permanent magnet and ferrous target. This causes an increase in the Hall voltage as a function of the distance between the ferrous target and the sensor.
Magnetic target detection
Ferrous target detection
Hall effect proximity sensors emit either analog or Boolean output signals. Analog output type Hall effect sensors convert the Hall voltage to a 0-10 V or 4-20 mA output which is (ideally) a linear function of the distance between the target and the sensor. Boolean output type Hall effect sensors utilize a comparator to convert the Hall voltage to a logic 0 or logic 1 output depending on whether a threshold voltage has been crossed. The comparator used in proximity sensors is called a Schmitt trigger and it differs from a regular comparator in that it has built in hysteresis. The practical effect of the hysteresis is illustrated below. As the target moves to within 5 mm of the sensor, the sensor is triggered high. However, the sensor will only be triggered low once the target moves to at least 6 mm away. This prevents an oscillation of the sensors output state around a particular value.
2. General characteristics of Hall effect proximity sensors
Hall effect proximity sensors are low cost sensors intended for measuring the proximity of either magnetic or non-magnetized ferrous objects (depending on the sensor configuration) at distances up to 20 mm. They are available with either analog or Boolean output signals. Hall effect proximity sensors have poor output linearity. As such, the analog models are not recommended for applications where an accuracy of greater than 10% of their full scale range is required. Hall effect proximity sensors are non-contact and solid state meaning that they are easy to assemble, do not experience mechanical wear and have an extremely long lifetime. They typically operate at sampling rates of 10-30 kHz. As well as having excellent ingress protection, their readings are not affected by contamination of the sensor surface or target object with liquid, dirt or non-ferrous, debris.
3. Input and output signals
Hall effect proximity sensors operate on a 4.5-30 Vdc supply voltage. Analog output sensors convert the Hall voltage to a 0-10 V or 4-20 mA output signal. If the sensor includes a built in permanent magnet (i.e. intended for detecting ferrous targets) then a 0 V output represents no target detected and a 10 V output represents a target extremely close to the sensors face. If the sensor does not include a built in magnet (i.e. is intended for detecting magnetic targets), then there are two common output configurations depending on whether the sensor is designed for detecting both north and south poles or only a single pole. If the sensor is designed for detecting a single pole, then 0 V signifies no magnetic field and 10 V signifies the maximal magnetic field. If the sensor is capable of detecting both magnetic poles then a 5 V output indicates no magnetic field, 0 V indicates the maximal magnetic field of one pole and 10 V the maximal magnetic field of the other pole.
Single pole detection
North and South pole detection
Boolean output sensors convert the Hall voltage to either 0 V for a logic 0 or the supply voltage (less the voltage drop across the transistor) for a logic 1, in accordance to whether a threshold Hall voltage has been crossed. If the sensors output is normally open (NO) then movement of the target to within sensing range will cause the sensor output to switch from logic 0 to logic 1. If the sensors output is normally open (NC) then movement of the target to within sensing range will cause the sensor output to switch from logic 1 to logic 0. Most Boolean output sensors have either a NO or an NC output, with the exception or 4 wire sensors which have both a NO and a NC output.
4. Applications of Hall effect proximity sensors
Hall effect proximity sensors are extremely common and used in a wide range of position measurement applications where accurate position measurement is NOT required. This includes environments with significant dirt and oil contamination. They are particularly popular for measuring motor shaft and gear speed because of their high sampling rates (as high as 30 kHz).
5. Typical specification
|Measurement range||1-20 mm|
|Sampling rate||10-30 kHz|
|Velocity||Limited only by sampling rate|
|Hysteresis||5-15% (Boolean models only)|
|Life time||Very high|
|Ambient temperature||-40 to +110°C typical (up to 150 °C available)|
|Supply voltage||4.5-30 Vdc|
|Output signal||Boolean or analog|
|Vibration resistance||10 g|
|Shock resistance||100 g|
|Ingress protection||IP67 typical (IP69 available)|
|Passive / active||Active|
|Contact / non-contact||Non-contact|
6. Purchasing tips
- Flush or non-flush housings: Proximity sensors are available in either flush or non-flush housings. Flush housings extend the entire length of the sensor, enabling the sensor face to be mounted ‘flush’ to a wall (hence the name). Non-flush housings do not extend the full length of the sensor. A flush housing also acts as an electromagnetic shield and so flush housed sensors are shielded whereas non-flush housed sensors are not. The shielding reduces the sensors measurement range and field of view.
- NO/NC: Boolean output Hall effect proximity sensors are available with either normally open (NO) outputs, normally closed (NC) outputs, or both.
- Number of wires: Analog output sensors may have 2 or 3 wires and Boolean output sensors may have 2, 3 or 4 wires. Sensors with only 2 wires often lack reverse polarity and short circuit protection. Boolean output sensors with 3 wires have either a normally open (NO) or normally closed (NC) output. Boolean output sensors with 4 wires have both a NO and a NC output.
- Adjustable threshold: Many Boolean output sensors enable the user to adjust the threshold measurement distance at which the sensor output changes state.
- PNP/NPN: The transistor used to switch output states (in response to the Schmitt trigger) can be of either PNP or NPN type (also known as current sourcing and current sinking types respectively). When interfacing the sensor to a PLC, it is vital that the sensor is of the opposite type to the PLC port e.g. a current sourcing sensor will only work with a current sinking PLC port.
- Magnetic field strength: The datasheets of Hall effect sensors intended for detecting magnets, will provide the range of magnetic field strength (units of Gauss) that the sensor can detect. The measurement distance can be calculated from the strength of the magnet.
7. Advantages of Hall effect sensors
Hall effect proximity sensors:
- Are normally the lowest cost proximity sensors, with the exception of passive proximity switches such as mechanical switches.
- Consume less power than other proximity sensors, making them well suited for battery powered applications.
- Are suitable for use in harsh environments (except for environments containing large magnetic fields).
- Have sampling frequencies as high as 30 kHz, higher than both inductive and capacitive proximity sensors (though not as high as eddy current sensors).
8. Disadvantages of Hall effect sensors
Hall effect proximity sensors:
- Are only capable of detecting magnetic and ferrous targets, limiting their range of applications.
- Have a slightly smaller maximum measurement range than many other types of proximity sensors.
- Have poor signal linearity (as do capacitive and inductive sensors) and are sensitive to changes in temperature. Although this is less of a problem for Boolean output sensors, analog output sensors are not recommended for applications where an accuracy or more than ±10% of their full scale range is required.
- Are sensitive to the magnetic field strength of the magnet. When using the sensor to detect a magnet, the detection range will depend on the strength of the magnet.
- Are sensitive to the size of the target. Using a target smaller than the face of the sensor will reduce the sensing range.
- Can be affected by external magnetic fields which may result in false readings.
9. Application tips
- Magnetic pole: A magnetic north pole produces a Hall voltage of opposite polarity to that produced by a magnetic south pole. When using a Hall effect sensor to detect a magnet, keep in mind that whilst some sensors are designed to work with either pols, others will only work with one pole. It is therefore necessary to setup the magnetic pole according to the sensor’s specification.
- Accuracy: The output linearity of Hall effect proximity sensors is very low at either extreme of their measurement range. As such, if using an analog output sensor, aim to avoid utilizing the first and last 10% of its measurement range.
- Installation: DO NOT countersink a non-flush sensor into a ferrous metal casing as this could result in a false signal as the sensor detects the metal casing.
- Multiple sensors: Multiple Boolean output sensors can be connected in series or parallel to a single process controller input port to achieve logic functions. Two sensors connected in series act as an AND gate. Two sensors connected in parallel act as an OR gate. The circuits used to implement parallel and series connections can be found here
- Leakage current: The leakage current (i.e. the residual current flowing when the output is at logic 0) of a 2 wire Boolean output sensor may result in a sufficiently high voltage to cause a reset error. This can be avoided by placing a bleeder resistor in parallel with the controller.