Hall effect proximity sensors are used to detect the proximity, presence or absence of a magnetic object using a critical distance. They function via an electrical potential that is developed across an axis transverse to an applied current flow in the presence of a magnetic field. As a magnetic target approaches the sensor the electrical potential increases and passes a threshold that marks a critical distance locating the target.

Vh = IB/ned

Vh = "Hall Effect Potential"
I = "Current"
B = "Magnetic Field"
n = "Electron Density"
e = "Electron Charge"
d = "Width of Metallic Strip"

Video Credit: nailphysics / CC BY 3.0

Applications

Hall effect proximity sensors are capable of rapid switching speeds and are commonly used to measure the rotational velocity of shafts, motors, gears and other fast moving targets. In some cases, they are employed in a limit switch fashion. In these cases, they are similar in appearance to a contact limit switch, but the sensor is separated from the switching mechanism and provides a limit for the travel detection signal.

Selection Criteria

When searching for hall effect proximity sensors, sensing performance and switch performance are the most important parameters to consider. Other considerations include body type, electrical connections, and various features.

Sensing Performance

Specifications for sensing performance include operating distance and repeatability.

• Rated operating distance is the critical distance at which switching takes place. It is important to choose a sensor that will operate in the required sensing range. This could be dictated by process requirements and mounting option.
• Repeatability is the distance within which the sensor repeatably switches. It is a measure of precision. Depending on the application, precision could be an important design criterion when selecting a sensor.

Electrical Switch Performance

Depending on the power options available, power requirements may be a key specification when selecting a sensor.

• The device can be powered by either an AC or DC power source.

Load configurations are important parameters to consider. Hall effect proximity sensors may switch an AC load or a DC load. DC load configurations can be NPN or PNP.

• NPN is a transistor output that switches the common or negative voltage to the load; load connected between sensor output and positive voltage supply.

• PNP is a transistor output that switches the positive voltage to the load; load connected between sensor output and voltage supply common or negative.

Wire configurations are 2-wire, 3-wire NPN, 3-wire PNP, 4-wire NPN and 4-wire PNP.

2-Wire DC circuitry typically includes a diode bridge in series with the sensor, allowing functionality either as NPN or PNP in a DC circuit.

Acceptable values of leakage current must be evaluated in this type of circuit.

Switch types can be normally open (NO) or normally closed (NC).

 Image Credit: Fargo Controls, Inc.

If the switch is a NAMUR type switch or a specialized switch for switching a resistive load. It will require an external amplifier.

Body Type

Hall effect proximity sensors are typically housed in either a barrel or rectangular housing, although limit switch, slotted or "U" channel switches and ring or "doughnut" configurations are also available. 

• A barrel body style is cylindrical in shape. The barrel of the sensor is typically threaded so that with the help of two lock-nuts the sensor can easily be adjusted closer or further from the target as necessary.
• A limit switch body style is similar in appearance to a contact limit switch. The sensor is separated from the switching mechanism and provides a limit of travel detection signal.
• A rectangular or block body style is a one piece rectangular or block shaped sensor.
• A slot style body is designed to detect the presence of a vane or tab as it passes through a sensing slot, or "U" channel.
• A ring shaped body style is a "doughnut" shaped sensor, where objects pass through the center of the ring.

Electrical Connections

Electrical connections for hall effect proximity sensors can be fixed cable, connector(s), and terminals.

• A fixed cable is an integral part of the sensor and often includes "bare" stripped leads.
• A sensor with connectors has an integral connector for attaching into an existing system.
• A sensor with terminals has the ability to screw or clamp down.

Features

There are a number of features that may be important to consider when selecting hall effect proximity sensors.

Field adjustable sensors are required when adjustments need to be made while the proximity sensor is in use. Potentiometers are commonly used to control the magnetic field strength or detection range.

Self-teaching proximity sensors can be auto-calibrated to trigger the switch at a specific target location.

Materials of construction and enclosure ratings must be considered when the sensor will be located outdoors or exposed to extreme temperatures, or under the influence of wet, humid, dusty, dirty or corrosive process conditions.

Enclosure ratings are specified by "National Electrical Manufacturers Association (NEMA)" or "Ingress Protection (IP)" ratings.

Resources 

HyperPhysics - Hall Effect

Image credits:

PHD Inc. | elobau sensor technology, Inc.