This article was updated on the 4th September 2019.
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A sensor is defined as a detector that measures the quantity of a signal or stimulus and then converts it into an electrical signal. Examples of sensor stimuli include temperature, humidity, pressure, force, chemicals and gases. A typical sensor output signal is measured as a voltage, current or electrical charge. For example, a temperature sensor senses variation in temperature expressed as a change in voltage.
Hall Effect sensors are commonly used in the automotive industry and can be found on most vehicles to measure the position of the crankshaft or camshaft. These sensors work on the principle that an electrical current flows through a detector triggering a magnetic field to push the electrical charge onto the opposite end of the conductor, a process that is referred to as a pulse. By measuring the number of pulses, the speed of a vehicle can be calculated.
The Hall Effect
The basic principle of the Hall Effect is based on a thin metal sheet of semiconducting material, such as copper, that carries electrical current, normally supplied by a battery device. The output connections are perpendicular to the direction of the current. When there is no magnetic field, the current distribution is uniform and the potential difference across the output is 0 volts.
When a perpendicular magnetic field (B) is present and positioned at a right angle to current flow, a voltage disturbing the current is exerted across the semiconducting metal plate, and this current flow can then be measured by probes. The potential difference in voltage across the output is termed the Hall voltage (VH). The Hall voltage is proportional to the vector cross product of the current (I) and the magnetic field (B). The video below introduces a visual aid to explain the basic principle of Hall Effect sensors.
Hall-Effect Based Current Sensors
We have described the mechanism of the Hall Effect across a metal platform; however, metals have a high carrier density (often referred to as carrier concentration) per unit volume, which makes it difficult for them to exhibit the Hall Effect at any given current. In contrast, semiconductor materials including silicon, germanium, and gallium arsenide are three commonly used materials with a low carrier density that display the Hall Effect more strongly.
The Hall Effect found its first applications with the advent of semiconducting materials in the 1950s. Hall Effect sensors can be applied to various types of sensing devices, where the quantity of a sensory stimulus incorporates or can incorporate a magnetic field. It should be noted that Hall Effect sensors require amplifiers for practical applications, as the order of Hall voltage is 7µV/Vs/gauss.
Design of Hall Effect Sensors
Hall Effect sensors comprise a thin piece of rectangular p-type semiconductor material such as gallium arsenide (GaAs), indium antimonide (InSb) or indium arsenide (InAs) passing a continuous current through itself. A bias current (I) is introduced via two contacts (the current contacts [CC1 and CC2] and the sense contacts [SC1 and SC2] – both types of currents are positions at the edge of the semiconducting plate).
When the device is placed within a magnetic field, the Hall voltage is detected between the sense contacts. This Hall voltage that appears between the sense contacts is directly proportional to the output (product) of the magnetic induction, which provides a measure of current density (j). By knowing the semiconductor plate thickness (t) and its width (w), the current density can be calculated:
J = I/tw
Hall Effects Sensor Outputs
Hall Effect sensors are available with either analog or digital outputs. Analog output sensors provide linear output values and are taken directly from the output of the operational amplifier with the output voltage being directly proportional to the magnetic field passing the Hall sensor.
Digital output sensors employ a Schmitt trigger with built-in hysteresis connected to the op-amp. The sensor device switches between the “off” and “on” condition, when the sensor output exceeds a preset value.
Classifications for Hall Effect Digital Output Sensors
Digital output sensors are classified into bipolar and unipolar sensors, based on their input characteristics. The input characteristics of a digital output sensor are defined as a Maximum Operate Point and Minimum Release Point. The output states to the digital sensor are “on” or “off.” These two states can be referred to as Hall Effect switches to indicate whether or not a magnetic field is present. The Hall Effect switch will behave like a sensor; when a magnetic field is present, a digital output is generated. A drop in the magnetic field forces the Hall Effect switch to drive the output inactive.
If the Maximum Operate Points and Minimum Release Points of a digital output sensor are positive (i.e., the south pole of a magnetic field), then the sensor is referred to as Unipolar; whereas, a bipolar sensor has a Maximum Operate Point (south pole) and a negative Minimum Release Point (north pole). Though three combinations of actual operate and release points are possible with a bipolar sensor, a true latching device using a bipolar sensor will always have a positive operate point and a negative release point.
Advantages of Hall Effect Sensors
The advantages of using Hall Effect sensors are as follows:
- Production of an output voltage signal independent of the rate of the detected field.
- Hall Effect sensors are not affected by ambient conditions, such as dust, humidity, and vibrations and are insensitive to some ambient conditions based on the principle that these sensors display a constant flow of electrical current, making their characteristics constant over time.
- Hall Effect sensors do not have contact with neighboring mechanical parts, making these sensors strong and sensitive enough to detect movement. These sensors do not wear over time thus maintain quality and unlimited use.
- Hall Effect sensors are built from semiconductor materials that display low carrier density, hence conductivity is smaller and their voltage is larger.
- Hall Effect sensors depend on carrier mobility, which eliminates any perturbations due to surface elements, and as a result these conductors reproducible and highly reliable.
- High-speed operation is possible.
- Hall sensors can measure zero speed.
- Hall sensors work over a wide temperature range, provide highly repeatable operation, and are capable of measuring a large current.
Disadvantages of Hall Effect Sensors
The Hall Effect sensor does have its disadvantages:
- The Hall Effect sensor is not capable of measuring a current flow at a distance greater than 10 cm; however, use of a magnet strong enough to generate a magnetic field wide enough may make this possible.
- Hall Effect sensors work on the principle of a magnetic field, making it possible for external magnetic fields to interfere and bias the measurement of current flow.
- Temperature affects the electrical resistance of the element and the mobility of majority carriers and also the sensitivity of Hall Effect sensors.
- Even with well-centered electrodes, the offset voltage still presents an output voltage in the absence of a magnetic field.
- An offset voltage occurs when there are physical inaccuracies and material non-uniformities. It can be as high as 100 mV for a 12 V source. To solve this problem, an additional control electrode would need to be added and through this, a necessary current can be injected to obtain a null output when no magnetic field is present.
Applications of Hall Effect Sensors
Hall Effect sensors are considered as magnetic sensors with a wide range of applications. Some of the most popular and effective ways of utilizing Hall effect sensing devices are listed below for both analog and digital output sensors.
Analog output sensor applications include:
- Current sensing
- Variable speed drives
- Motor control protection/indicators
- Power supply sensing
- Motion sensing
- Diaphragm pressure gage
- Flow meters
- Direct current electricity
- Encoded switches
- Rotary encoders
- Voltage regulators
- Ferrous metal detectors (biased Hall)
- Vibration sensors
- Magnetic toner density sensor
- Rotational speed sensor
Digital output sensor applications include:
- Wireless communication
- Pressure sensors
- Proximity sensors
- Flow sensors
- Valve position sensors
- Lens position sensors
- Shaft position sensors
Although Hall Effect sensors have been generally made using semiconductor materials, a recent study investigated the use of graphene for making these sensors. The results showed that graphene-based sensors showed a high magnetic field sensitivity, but there was a significant drift in the sensor offset voltage with time and temperature. Commercialization of these sensors can only happen if these drifts are minimized.
Another effort reported self-compensating Hall Effect sensors for use in large telecom platforms. This study improved traditional Hall Effect sensors by compensating for the effects of temperature, ageing and radiation drifts, reducing errors in measurements. This allowed for a galvanically isolated current sensor for measuring high current values with very low losses and power dissipation.
One recent application of the Hall Effect sensor is in computer keyboards. Although they are used in keyboard applications was studied in the past, the high price prevented the commercialization of keyboards based on the sensors. The decrease in price because of mass production methods has enabled such applications. A magnet switch was designed using Hall Effect sensors for analog input. The Hall Effect sensor was customized for reading a consistent analog input range. Use of this configuration allowed the keyboard to be bounce-free, have smoother clicks and can last for about 100 million clicks.
Hall Effect sensors were developed in 1950 and have come a long way to become a major requirement for several industrial and automotive applications.
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