Hall Effect & Reed Switch -- Magnets for speed and position sensing using magnetism
Magnetic sensors interact with magnetic fields from electromagnets, solenoids and permanent magnets. They do not require any physical contact making them potentially highly reliable with long lifecycle due to no wear contactless position sensing. Magnetic sensors can work through most non-magnetic barriers (materials capable of having eddy currents would have limitations e.g. aluminium, copper).
Some of the magnetic sensors have a voltage induced in them e.g. Hall Effect devices in Gaussmeters the voltage is proportional to the magnitude of the applied magnetic field from the magnet. The magnitude of the applied magnetic field from the magnet is a function of the magnet size and shape and the magnetic material used (magnet type, magnet grade, temperature of the magnet).
Magnetic field sensors are used to give information relating to speed, position, movement, rotational angle, etc.
Magnetic field sensors include:-
Hall Effect Devices
Giant Magneto Resistive Devices (GMR)
Variable Reluctance Devices (VR)
Reed Switch Magnets
Reed Switches are amongst the simplest of all the magnetic field detection devices. It consists of two thin ferromagnetic plates (usually nickel-iron or nickel-cobalt) called reeds within a glass ampoule (casing) which overlap each other except for a small gap between them so they are not quite touching.
When a magnetic field nears the reed switch, a magnetic field is induced within the reeds, magnetising them. Since they carry magnetism because they have high magnetic permeability, the two reeds magnetically attract each other closing the gap until they mechanically touch (when the applied magnetic field is strong enough to cause the magnetic pull to exceed the spring force trying to open them out). The reed switch is closed so, if in an electrical circuit, would allow current to flow through it. Reed switches close when the magnetic field is large enough they activate regardless of whether a North or a South pole is present. As soon as the magnetic field is reduced enough the spring force exceeds the magnetic pull force and the reed switch opens again and any electrical circuit is made open circuit.
A typical example is in security systems a magnet holds the Reed Switch closed when a window closed; when the window opens the magnet is taken away and the switch opens changing the electrical circuit and hence setting off the alarm.
Although a few versions exist, the most commonly used is of the above description which is called a SPST-NO (Single Pole Single Throw Normally Open Reed Switch).
One issue with reed switches is how they are rated. Reed Switch Suppliers use an Ampere-turn rating (A-t) for activating their reed switches (due to their using electromagnet coils with a certain number of turns and an applied current). In the Magnetics Industry the units are Gauss, Tesla or perhaps even Oersted (units of magnetic field used for magnets). The units are not easily interchangeable. A very rough guide is:-
B(Gauss)=0.4 x pi x NI(Ampere Turns) / L(cm) x 1.2(safety factor)
Where L is derived from the coil that the Reed Switch manufacturer uses. The coil may perhaps be a standard coil known as a Narm 1 coil that has a length, L, of 1.039 cm if the coil length is not known, using L=1.039 is a good estimate to consider using.
Simplified rough guide (assuming L=1.039cm)
B(Gauss)=1.451 x NI(Ampere Turns)
However, changing the reed switch terminal lengths, bending the terminals and altering the direction from which the magnetic is brought towards the Reed Switch can all affect the above rough guide. So the Gauss value is shown to be anything from 1 to 3 times the NI value (some Reed Switch companies even suggest up to 10 times). So perhaps testing with sample magnets beforehand may be advised if the Reed Switch company only offers Ampere turn data.
Sometime a magnet is offered with the Reed Switch this is fine. However the trigger positions for the Reed Switch may not be ideal for your application, in which case you may be able to change to another magnet type or magnet size to change the operation zone of the Reed Switch. If you have a magnet of known size, known material type and known material grade and you know how far the Reed Switch is from the magnet before it activates, it is often relatively easy to work out a new magnet shape for activating at a different distance away from the magnet.
We will give Gauss values (1 Gauss = 1 Oersted = 0.0001Tesla) and have assumed 1A-t~1.451Gauss (bearing in mind the A-t to Gauss conversion may need altering to suit your application).
Hall Effect Magnets
Hall Effect devices usually have three connections a Vcc (dc power supply positive terminal), Ground (dc power supply negative, zero or ground) and Vo (dc output voltage).
Hall Effect devices differ to Reed Switches in that they are more sensitive to the magnitude, direction and polarity of the magnetic fields being applied. Gaussmeters uses Hall Effect devices to operate.
The Hall Effect device is a semiconductor which has an electrical current flowing through the semiconductor - a voltage is created (Hall Voltage) when a magnetic field is perpendicular to the active element of the semiconductor. The Hall Voltage is proportional to the magnitude of the perpendicular magnetic field. The magnetic field causes the charge carriers, electrons and holes in the semiconductor to move to one side of the semiconductor, creating a potential difference across the semiconductor which is the Hall Voltage. Usually, Hall Effect devices output zero voltage output when no magnetic field is applied.
Some Hall Effect devices only work when a South Pole of a magnet face them; others only work with a North Pole facing.
Hall Effect devices are sensitive to the angle of the magnetic field relative to the active element of the semiconductor. When the field is perpendicular, maximum voltage is possible, as the angle reduces from 90 degrees, the Hall Voltage will fall by a ratio of the sine of the angle (sine90=1, sine60=0.866, sine45=0.707, etc).
Ideally the voltage output is linearly proportional to the applied field strength. The voltage is also linked to the perpendicularity of the applied field - a Sine (angle) relationship exists e.g. maximum output when the field is 90 degrees to the semiconductor active element. Some electronic chips have multiple Hall Effect Devices to allow measurement of North and South Magnetic fields. Some Hall Effect chips contain more than one device and allow applications such as 3D field mapping to be possible. Other chip variants also exist e.g. some Hall Effect Devices allow temperature correction.
Some Hall Effect devices give an output of half the supply voltage when no magnetic field is present then increase of decrease the voltage depending on whether a North or South magnetic field is applied.
Some Hall Effect devices have trigger levels to provide a digital high or low to give clean digital switching they are often known as Hall Effect Switches.
Some Hall Effect devices have trigger levels to provide latching ability linked to a digital high or low output to give clean digital switching (staying on one state with a North and not changing state until a South is applied) they are often known as Hall Effect Latches.
Some Hall Effect devices have Schmitt triggers built into them to help avoid "chattering" when the device is near its trigger value to change state.