Speed sensor: Working principle | Types | Applications

Typical view of Speed sensor 

What is speed sensor?

The speed sensor belongs to the tachometer category. It is a device used to measure the speed of wheel rotation of a vehicle. The speed sensor was initially used to replace the mechanical connection between the rotating wheels and the speedometer, reducing the use of cables and facilitating construction by reducing rotating parts. These sensors also generate data that allows automatic driving to take place.

The speed of an object is the magnitude of the change of its position; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance travelled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as the duration of the time interval approaches zero. Speed has the dimensions of distance divided by time. The SI unit of speed is the meter per second, but the most common unit of speed in everyday usage is the kilometer per hour.

Major types of speed sensors:

There are many types of speed sensors, here we will see about the sensor which is the most important of them;

a. Hall Effect Sensor

b. Inductive Sensor

c. Optical sensor

d. Reed switch

e. Potentiometers

Working principle of speed sensor:

a. Hall Effect Sensor:

Hall Effect sensors are highly sensitive to magnetic fields. They can be used to measure linear and rotary as well as for speed and direction sensing. The main component of these sensors is the Hall IC and the permanent magnet. The Hall Effect is based on thin semiconductor chips. If such a current-carrying plate is penetrated vertically by permanent magnet, charge carriers are deflected perpendicular to the field. The current, provided by an external power supply is deflected from its otherwise straight path. So, a voltage, can be detected which occurs across the current direction between two opposite edge points of the plate. The output voltage, is proportional to the induction, and the current. This output voltage is sent to an electronics board and converted to a equal speed value and screened. 

The magnetic field generated by the permanent magnet is affected as it approaches the external ferromagnetic material sensor. Especially for speed sensing, the specifications of the target wheel and the air gap must be compatible with the Hall sensor.

b. Inductive Sensor:

An inductive Encoder also known as an Incoder, is a contactless speed and positioning sensor suitable for linear and angular displacements. Based on the electromagnetic induction principle the sensor measures. Main components of an inductive sensor are spools or printed circuits on board. When a metal target approaches the inductor a change of magnetic field happens. As per the law of induction, the sensor creates a voltage at outlet, which is proportional to the changing of a magnetic flux. This allows a current to flows through the inductor and connected wires to the output device. The sensing range depends on the type of target material i.e ferrous metals allow a longer sensing range than nonferrous metals. The electronics board are be placed in proper way from the sensing area which allows for the use of the Incoder in rough environments. 

c. Optical sensor:

Optical sensors are usually known as encoders and incremental position sensors for measuring angular and linear. They are also suitable for speed measurement. Main components are an LED, a optical disk with a reference mark, and as a photo sensor with a squaring circuit. A generated light beam provided by the LED is shone through a grating of the optical disk. The resulting light is measured by a photo sensor and is automatically converted into a digital output.

d. Reed switch:

The reed switch was invented by Bell Laboratories in the 1930s. Compare with other mentioned sensors in this topic, the reed switch can only detect the end position of a target, further it can be used as a speed sensor as it counts switching operation over time. The switching mechanism of the reed switch consists of two ferromagnetic blades which are separated by a small gap. These ferromagnetic blades are lightweight, magnetic and flexible. When a magnet is brought close to these blades, the two blades pull towards each other. Once the blades are closed the normally open (NO) contacts that allow electricity to flow. Some reed switches do not have a ferromagnetic contact, such switches typically produce a closed (NC) output. When a magnet is brought close to these blades the contact is disconnected and the switch is removed from the contact.

e. Potentiometers:

Potentiometers use a resistive element in the form of a mechanical sliding contact for measuring linear or angular displacements. The slider is always in contact with a resistance path. The extent of resistance changes according to the movement of the slider. In addition to the mechanical part, every potentiometer needs an external power source so that the mechanical movement can be converted into an electrical signal. The voltage output depends on the extent of resistance and therefore on the rotation angle or covered distance. The measurement range, output signal and electrical connections are customizable and available by many electrical component manufacturers.

Applications of speed sensor:

Automotive:

The applications of speed sensors in the automobile industry are innumerable, in particular Crankshaft, Transmission speed, Engine speed, dynamometers, and Performance vehicles, speed and position monitoring. 

Aviation/Aerospace:

The sensor is used to control and monitor the fan control, solenoid, test equipment, antenna, engine control and switches.

Off highway, agriculture, construction:

Dynamometers, Crankshaft, Transmission control, Engine speed control, Cement mixers, lifters, tractors, harvesting machines, trucks. OEM and assembly manufacturing companies in this industry that require data acquisition for their rotational or flywheel applications. Also proximity switches used for ON / OFF functions used in industrial machinery.

Railroad:

Locomotive Speed control, Diesel engines, Transmission, Switches. Typically magnetic speed sensors used for speed control of the diesel engines. There are also some Hall Effect applications such as proximity switches for control systems.

Power generation:

Gen-sets, Panels, Diesel engines, Power turbines, Wind Mills. Hall Effect speed sensors are used for measuring the speed of the flywheel.

Military:

Engine control, Bradley transmission, various light armoured vehicles, GPS systems, Missiles, Mining equipment. Range of Hall Effect and induction speed sensors for different applications requiring data acquisition for speed and position.

Biotech:

Centrifuge Speed measurement, Proximity switches optical and Hall Effect speed sensors used for different medical equipment using control systems. Hall Effect proximity switch is also used for these systems.

Exercise equipment:

Speed control, and Distance measurement. Potentiometer and Hall Effect magnetic sensors used for reading the speed on Treadmills, elliptical, and step masters. Any other machine that requires sensing speed, direction and distance can be a potential application for magnetic sensors.