EAEP-Electrical and Electronics Portal

Speed sensor: Working principle | Types | Applications

2020-10-19T07:22:00.000-07:00

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.

Reed switch: working principle, Types, and applications

2020-10-09T03:02:00.001-07:00

Reed switch

What is a reed switch?

Magnetic reed switch an electrical switch driven by a magnet. It consists of a pair of ferromagnetic flexible metal contacts in a hermetically sealed glass envelope. There are two types of configuration: Normally open (NO), closes when a magnetic field is brought near and normally close (NC) open when a magnetic field is brought near. When the magnetic field is removed, the contacts in the reed switch return to their original position.

It was invented at Bell Telephone Laboratories in 1936 by Walter B. Ellwood.

How a Reed Switch Work?

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.

Types of Reed switch:

Reed switches classified in two basic types: Normally Open and Normally Closed.

Normally Open (NO) Reed Switches:

In a Normally Open reed switch design, the switch defaults to an open position. When an outside magnet approaches, it polarizes the two contacts, allowing them to attract each other and closes the contacts. This closes the circuit, allows the electrical current to flow. When the outside magnet is removed, the stiff contacts spring back into their original position, thus breaking the circuit and stop the electrical current flow.

Normally close (NC) Reed Switches:

Normally Closed reed switches operate on the same principle, except that the switch uses a built-in magnet to keep its two contacts in a close state. This means that the switch’s default position is to keep the circuit closed, allowing the electrical current to flow continuously. When a magnet with reversed polarity approaches, it collapse out the magnetic field of the built-in magnet. This causes the two contacts to lose polarity, and they spring apart from each other to their default, non-magnetized positions. This breaks the circuit, stopping the electrical current flow.

Difference between Hall effect and reed switch:

A major difference between reed switches and Hall effect sensors is the proper orientation required for magnet. Hall effect sensors activate when a magnetic field that is perpendicular to the solid-state sensor is applied. Most look for the magnet's South Pole to be facing an indicated location on the sensor. If you turn the magnet backwards or sideways the sensor won't activate.

Reed switches are an electrical switch with moving parts. It consists of two ferromagnetic contact separated by a small gap. In the presence of a magnetic field they will touch each other, making electrical contact. In other words, the magnetic axis of the magnet should be parallel to the long axis of the reed switch.

Application examples of reed switches:

Reed Switches in Security Systems:

Reed switches can be used to signal the position of doors, elevators, windows, and gates. Reed switch sensors can limit access to equipment panels, prevent removal of valuable system components, sense the accumulation of water or other fluids or sense seismic movement.

Reed switches in home appliances:

Reed switches are used as liquid level sensors in coffee makers, dishwashers, washing machines and water heaters, or as heat sensors in cookers or steamers. They can also be used as proximity sensors on utility doors, access panels and bicycle odometers.

Reed Switches in Automobiles:

Reed switches are used in fluid level sensors to monitor brake fluid reservoirs and oil levels. They are also used in speed sensors for engine control and power steering. Automatic door locks, airbags, parking brakes, seat, door, and hood proximity sensors also use reed switches.

Reed Switches in Robotics and Automation:

Reed switches are used as proximity sensors to control robotic motion, speed, location and angle. They can sense whether valves, tracks, hoppers, and doors are open or closed, or if containers or conveyors are full or empty. Reed sensors report the position and movement of products in vending machines.

Reed Switches in very Harsh Environments:

Reed switches are hermetically sealed, durable, proven, reliable and precise. They can tolerate wide swings in temperature and to of shock. Reed switches tolerate high humidity, sea salt spray, toxic gases, dangerous chemicals, and dust or partial laden atmospheres. Reed switches do not need to be powered (battery) in the off position.

Advantages of reed switch:

Reliability and durability
Precision in magnetic sensitivity
Simple in design
As they are triggered by magnetism, there is no voltage required
As the contacts of a reed switch are sealed within a glass tube they are protected from atmospheric corrosion.