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Using Hall-Effect Type Sensors Effectively

We are familiar with appliances such as wine coolers, freezers, and refrigerators. They keep out beverages and food cold, extending their useful life. Most often, these appliances have lights that illuminate the insides when the user opens their doors. Since the lights only need to be on when the user opens the door, usually, the designer of such appliances place a sensor to detect the opening and closing of the door.

A sensor of the Hall-effect type can detect the position of the door. In refrigerators, the position of the sensor is within the frame, while a permanent magnet is placed on the door directly opposite the Hall-effect type sensor. For refrigerators with multiple doors, each door needs a magnet and for the detection, each magnet must have a corresponding sensor placed in the frame. The adjustment of proximity of each Hall-effect type sensor and magnet pair is such that the Hall-effect type sensor detects the magnet only as the door closes completely.

An electronic control unit inside the electronics assembly of the refrigerator monitors the output from the Hall-effect type sensors and turns the lights on or off as necessary. Hall-effect type sensors can detect a variety of proximity- and position-sensing applications such as when there is a need to discover the proximity of a moving part relative to a sensor placed in a fixed location.

For instance, Hall-effect type sensors can help to stop the motor opening or closing a garage door once the door has reached its desired position. Typically, this needs a system of two Hall-effect type sensors to detect the two dominant positions of the door—open or closed. Each sensor also needs a corresponding magnet to trigger it.

The position of one of the magnets on the drive chain of the garage door opener places it directly next to the sensor that detects a closed door. The position of the other magnet, also on the drive chain, is such that the drive chain brings it next to the other Hall-effect type sensor as the door opens completely.

Hall-effect type sensors are preferable to other sensors such as reed relays, as the former has no moving electrical contacts, resulting in long life and improved reliability. Other applications that use Hall-effect type sensors effectively are vending machines, security locks on doors, vacuum cleaners, washing machines, dishwashers, and similar applications requiring door- and lid-position sensing.

A flow switch is another application that benefits from the use of a Hall-effect type sensor, which detects the motion of a piston, paddle wheel, or a valve fitted with a permanent magnet. For instance, this arrangement suits tankless water heater units, where the flow sensor has a permanent magnet fixed to a piston. The increasing presence of water pressure in the system moves the piston and its associated magnet near to a permanently positioned Hall-effect type sensor. This causes the output of the Hall-effect type sensor to change and it signals the presence of flowing water.

Similarly, a turbine can have a magnet attached to its blades. As the blades rotate, the magnet passes by a fixed Hall-effect type sensor. The speed at which the blades rotate is proportional to the fluid flowing through the turbine.

How do Sensors Measure Gear Tooth Speed and Direction?

Measuring speed of gears is an important factor in various industries, especially in pharmaceutical, tobacco, printing, woodworking, paper, textile, food and others where rotational machinery predominates. Gear speed measurements also necessary in pumps, blowers, mixers, exhaust and ventilation fans, wheel-slip measurement on autos and locomotives, flow measurement on turbine meters and many more.

The most common gear tooth sensors detect a change in the magnetic field for determining the speed and direction. Usually, these are of three types – the Hall Effect, magneto-resistive and the Variable Reluctance. There are optical types of sensors as well, detecting a change in light levels as the gear rotates past the sensor.

Sensors using magnetic properties are good for measuring speed and direction of gears made of ferrous metals. All these sensors are non-contact type and sensitive to detect the presence of gear teeth passing in front of the sensor. As a gear tooth comes close to the magnetic sensor, its output flips and the electrical level at its output changes state. The output remains steady as long as the gear tooth is within the detectors sensing zone. As the tooth passes out of this zone, the output flips back. Therefore, a magnetic sensor placed in front of a rotating gear, the output from the sensor will be a series of electrical pulses.

There are several advantages when using magnetic sensors. Apart from the sensors being non-contact type, they are robust, hermetically sealed and can withstand unregulated power supply. Most manufacturers make then RoHS and IP67 compliant. That means no lead or other toxic materials are used for manufacturing these sensors and dust or liquid will not enter their enclosure. That makes such sensors suitable for use in food processing industries.

For measuring the speed of gears made of non-magnetic material, engineers often use optical sensors. The most common sensor of this type is the optical interrupters. Gear teeth interrupt a light beam from an LED source and the detector produces a corresponding electrical output. A continuously rotating gear in front of the sensor therefore, creates a similar series of electrical pulses as the output from magnetic sensors do.

The functioning of optical speed or proximity sensors is dependent of the dust and dirt level of the environment where they are used. Therefore, their range of applications is somewhat restricted as compared to magnetic sensors.

Measurement of direction involves a reference point, which means two sensors need to be used, with one of them being the reference sensor. An electronic circuit measures the time gap between the responses from each sensor. As the gear tooth passes in front of both sensors, one of them will change output before the other. If sensor A happens to trigger before sensor B does, the electronic circuit determines the gear is moving from A towards B. In case the output of sensor B switches before sensor A does, then the gear is moving from B towards A.

Usually, the sensors provide separate digital outputs for speed and direction. Their measuring capability may extend from detecting near zero speed up y 15 kHz.

How do Sensors Measure Angle?

An angle is the degree of rotation of an object from a reference position about a central axis. In the engineering world, there are two types of angles requiring measurement. One is the physical or mechanical characteristic, such as the rotation of a shaft with respect to its bearing or housing. The other is a mathematical term such as the angle between two phases of alternating voltage system. Usually, sensors measure angles in a format that a computer or a machine can understand, interpret and utilize.

It is also a common practice to convert a physical characteristic into a rotational mechanical movement to measure linear displacement. For example, the distance traveled by a shaft can be translated into rotational movement by a rack and pinion arrangement. The angular position sensor attached to the arrangement then interprets the angular movement in proportion to the linear movement of the shaft.

In the market, you will find different sizes and forms of angle positioning sensors using various technologies. Generally speaking, these sensors are versatile and one can use them in all kinds of applications, such as in agriculture, commercial equipment, off-road vehicles and in automotive industries. Most of the applications above require a product suitable for operating in harsh environments, including moisture, dirt, dust, extreme temperatures and more.

For example, Forklift Position sensors measure the angle of the forks on a forklift truck. According to OSHA, one of the primary causes for tip-over accidents on forklifts is excessive speed when the machine is turning or rounding a corner. The angle position sensor on the truck helps it to remain within a safe speed and prevents overturning. This particular application also prevents accidents from unbalanced loads and limits the operation of the machine when the load is improperly positioned or balanced.

The simplest form of measuring angle is by using the gear tooth sensor. By sensing the teeth to count the rotation of a gear or wheel, engineers monitor and limit speed. Another common form of angular position measurement utilizes potentiometers. Other more sensitive and rugged types of angle sensors use optical or magnetic technology.

Traditional rotary encoders use an LED transmitter, a coded disc and a photo sensor to detect angular movement. The disc is coded with opaque and transparent sections, which transmit light in a specific manner to the photo sensors depending on the position of the disc. The photo sensor converts the light falling on to it into an electrical code. This allows the encoder to detect rotation, position, angle, etc.

Sensors that are more rugged use the Hall-Effect technology for measuring angle. This technology uses magnetic field sensing and does not require the critical positioning necessary for the components using optical methods. In both methods, accuracy of an angle sensor depends largely on its resolution. The higher the resolution, the more precise is the detection of angular movement. Sensors measuring angles using Hall-Effect technology can perform without physical contact, thereby remaining unaffected by vibration and abrupt movements. These sensors also have the added benefits of virtually unlimited lifespan.