As time and technology progressed however, the need for other types of accurate speed sensors developed. How can a magnet detect and transmit the speed of a moving object?

It is not just the magnet in a magnetic speed sensor that is used to determine speed but an electrical current that surrounds the magnet as well. An Electrical Current

Gear Toothed Magnetic Sensor

Often a gear is used in conjunction with a magnetic speed sensor. The faster the gear spins the faster the electrical pulses the sensor sends and thus a speed reading is made.

What is An LVDT?

The letters LVDT are an acronym for Linear Variable Differential Transformer, a common type of electromechanical transducer that can convert the rectilinear motion of an object to which it is coupled mechanically into a corresponding electrical signal. LVDT linear position sensors are readily available that can measure movements as small as a few millionths of an inch up to several inches, but are also capable of measuring positions up to ±20 inches (±0.5 m).

The transformer’s internal structure consists of a primary winding centered between a pair of identically wound secondary windings, symmetrically spaced about the primary. The coils are wound on a one-piece hollow form of thermally stable glass reinforced polymer, encapsulated against moisture, wrapped in a high permeability magnetic shield, and then secured in a cylindrical stainless steel housing. This coil assembly is usually the stationary element of the position sensor.

The moving element of an LVDT is a separate tubular armature of magnetically permeable material called the core, which is free to move axially within the coil’s hollow bore, and mechanically coupled to the object whose position is being measured. This bore is typically large enough to provide substantial radial clearance between the core and bore, with no physical contact between it and the coil.

In operation, the LVDT’s primary winding is energized by alternating current of appropriate amplitude and frequency, known as the primary excitation. The LVDT’s electrical output signal is the differential AC voltage between the two secondary windings, which varies with the axial position of the core within the LVDT coil. Usually this AC output voltage is converted by suitable electronic circuitry to high level DC voltage or current that is more convenient to use.

Why Use An LVDT?

LVDTs have certain significant features and benefits, most of which derive from its fundamental physical principles of operation or from the materials and techniques used in its construction.

Friction-Free Operation

One of the most important features of an LVDT is its friction-free operation. In normal use, there is no mechanical contact between the LVDT’s core and coil assembly, so there is no rubbing, dragging or other source of friction. This feature is particularly useful in materials testing, vibration displacement measurements, and high resolution dimensional gaging systems.

Infinite Resolution

Since an LVDT operates on electromagnetic coupling principles in a friction-free structure, it can measure infinitesimally small changes in core position. This infinite resolution capability is limited only by the noise in an LVDT signal conditioner and the output display’s resolution. These same factors also give an LVDT its outstanding repeatability.

Unlimited Mechanical Life

Because there is normally no contact between the LVDT’s core and coil structure, no parts can rub together or wear out. This means that an LVDT features unlimited mechanical life. This factor is especially important in high reliability applications such as aircraft, satellites and space vehicles, and nuclear installations. It is also highly desirable in many industrial process control and factory automation systems.

Overtravel Damage Resistant

The internal bore of most LVDTs is open at both ends. In the event of unanticipated overtravel, the core is able to pass completely through the sensor coil assembly without causing damage. This invulnerability to position input overload makes an LVDT the ideal sensor for applications like extensometers that are attached to tensile test samples in destructive materials testing apparatus.

Single Axis Sensitivity

An LVDT responds to motion of the core along the coil’s axis, but is generally insensitive to cross-axis motion of the core or to its radial position. Thus, an LVDT can usually function without adverse effect in applications involving misaligned or floating moving members, and in cases where the core doesn’t travel in a precisely straight line.

Separable Coil And Core

Because the only interaction between an LVDT’s core and coil is magnetic coupling, the coil assembly can be isolated from the core by inserting a non-magnetic tube between the core and the bore. By doing so, a pressurized fluid can be contained within the tube, in which the core is free to move, while the coil assembly is unpressurized. This feature is often utilized in LVDTs used for spool position feedback in hydraulic proportional and/or servo valves.

Environmentally Robust

The materials and construction techniques used in assembling an LVDT result in a rugged, durable sensor that is robust to a variety of environmental conditions. Bonding of the windings is followed by epoxy encapsulation into the case, resulting in superior moisture and humidity resistance, as well as the capability to take substantial shock loads and high vibration levels in all axes. And the internal high-permeability magnetic shield minimizes the effects of external AC fields.

Both the case and core are made of corrosion resistant metals, with the case also acting as a supplemental magnetic shield. And for those applications where the sensor must withstand exposure to flammable or corrosive vapors and liquids, or operate in pressurized fluid, the case and coil assembly can be hermetically sealed using a variety of welding processes.

Ordinary LVDTs can operate over a very wide temperature range, but, if required, they can be produced to operate down to cryogenic temperatures, or, using special materials, operate at the elevated temperatures and radiation levels found in many nuclear reactors.

Null Point Repeatability

The location of an LVDT’s intrinsic null point is extremely stable and repeatable, even over its very wide operating temperature range. This makes an LVDT perform well as a null position sensor in closed-loop control systems and high-performance servo balance instruments.

Fast Dynamic Response

The absence of friction during ordinary operation permits an LVDT to respond very fast to changes in core position. The dynamic response of an LVDT sensor itself is limited only by the inertial effects of the core’s slight mass. More often, the response of an LVDT sensing system is determined by characteristics of the signal conditioner.

Absolute Output

An LVDT is an absolute output device, as opposed to an incremental output device. This means that in the event of loss of power, the position data being sent from the LVDT will not be lost. When the measuring system is restarted, the LVDT’s output value will be the same as it was before the power failure occurred.

How does an LVDT work?

The LVDT’s primary winding, P, is energized by a constant amplitude AC source. The magnetic flux thus developed is coupled by the core to the adjacent secondary windings, S1 and S2 . If the core is located midway between S1 and S2 , equal flux is coupled to each secondary so the voltages, E1 and E2 , induced in windings S1 and S2 respectively, are equal. At this reference midway core position, known as the null point, the differential voltage output, (E1 – E2 ), is essentially zero.

If the core is moved closer to S1 than to S2 , more flux is coupled to S1 and less to S2 , so the induced voltage E1 is increased while E2 is decreased, resulting in the differential voltage (E1 – E2). Conversely, if the core is moved closer to S2 , more flux is coupled to S2 and less to S1 , so E2 is increased as E1 is decreased, resulting in the differential voltage (E2 – E1).

The top graph shows how the magnitude of the differential output voltage, EOUT, varies with core position. The value of EOUT at maximum core displacement from null depends upon the amplitude of the primary excitation voltage and the sensitivity factor of the particular LVDT, but is typically several volts RMS. The phase angle of this AC output voltage, EOUT, referenced to the primary excitation voltage, stays constant until the center of the core passes the null point, where the phase angle changes abruptly by 180 degrees, as shown in the middle graph.

This 180 degree phase shift can be used to determine the direction of the core from the null point by means of appropriate circuitry. This is shown in the bottom graph, where the polarity of the output signal represents the core’s positional relationship to the null point. The figure shows also that the output of an LVDT is very linear over its specified range of core motion, but that the sensor can be used over an extended range with some reduction in output linearity. The output characteristics of an LVDT vary with different positions of the core. Full range output is a large signal, typically a volt or more, and often requires no amplification. Note that an LVDT continues to operate beyond 100% of full range, but with degraded linearity.

LVDTs and their support electronics

Although an LVDT is an electrical transformer, it requires AC power of an amplitude and frequency quite different from ordinary power lines to operate properly (typically 3 V rms at 3 kHz). Supplying this excitation power for an LVDT is one of several functions of LVDT support electronics, which is also sometimes known as LVDT signal conditioning equipment.

Other functions include converting the LVDT’s low level AC voltage output into high level DC signals that are more convenient to use, decoding directional information from the 180 degree output phase shift as an LVDT’s core moves through the null point, and providing an electrically adjustable output zero level.

A variety of LVDT signal conditioning electronics is available, including chip-level and board-level products for OEM applications as well as modules and complete laboratory instruments for users.

The support electronics can also be self-contained, as in the DC-LVDT shown here. These easy-to-use position transducers offer practically all of the LVDT’s benefits with the simplicity of DC-in, DC-out operation. Of course, LVDTs with integral electronics may not be suitable for some applications, or might not be packaged appropriately for some installation environments.

Macro Sensors offers an extensive line of LVDTs including AC- and DC-operated versions, linear and rotary sensors, free core and spring-loaded technology, hermetically sealed and high temperature resistance units as well as custom products. Macro Sensors’ extensive line of LVDT-based linear and rotary sensors are used for linear position measurement and feedback in a variety of industrial applications including factory automation, motion control systems, metal fabricating, automotive assembly as well as power plants, gas/steam turbines. For a catalog of Macro Sensors complete line of LVDTS, refer to the web site at: http://www.macrosensors.com

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bit.ly – This tutorial, provided by Digi-Key and Honeywell, will provide a general overview of magnetic sensors and their attributes, as well as explain the benefits of magnetic sensors.

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Macro Sensors offers an extensive line of LVDTs including AC- and DC-operated versions, linear and rotary sensors, free core and spring-loaded technology, hermetically sealed and high temperature resistance units as well as custom products. Macro Sensors’ extensive line of LVDT-based linear and rotary sensors are used for linear position measurement and feedback in a variety of industrial applications including factory automation, motion control systems, metal fabricating, automotive assembly as well as power plants, gas/steam turbines. For a catalog of Macro Sensors complete line of LVDTS, refer to the web site at: http://www.macrosensors.com

The automotive industry has gone along way in terms of developments made in the design and performance of automobiles. Year after year, different car makers introduce more and more feature on their cars not only to appease their customers but also to provide good performance and enhanced safety.

Currently, more and more electronic control systems are being employed in the production of automobiles. In the production of the said advanced features, sensors play key roles.

The most common areas of a vehicle which is equipped with the electronic control systems are the safety and driver convenience features. For these to function properly, the presence of sensors is vital. These sensors act as switches for other electronically controlled feature of a system in an automobile.

As a result of the increasing demand for advanced safety and driver convenience features, the demand for sensors also increases. These sensors are commonly used to detect position, displacement, rotary position, linear position, timing, and angles. Navigation and safety features like the anti-lock braking system are also where these magnetic sensors are widely used.

In a typical anti-lock braking system, a central electronic circuit and four speed sensors are needed. These sensors play a key role in keeping a car’s occupants safe in emergency braking. The sensor, tells the central electronic unit how fast the wheels, are going while braking. It is the job of the central electronic unit to monitor and keep all the four wheels of a vehicle running in almost uniform speed.

The sensors, four of them, one for each wheel, tells the central electronic unit which wheel is slowing down at a faster rate than the others, the unit will then ease the brake on that particular wheel. This technology allows a vehicle to slow down easily without locking the wheels. Locking the wheels means the driver will loose control over the car. This will then translate to heightened risk of being involved in an accident.

With more and more car buyers looking for safety features on vehicles, car manufacturers continually improve the safety of their vehicle. To improve the safety of a car, certain features are added to it like electronic stability control. The system is designed to aid a driver on situations where there is a possibility of one loosing control of the steering system.

The ESC as it is called helps the driver by controlling oversteer and understeer and also controlling the amount of pressure exerted on the brakes. With this technology, drivers face less risk of oversteering or understeering their car. In this technology, sensors also play a major role since they are the components which tells the main electronic control whether the driver is steering properly or putting excessive force on the steering wheel as well as the brakes.

Sensors are widely used in an automobile. Different systems of a car use sensors. There are sensors that measure the pressure inside the combustion chamber – there are sensors which detects the temperature of the engine. There are many uses for sensors currently. But the advent of technology has made the sensor even more important.

As consumers look for more and more features that will improve the safety, handling, stability, performance, comfort, convenience, and other attributes of a vehicle, more and more sensors will surely be developed to be used on different applications which are yet to be developed. The market for these sensors will continue to grow as car manufacturers find more ways to integrate sensor-related features on their production vehicles. The sensors are as indispensable as a Ford Hedman header is in a Ford vehicle, and that fact makes the sensor one of the most in demand vehicle component in the market.

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A hall effect sensor Melexis 90217 is connected to an Arduino board. It detects change in magnetic field and sends the signal back to a Processing program in the host PC.

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Mike Bartley, 49, is a professional automotive journalist domiciled in Irvine, CA. He travels from one state to another to cover the hottest auto shows, racing events and automotive revelations. His penned compositions cover press releases, reviews, and suggestions. Where the auto action is, that’s exactly where you can find Mike.

For centuries speed sensors have been used to determine the speed of moving objects. In fact, the very first primitive speed sensors were lengths of rope with a knots tied in them that were tossed over the sides of moving ships to determine how many “knots” the ship was traveling at. However; the advent of the motorized wheeled carriage created the need for a more advanced mechanical speed sensor, such as the type that used a gear and a cable to run a speedometer on an automobile.

A Technological Need

As time and technology progressed however, the need for other types of accurate speed sensors developed. This in turn led to the development of what is often referred to as the magnetic speed sensor. So how do they work? How can a magnet detect and transmit the speed of a moving object?

The Hall Effect

It is not just the magnet in a magnetic speed sensor that is used to determine speed but an electrical current that surrounds the magnet as well. There is a certain electrical phenomena called the “Hall effect” that is used to determine the speed of an object with a magnet.

An Electrical Current

In short, when an electrical current is ran near a magnet and the magnet detects ferrous metal such as iron or steel the electrical current is effected. This electrical effect can then be transmitted by wires to a speed gage where it can be displayed.

Gear Toothed Magnetic Sensor

Often a gear is used in conjunction with a magnetic speed sensor. As the gear spins or turns, each spline or tooth in it will be detected by the magnet as it passes and a corresponding electrical pulse is sent out. The faster the gear spins the faster the electrical pulses the sensor sends and thus a speed reading is made.

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Behind the scenes of Discovery Channel’s Prototype This from the Virtual Sea Adventure episode. Final test of the control system for our underwater ROV controller. Control is achieved by using two Melexis MLX90333 3-D Joystick Position Sensors. The user, under water, wears gloves with magnets embedded in the thumbs and moves his thumbs over the sensors to control the various aspects of the ROV. The analog outputs of the sensor’s X- and Y-axis are fed through an A/D, into a BASIC Stamp …

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Written by Rosa Telipten. Now you can learn all you wanted to know about Magnetic Speed Sensors and you will even find articles on Variable Reluctance Pickup