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A variable reluctance sensor (commonly called a VR sensor) is a transducer that measures changes in magnetic reluctance. When combined with basic electronic circuitry, the sensor detects the change in presence or proximity of ferrous objects.
A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another.
Magnetic reluctance, or magnetic resistance, is a concept used in the analysis of magnetic circuits. It is defined as the ratio of magnetomotive force (mmf) to magnetic flux. It represents the opposition to magnetic flux, and depends on the geometry and composition of an object.
With more complex circuitry and the addition of software and specific mechanical hardware, a VR sensor can also provide measurements of linear velocity, angular velocity, position, and torque.
In physics, angular velocity refers to how fast an object rotates or revolves relative to another point, i.e. how fast the angular position or orientation of an object changes with time. There are two types of angular velocity: orbital angular velocity and spin angular velocity. Spin angular velocity refers to how fast a rigid body rotates with respect to its centre of rotation. Orbital angular velocity refers to how fast a point object revolves about a fixed origin, i.e. the time rate of change of its angular position relative to the origin. In general, angular velocity is measured in angle per unit time, e.g. radians per second. The SI unit of angular velocity is expressed as radians/sec with the radian having a dimensionless value of unity, thus the SI units of angular velocity are listed as 1/sec. Angular velocity is usually represented by the symbol omega. By convention, positive angular velocity indicates counter-clockwise rotation, while negative is clockwise.
A VR sensor used as a simple proximity sensor can determine the position of a mechanical link in a piece of industrial equipment.
A proximity sensor is a sensor able to detect the presence of nearby objects without any physical contact.
A Crankshaft position sensor (in an automobile engine) is used to provide the angular position of the crankshaft to the Engine control unit. The Engine control unit can then calculate engine speed (angular velocity).
A crank sensor is an electronic device used in an internal combustion engine, both petrol and diesel, to monitor the position or rotational speed of the crankshaft. This information is used by engine management systems to control the fuel injection or the ignition system timing and other engine parameters. Before electronic crank sensors were available, the distributor would have to be manually adjusted to a timing mark on petrol engines.
An engine control unit (ECU), also commonly called an engine control module (ECM), is a type of electronic control unit that controls a series of actuators on an internal combustion engine to ensure optimal engine performance. It does this by reading values from a multitude of sensors within the engine bay, interpreting the data using multidimensional performance maps, and adjusting the engine actuators. Before ECUs, air-fuel mixture, ignition timing, and idle speed were mechanically set and dynamically controlled by mechanical and pneumatic means.
Speed sensors used in automobile transmissions, are used to measure the rotational speed (angular velocity) of shafts within the transmission. The Engine control unit or Transmission control unit (depending on the particular automobile) uses these sensors to determine when to shift from one gear to the next.
A transmission control unit or TCU is a device that controls modern electronic automatic transmissions. A TCU generally uses sensors from the vehicle as well as data provided by the engine control unit (ECU) to calculate how and when to change gears in the vehicle for optimum performance, fuel economy and shift quality.
A pickup used in an electric guitar (or other musical instrument) detect vibrations of the metallic "strings". See Pickup (music technology) for details of this application.
This sensor consists of a permanent magnet, a ferromagnetic pole piece, and coil of wire.
VR sensors need waveform shaping for their output to be digitally readable. The normal output of a VR sensor is an analog signal, shaped much like a sine wave. The frequency and amplitude of the analog signal is proportional to the target's velocity. This waveform needs to be squared up, and flattened off by a comparator like electronic chip to be digitally readable. While discrete VR sensor interface circuits can be implemented, the semiconductor industry also offers integrated solutions. Examples are the MAX9924 to MAX9927 VR sensor interface IC from Maxim Integrated products, LM1815 VR sensor amplifier from National Semiconductor and NCV1124 from ON semiconductor. An integrated VR sensor interface circuit like the MAX9924 features a differential input stage to provide enhanced noise immunity, Precision Amplifier and Comparator with user enabled Internal Adaptive Peak Threshold or user programmed external threshold to provide a wide dynamic range and zero-crossing detection circuit to provide accurate phase Information.
To measure angular position or rotational speed of a shaft, a toothed ring made of ferrous material can be attached to the shaft. As the teeth of the rotating wheel (or other target features) pass by the face of the magnet, the amount of magnetic flux passing through the magnet and consequently the coil varies. When the gear tooth is close to the sensor, the flux is at a maximum. When the tooth is further away, the flux drops off. The moving target results in a time-varying flux that induces a proportional voltage in the coil. Subsequent electronics are then used to process this signal to get a waveform that can be more readily counted and timed. This system has been employed in automotive electronic ignition and ABS braking. By attaching two reluctor rings to a shaft, the torque can be measured. The tooth spacing on reluctor rings may be uniform, or uneven.
Although VR sensors are based on very mature technology, they still offer several significant advantages. The first is low cost - coils of wire and magnets are relatively inexpensive. Unfortunately, the low cost of the transducer is partially offset by the cost of the additional signal-processing circuitry needed to recover a useful signal. And because the magnitude of the signal developed by the VR sensor is proportional to target speed, it is difficult to design circuitry to accommodate very-low-speed signals. A given VR-sensing system has a definite limit as to how slow the target can move and still develop a usable signal. An alternative but more expensive technology is Hall effect sensor. Hall effect sensors are true zero-rpm sensors and actively supply information even when there's no transmission motion at all.
One area in which VR sensors excel, however, is in high-temperature applications. Because operating temperature is limited by the characteristics of the materials used in the device, with appropriate construction VR sensors can be made to operate at temperatures in excess of 300 °C. An example of such an extreme application is sensing the turbine speed of a jet engine or engine cam shaft and crankshaft position control in an automobile.
A stepper motor, also known as step motor or stepping motor, is a brushless DC electric motor that divides a full rotation into a number of equal steps. The motor's position can then be commanded to move and hold at one of these steps without any position sensor for feedback, as long as the motor is carefully sized to the application in respect to torque and speed.
An alternator is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. For reasons of cost and simplicity, most alternators use a rotating magnetic field with a stationary armature. Occasionally, a linear alternator or a rotating armature with a stationary magnetic field is used. In principle, any AC electrical generator can be called an alternator, but usually the term refers to small rotating machines driven by automotive and other internal combustion engines. An alternator that uses a permanent magnet for its magnetic field is called a magneto. Alternators in power stations driven by steam turbines are called turbo-alternators. Large 50 or 60 Hz three-phase alternators in power plants generate most of the world's electric power, which is distributed by electric power grids.
A tachometer is an instrument measuring the rotation speed of a shaft or disk, as in a motor or other machine. The device usually displays the revolutions per minute (RPM) on a calibrated analogue dial, but digital displays are increasingly common. The word comes from Greek ταχος and metron ("measure"). Essentially the words tachometer and speedometer have identical meaning: a device that measures speed. It is by arbitrary convention that in the automotive world one is used for engine and the other for vehicle speed. In formal engineering nomenclature, more precise terms are used to distinguish the two.
A Hall effect sensor is a device that is used to measure the magnitude of a magnetic field. Its output voltage is directly proportional to the magnetic field strength through it.
An ignition system generates a spark or heats an electrode to a high temperature to ignite a fuel-air mixture in spark ignition internal combustion engines, oil-fired and gas-fired boilers, rocket engines, etc. The widest application for spark ignition internal combustion engines is in petrol (gasoline) road vehicles such as cars and motorcycles.
A brushless DC electric motor, also known as electronically commutated motor and synchronous DC motors, are synchronous motors powered by direct current (DC) electricity via an inverter or switching power supply which produces an alternating current (AC) electric current to drive each phase of the motor via a closed loop controller. The controller provides pulses of current to the motor windings that control the speed and torque of the motor.
A rotary variable differential transformer (RVDT) is a type of electrical transformer used for measuring angular displacement.
Torsional vibration is angular vibration of an object—commonly a shaft along its axis of rotation. Torsional vibration is often a concern in power transmission systems using rotating shafts or couplings where it can cause failures if not controlled. A second effect of torsional vibrations applies to passenger cars. Torsional vibrations can lead to seat vibrations or noise at certain speeds. Both reduce the comfort.
A gear train is a mechanical system formed by mounting gears on a frame so the teeth of the gears engage.
A wheel speed sensor or vehicle speed sensor (VSS) is a type of tachometer. It is a sender device used for reading the speed of a vehicle's wheel rotation. It usually consists of a toothed ring and pickup.
A torque sensor, torque transducer or torque meter is a device for measuring and recording the torque on a rotating system, such as an engine, crankshaft, gearbox, transmission, rotor, a bicycle crank or cap torque tester. Static torque is relatively easy to measure. Dynamic torque, on the other hand, is not easy to measure, since it generally requires transfer of some effect from the shaft being measured to a static system.
In a spark ignition internal combustion engine, Ignition timing refers to the timing, relative to the current piston position and crankshaft angle, of the release of a spark in the combustion chamber near the end of the compression stroke.
An amplidyne is an electromechanical amplifier invented prior to World War II by Ernst Alexanderson. It consists of an electric motor driving a DC generator. The signal to be amplified is applied to the generator's field winding, and its output voltage is an amplified copy of the field current. The amplidyne is used in industry in high power servo and control systems, to amplify low power control signals to control powerful electric motors, for example. It is now mostly obsolete.
Electromagnetic clutches operate electrically but transmit torque mechanically. This is why they used to be referred to as electro-mechanical clutches. Over the years, EM became known as electromagnetic versus electro-mechanical, referring more about their actuation method versus physical operation. Since the clutches started becoming popular over 60 years ago, the variety of applications and clutch designs has increased dramatically, but the basic operation remains the same today.
A velocity receiver is a sensor that responds to velocity rather than absolute position. For example, dynamic microphones are velocity receivers. Likewise, many electronic keyboards used for music are velocity sensitive, and may be said to possess a velocity receiver in each key. Most of these function by measuring the time difference between switch closures at two different positions along the travel of each key.
An inertial navigation system (INS) is a navigation device that uses a computer, motion sensors (accelerometers) and rotation sensors (gyroscopes) to continuously calculate by dead reckoning the position, the orientation, and the velocity of a moving object without the need for external references. Often the inertial sensors are supplemented by a barometric altimeter and occasionally by magnetic sensors (magnetometers) and/or speed measuring devices. INSs are used on vehicles such as ships, aircraft, submarines, guided missiles, and spacecraft. Other terms used to refer to inertial navigation systems or closely related devices include inertial guidance system, inertial instrument, inertial measurement unit (IMU) and many other variations. Older INS systems generally used an inertial platform as their mounting point to the vehicle and the terms are sometimes considered synonymous.
Positional tracking detects the precise position of the head-mounted displays, controllers, other objects or body parts within Euclidean space. Positional tracking registers the exact position due to recognition of the rotation and recording of the translational movements. Since virtual reality is about emulating and altering reality it’s important that we can track accurately how objects move in real life in order to represent them inside VR. Defining the position and orientation of a real object in space is determined with the help of special sensors or markers. Sensors record the signal from the real object when it moves or is moved and transmit the received information to the computer.