Air Gap - The distance between the face of the magnet and the face of the sensor.

Total Air Gap - The distance between the face of the magnet and the active sensing element inside the sensor housing.

Anisotropic Magnet - A magnet having a preferred direction of magnetic orientation, so that the magnetic characteristics are optimum in one preferred direction.

Bipolar Sensor - A sensor that requires one pole (North or South, usually South) to turn on and an opposite pole to turn off. These sensors can be either latching or non-latching.

Closed Circuit - This exists when the flux path external to a permanent magnet is confined within high permeability materials that compose the magnet circuit.

Coercive Force (Hc) - The demagnetizing force, measured in Oersteds, necessary to reduce observed induction, B, to Zero after the magnet has previously been brought to saturation.

Current sinking - A transistor configuration where the load is connected between the output and a supply voltage. When the transistor is "on", the current flows from the transistor into the load.

Duty cycle (Latching Bipolar Hall Effect) - The relationship of a sensor "on" time to the total operate/release time cycle. A 50% duty cycle represents a situation where the sensor "on" time is equal to the sensor "off" time.

Gauss - The CGS unit of magnetic induction and flux density. One Gauss equals one Maxwell per square centimeter.

Hall Effect - The creation of a voltage between the two edges of a current carrying conductor whose faces are perpendicular to a magnetic field.

Hysteresis - The difference between the operate point and the release point.

Latching (Hall Effect) - A device that remains in a digital output (high or low) state after the magnetic operate field is removed. An opposite magnetic field (release point) is required to change the output state of the device.

Magnetic flux - The physical indication of a magnetic condition existing in a material or medium that is subject to a magnetizing influence. The CGS unit for flux is the Maxwell (M); the SI unit is the Weber (Wb).

Operate point - The Gauss level required to turn a sensor "on".

Release point - The Gauss level required to turn a sensor "off".

Unipolar sensor - A non-latching sensor that requires only one pole (South) to operate and release.

Curie Temperature (Tc) - The temperature at which the parallel alignment of elementary magnetic moments completely disappears, and the material is no longer able to hold magnetization.

Demagnetization Curve - The second quadrant of the hysteresis loop, generally describing the behavior of magnetic characteristics in actual use, also known as the B-H curve.

Eddy Currents - Circulating electrical currents that are induced in electrically conductive elements when exposed to changing magnetic fields, creating an opposing force to the magnetic flux. Eddy currents can be harnessed to perform useful work (such as damping of movement), or may be unwanted consequences of certain designs which should be accounted for or minimized.

Ferromagnetic Material - A material whose permeability is very much larger than 1 (from 60 to several thousand times), and which exhibits hysteresis phenomena.

Fringing Fields - Leakage flux particularly associated with edge effects in a magnetic circuit.

Induction (B) - The magnetic flux per unit area of a section normal to the direction of flux. Measured in Gauss, in the cgs system of units.

Intrinsic Coercive force (Hci) - Measured in Oersteds in the cgs system, this is a measure of the material's inherent ability to resist demagnetization. It is the demagnetization force corresponding to zero intrinsic induction in the magnetic material after saturation. Practical consequences of high Hci values are seen in greater temperature stability for a given class of material, and greater stability in dynamic operating conditions.

Irreversible Loss - Defined as the partial demagnetization of a magnet caused by external fields or other factors. These losses are only recoverable by remagnetization. Magnets can be stabilized to prevent the variation performance caused by irreversible losses.

Isotropic Magnet - A magnet material whose magnetic properties are the same in any direction.

Magnetic Field Strength (H) - A measurement of the magnetic ability to induce a magnetic field at a given point. This is measured in Oersteds.

Magnetizing Force (H) - The magnetomotive force per unit length at any point in the magnetic circuit. this is measured in Oersteds.

Magnetomotive Force (F) - The magnetic potential difference between any two points.

Maxwell - A unit of magnetic flux. One Maxwell is one line of magnetic flux.

Oersted, Oe - A unit of magnetic field strength or magnetizing force.

Orientation Direction - The direction in which an anisotropic magnet should be magnetized in order to optimize the magnetic properties.

Saturation - This is a condition where all magnetic moments have become oriented in one direction.

Stabilization - Exposing a magnet to demagnetizing influences which are expected in the application in order to prevent irreversible loss during the operation of the magnet.

Accuracy - A measure of how close the output is to where it should be. It is usually expressed in units of distance, such as ±30 arcseconds or ±0.0001 inch. If it's expressed as a percent, make sure to state whether it's a percent of full scale (not usually meaningful with a rotary encoder) or a percent of nominal resolution.

Angular Misalignment - The maximum deviation in perpendicularity between the encoder shaft and the face of the mounting surface. It is the total of shaft misalignment, shaft runout and mounting face runout.

Bit - An abbreviation for BInary digiT; it refers to the smallest element of resolution.

CPR - mean either cycles/rev or counts/rev. To avoid confusion, this term should not be used.

Error - the algebraic difference between the indicated value and the true value of the input.

Frequency Response - the encoder's electronic speed limit, expressed in kilohertz (1 kHz = 1000 Hz = 1000 cycles/sec). For calculations, rotational speed must be in rev/sec (rps = rpm/60); linear speed must be either in/sec or mm/sec, depending on the scale line count.
(cycles/rev) x (rev/sec)/1000 = kHz

Index Signal - A once-per-rev output used to establish a reference or return to a known starting position; also called reference, marker, home, or Z

Interpolation - An electronic technique for increasing the resolution from the number of optical cycles on the disc or scale to a higher number of quadrature square waves per revolution or per unit length. These square waves can then be quadrature decoded.

Measuring Step - The smallest resolution element; it assumes quadrature decode.

PPR - Pulses per revolution. Commonly (but mistakenly) used instead of cycles/rev when referring to quadrature square wave output.

Quadrature - the 90-electrical-degree phase relationship between the A and B channels of incremental encoder output.

Quadrature Decode (or 4X Decode) - The common practice of counting all 4 quadrature states (or square wave transitions) per cycle of quadrature square waves. Thus, an encoder with 1000 cycles/rev, for example, has a resolution of 4000 counts/rev.

Quantization Error - Inherent in all digital systems; it reflects the fact that you have no knowledge of how close you are to a transition. It is commonly accepted as being equal to ±1/2 bit.

Quantum (plural: quanta) - Equivalent to bit. It is the smallest resolution element. (Quanta and Bit are more commonly used with absolute encoders; counts/rev or measuring steps are more common with incremental encoders.)

Repeatability - A measure of how close the output is this time to where it was last time, for input motion in the same direction. It's not usually specified explicitly, but it is included in the accuracy figure. (As a rule of thumb, the repeatability is generally around 1/10 the accuracy.)

Resolution - The smallest movement detectable by the encoder. It can be expressed in either electrical terms per distance (e.g., 3600 counts/rev or 100 pulses/mm) or in units of distance (e.g., 0.1° or 0.01 mm).

Slew Speed - The maximum allowable speed from mechanical considerations. It is independent of the maximum speed dictated by frequency response.

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