Find a comprehensive and useful glossary section explaining technical terms related to both Motion Control and Photonics.

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Motion Control Glossary

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Absolute accuracy – Maximum difference between the target position and the actual position. Accuracy is limited by backlash, hysteresis, drift, nonlinearity of drive or measurement system, tilt, etc. The best absolute accuracy is achieved with direct metrology sensor systems. In such systems, the position of the platform itself is measured, with, for example, an interferometer or linear encoder so that mechanical play within the drive train does not affect the position measurement. Indirect metrology systems (e.g. rotary encoders on the motor shaft) or open-loop stepper-motor-driven stages, have significantly lower absolute accuracies. Independent of this fact, they can still offer high resolutions and repeatabilities.

Amplifier classification – PI uses the following amplifier classifications: Charge-controlled, switched (class D), linear.

Amplifier resolution – Only for digitally controlled amplifiers: Measurement of the smallest digital output value (LSB) in mV.

Average current – For multi-axis controllers, it is specified per channel. Measured value. It is available reliably over a longer period.


Backlash – The position error that appears upon reversing direction. Backlash is caused by mechanical play in the drive train components, such as gearheads or bearings, or by friction in the guiding system. Unlike hysteresis, it can lead to instability in closed-loop setups because it causes a deadband in the control loop. The backlash depends on temperature, acceleration, load, leadscrew position, direction, wear, etc. Backlash is suppressed by the preload of the drive train. A position measurement method, that can detect the position of the platform directly, eliminates all errors in the drive train (direct metrology). The data table shows typical measured values. Data for vacuum versions can differ.

Bandwidth – Measured value. The frequency in kHz, with which the amplitude decreased by -3 dB, is specified. Large signal values: With maximum output voltage. Small signal values: With an output voltage of 10 Vpp. The values are displayed in the amplifier operating diagram.

Bidirectional repeatability – The accuracy of returning to a position within the travel range after any change in position. Effects such as hysteresis and backlash affect bidirectional repeatability if the system does not have direct metrology.

See also  >> Unidirectional repeatability


Crosstalk: Pitch/yaw, straightness/flatness – Deviation from the ideal straight motion measured along the entire travel range; it is a pitch around the Y-axis and a yaw around the Z axis with the motion being in the X direction (orthogonal coordinate system). The data table shows typical measured values as ± values. The straightness (in relation to Y) and flatness (in relation to Z) are specified in absolute μm values.

Capacitive base load (internal) – For switching amplifiers. Stabilizes the output voltage even without connected capacitive load (piezo actuator). The possible output power of a piezo controller/driver depends on internal and external capacitive loads.

Current consumption – Current consumption of the system on the supply end. It is specified for the controller without load. Alternatively, power consumption.

Closed-loop operation – A closed-loop operation requires processing the results of a position feedback system. A control algorithm then compares the target position with the measured actual position. The closed-loop control provides a better repeatability and positional stability.

Control input voltage range – Also input voltage; for piezo controller/driver. Recommended range from -2 to 12 V. The usual gain value of 10 V leads to an output voltage of -20 to 120 V. Most PI controllers allow for an input voltage range of -3 to 13 V.

Cosine error – The cosine error is a cumulative position error in linear systems that occurs when a drive system is misaligned in regard to the driven part. The error is calculated by multiplying the change in position with the difference between 1 and the cosine of the angular error

Current limitation – Short-circuit protection.


Design resolution – The theoretical minimum movement that can be made. Design resolution must not be confused with minimum incremental motion. In indirect position measurement methods, values for spindle pitch, gear ratio, motor or sensor/encoder resolution, for example, are included in the calculation of the resolution; normally it is considerably below the minimum incremental motion of a mechanical system. Indirect measurement methods, the resolution of the sensor system is specified.

Degree of freedom – A degree of freedom corresponds to an active axis of the positioning system. An XY positioning stage has two degrees of freedom, a Hexapod six.

Drive type – Defines the types of drive supported by the controller/ driver, such as DC motors, piezo stepping drives, piezo actuators.

Dynamic resolution –For capacitive sensors. Measured in the nominal measuring range, bandwidth see data table.

See also  >> Static resolution

Defining linear and rotational axes – What the mathematical letters correspond to

X – Linear motion in positioning direction
Y – Linear motion perpendicularly to the X axis
Z – Linear motion perpendicularly to X and Y

θX – Rotation around X
θY – Rotation around Y
θZ – Rotation around Z


Eccentricity – The deviation between theoretical and actual rotational axis of a rotary stage.

Encoder input – Maximum bandwidth (-3 dB) of the input signals for the encoder input.


Flatness – See >> Crosstalk
Friction – See >> Stick-slip effect


Guiding accuracy, guiding error – The guiding error represents the deviation of the stage platform from the planned trajectory perpendicularly to the positioning direction and tilt around the axes. For a single-axis linear stage, it is unwanted motion in all five degrees of freedom. For a translation in X, linear runout occurs in Y and Z, tip and tilt occur in X (θX, roll), Y (θY, pitch) and Z (θZ, yaw). Guiding errors are caused by the guiding system itself, by the way the stage is mounted (warping) and the load conditions (e.g. torques).


Holding force, de-energized – Piezomotor linear drives are self-locking at rest, they do not consume current and do not generate heat. If they are switched off for a longer time, the holding force decreases. This is typical for piezo motors. The minimum holding force in long-term operation is specified.

Hysteresis – Hysteresis is a position error that occurs when reversing direction. It is due to elastic deformation, such as friction-based tension and relaxation. Hysteresis of a positioning system varies greatly with load, acceleration and velocity.


Input level – Permissible input level for digital interfaces.


Lateral force – Maximum permissible force orthogonally to the positioning direction. This value is valid when applied directly to the moving platform, and is reduced when the force applies from ontop of the platform.

Limit switches  – Each limit switch sends an overtravel signal on a dedicated line to the controller. The controller then interrupts the motion avoiding that the stage gets damaged when the hard travel stop is reached. PI stages have mechanical, noncontact optical or Hall-effect limit switches. Function: Optical, magnetic.

Linearity error –Value gained with external, traceable measuring device. Defines the maximum deviation from an ideal straight motion. The value is given as a percentage of the entire measuring range. The linearity error does not influence the resolution or repeatability of a measurement. Measurement of the linearity error: The target and measured actual values of the positions are plotted against each other, a straight line is drawn through the first and last data point, and the maximum absolute deviation from this straight line is determined. A linearity error of 0.1% corresponds to an area of ±0.1% around the ideal straight line. Example: A linearity error of 0.1% over a measuring range of 100 μm produces a possible maximum error of 0.1 μm.

Linearization – Integrated method, e.g. ILS, polynomials to the nth degree, sensor linearization.

Load capacity – Maximum load capacity vertically if the stage is mounted horizontally. The contact point of the load is in the center of the platform.


Measurement range extension factor – For capacitive sensors, used by PI.

Measured values – Measured values, such as backlash and repeatability, are determined based on the VDI standard 3114.

MTBF – “Mean Time Between Failure” measures lifetime and reliability of the stage.

Material – Micropositioning stages are normally made of anodized aluminum or stainless steel. Small amounts of other materials may be used (for bearings, preload, coupling, mounting, etc.). On request, other materials, such as nonmagnetic steel or Invar, can be used.

Min. incremental motion  – The smallest motion that can be repeatedly executed is called minimum incremental motion, or typical resolution, and is determined by measurements. The data table shows typical measured values. The minimum incremental motion differs in most cases strongly from the design resolution which can be considerably smaller in numerical values. Repeatable motions in nanometer and sub-nanometer range can be carried out using piezo stage technology and friction-free flexure guides.

See >> Design resolution


Noise – For capacitive sensors. In extended measurement ranges, noise is considerably higher than in the nominal measurement range.


Operating limits – Values measured at an ambient temperature of 20°C. A sine is used as a control signal in open-loop operation. The amplifier works linearly within the operating limits, in particular without thermal limitation.

Operating temperature range – In any case, the device can be operated safely in the maximum permissible temperature range. To avoid internal overheating, however, full load is no longer available above a certain temperature (maximum operating temperature under full load). All technical data specified in the datasheet refer to room temperature (22 ±3°C).

Operating voltage – Allowed control input voltage range (also input frequency) for the supply of the device.

Open-loop operation – Operation without processing the position sensor and without control loop. Stages with stepper motors execute precise and repeatable steps; therefore, they do not need any closed-loop control. The closed-loop control provides a better repeatability and positional stability.

Output voltage – The output voltage of piezo controllers shows variations of only a few millivolt and is particularly stable in the long term.

Orthogonality – See >> Perpendicularity

Overtemperature protection – Switch-off temperature for voltage output. No automatic restart.


Parallel kinematics – Multi-axis system, in which all actuators act directly on the same moving platform. The advantages if compared to serial kinematics are a lower mass moment of inertia, no moved ca bles, lower center of gravity, no cumulated guiding errors, more compact structure.

Peak current – Only available for very short times, typically under a few milliseconds. It is used to estimate the possible dynamics with a certain capacitive load. Note: In this case, the piezo controller/ driver does not necessarily work linearly.

Perpendicularity, orthogonality – Perpendicularity describes the deviation from an ideal 90° angle of the X, Y and Z motion axes.

Pitch / yaw – See >> Crosstalk

Power consumption – Maximum power consumption under full load.

Precision – Precision is a term not clearly defined and is used by different manufacturers in different ways for repeatability, accuracy or resolution. PI uses the term for a high, but not quantified, accuracy.

Precision Class – Product in the Precision Class are suited for high-precision positioning applications.

Profile Generator – Linear interpolation, point-to-point, trapezoid, double bends. For several axes: Electronic gearing.

Pulse width modulation (PWM) – The PWM mode is a highly effective amplifier mode in which the duty cycle is varied rather than the amplitude of the output signal.

Push / pull force, max – Maximum force in direction of motion. Some stages may reach higher forces but with limited lifetimes.


Reference Class – The highest demands for sensor resolution and positioning and guiding precision are satisfied by the Reference Class.

Reference point switch – Many stages are equipped with direction-sensing reference point switches, which are located at about the midpoint of the travel range. It is recommended to approach the reference point switch always from the same direction to obtain best position repeatability. Function: Optical, magnetic.

Resolution – Position resolution relates to the smallest change in displacement that can still be detected by the measuring devices. The uncontrolled resolution in piezo nanopositioning systems and piezo actuators is basically unlimited because it is not limited by static or sliding friction. Instead, the equivalent to electronic noise is specified.

See >> Design resolution and >> Min. incremental motion

Ripple, noise, 0 to 100 kHz – Ripple of voltage in mVpp with unique frequency. Noise over the entire frequency range.

Rise time – Time constant of the controller/driver. The time required for increasing the voltage range from 10% to 90%.


Sensor bandwidth Measured value. The frequency, with which the amplitude decreased by -3 dB, is specified.

Sensor resolution – The sensor can be the critical element in position resolution, for this reason the sensor resolution can be specified separately if necessary. Rotary encoder: Impulses per screw rotation. Linear encoder: Smallest motion still detected by the sensor system used.

Straightness – See >>Crosstalk

Serial kinematics – Multi-axis system design in which each actuator drives its own separate platform. Advantages are simpler mechanical assembly and control algorithms. Disadvantages compared to parallel kinematics are poorer dynamic performance, no integrated parallel metrology possible, cumulative guiding errors, less accuracy.

Suggested capacitive load – For switching amplifiers. The possible output power of a piezo controller/driver depends on internal and external capacitive loads.

Standard Class Especially cost-favorable positioning systems are referred to as the Standard Class.

Static resolution – For capacitive sensors. Measured with a bandwidth of 10 Hz, nominal measuring range.

Stick-slip effect, friction – This effect limits the minimum incremental motion. It is produced during the transition from static to sliding friction and causes a motion. Friction-free drives, such as piezo actuators with flexure guides, are not affected by stick-slip effects so that resolutions in the sub-nanometer range are possible.


Tilt – See >> Crosstalk

Travel range – The maximum possible travel range is limited by the length of the drive screw. The distance between the limit switches, if any, determines the travel range.


Unidirectional repeatability – The accuracy of returning to a given position from the same direction. Because unidirectional repeatability is almost unaffected by backlash and hysteresis, it is often considerably better than “bidirectional repeatability”.


Velocity, max. – This is the short-term peak value for horizontal mounting, with no load, and not intended for continuous operation. Average and permanent velocities are lower than the peak value and depend on the external conditions of the application. Data for vacuum versions can differ.


Wobble – Wobble describes tilting of the rotary stages around the rotational axis in each revolution.


Yaw / pitch – See >> Crosstalk

* The Motion Control glossary is a rendition from Physik Instrumente (PI) GmbH glossary section (PI Glossary for Technical Terms)

Photonic Glossary

To find the word of your choice in the glossary below, click the corresponding starting letter of the first word, to bring up the correct section.


Background Temperature (TB) °K – The effective temperature of all radiation sources, exclusive of the chosen system source or target, in the detector field of view.

Beam Angle – Generally specified as the off-axis angle where the output power drops to 50% of the peak value. Can be specified from 50% to 50% point, or peak to 50%. Generally speaking, if the value is referred to as Half Intensity Beam Angle or FWHM, the value is from 50% to 50% points.

Bias Voltage (VB) volts – The voltage applied to the detector circuit, usually in DC volts. Total bias on CSI test data reports is split between the detector and load resistor in series. The split is based on relative resistance match.

  • Optimum Bias – The value of bias voltage which yields maximum signal-to-noise ratio.
  • Maximum Bias – The value of bias voltage which yields maximum signal.

Candela – A measurement of luminous intensity. Visible LEDs are usually specified in Candela (cd) or millicandela (mcd). The angle of measurement is critical when comparing lensed (narrow beam) products from different vendors. Value is pegged to the human eye response, making the peak wavelength a critical factor in the final value. Infrared LEDs have a value of nearly zero, because they do not emit appreciable levels of visible light.

Centroid Wavelength – The wavelength value where half of the light energy is at shorter and half the energy is at longer wavelengths. Value is stated in nanometers (nm) or microns (µ). This value is of interest to people in the test and measurement industries. Not commonly specified for standard LED products of any wavelength.

Cutoff Wavelength (λc) µm – The wavelength at which detector D* has degraded to one half of its peak value.

Cutoff Frequency (fc) Hz – A measure of detector response speed. It is defined as the frequency at which detector signal decreases to 0.707 of its maximum value when it is exposed to a square wave pulse of radiation.


D-Star (D*) cm Hz1/2 W-1 – The figure of merit which essentially describes IR detector signal-to-noise ratio (S/N), normalized to a detector active area (Ad) of 1cm2 and a noise equivalent bandwidth Δf of 1Hz. It is also a function of source radiation temperature, chopping frequency (f), detector operating temperature, field of view and background temperature. 

where signal (S) and noise (N) are in volts or amps, H is in rms watts/cm2 and is the flux density of source radiation incident on Ad. Unless specified otherwise, detector field of view is 2 steradians and background temperature is 298°K. 

  • Blackbody D Star (D*BB) – The D* of a detector irradiated by a blackbody source at a given temperature, and expressed, for example, as D* (500°K, 1000Hz, 1Hz), where 500°K is the blackbody source temperature, 1000Hz is the chopping frequency and 1Hz is the noise equivalent bandwidth. 
  • Peak D Star (D*λpk) – The D* of a detector irradiated by a monochromatic source operating at a wavelength where the detector has its maximum S/N, and expressed, for example, as D*(λpk, 1000Hz,1Hz). D*λpk = (C.F.) D*BB, where (C.F.) is a conversion factor which is a function of detector cutoff wavelength and spectral response. D*λpk is usually not a directly measured parameter in IR detector test data and is calculated using a predetermined C.F. for a given detector chemistry and operating temperature.

Dark Current  Usually abbreviated as ID. The current flowing through a reverse biased photodiode when light is not incident upon a photodiode. Higher reverse bias voltages result in higher dark currents. Dark current is not present in zero photodiode bias circuits (see shunt resistance).

Dark Resistance (Rd) ohms – The resistance of the detector, at a given detector temperature, with no incident source radiation. 

Dominant Wavelength – The color, or perceived wavelength of a light source by the human eye. Also called the hue wavelength. Most visible LEDs are specified by the dominant wavelength.


Element Size and Area (L,W) mm and (Ad) cm The length (L) and width (W) of the active area of the detector element. L is the distance between detector electrodes and is expressed first for rectangular elements (e.g. for a 1 mm x 2 mm detector, the distance between electrodes is 1 mm). The active area (Ad) of the detector generates signal and noise, and Ad = LxW. 

Element Spacing and Pitch mm – Element spacing is used to describe the non-active distance between active detectors in an array. Pitch is used to denote the center-to-center distance between active detectors in an array. 

Element Temperature (Td) °C – The actual detector element operating temperature. For uncooled detectors, this is usually ambient temperature. 


Field of View (FOV) Steradians – The solid angular measure within which the detector can respond. Measured from the diagonal of the active area to the limiting aperture. 

Full-Width-Half-Max – Usually abbreviated as FWHM. Used most commonly when discussing beam angle or spectral bandwidth. In both cases it refers to the distance from 50% to 50% point, or and –3db to and –3db point. Beam angle value is specified in degrees and spectral bandwidth values are specified in nanometers.


Load Resistor (RL) ohms – A resistor in the bias circuit which is connected in series with the detector. 


Junction Capacitance – P-N junctions have an inherent capacitance similar to a parallel plate capacitor. The junction capacitance is proportional to the active area of the semiconductor. In the case of photodiodes, the junction capacitance can be reduced by reverse biasing it. The junction capacitance in conjunction with the inherent series resistance of the diode is not the limiting factor for the response time of the device (see response time).


Light Emitting Diodes – Commonly abbreviated as LED, LEDs, or IRLEDs in the case of infrared light emitting diodes. Refers to any diode which when forward biased converts electrons (electrical current) to photons (light) in a non-coherent waveform.

Load Resistor (RL) ohms – A resistor in the bias circuit which is connected in series with the detector. 

Lumens – A measurement of total visible energy emitted from a point source. The output is measured in an integrating sphere with a detector whose spectral sensitivity approximates the human eye. This value is not commonly specified for LEDs.


Noise Equivalent Power (NEP) Watts – The amount of incident signal radiation (W) required to yield a detector signal-to-noise (S/N) ratio of one. NEP is a function of source temperature, chopping frequency, noise equivalent bandwidth, field of view and background temperature. 

where signal (S) and noise (N) are in volts or amps and W is the incident signal radiant power in watts. It is expressed, for example, as NEP (500°K, 1000Hz, 1Hz) where 500°K is the blackbody source temperature, 1000Hz is the chopping frequency and 1Hz is the noise equivalent bandwidth. Since noise is proportional to the square root of the bandwidth, if normalized to


Peak Wavelength – The wavelength value with the highest amount of energy radiating from the source. Most commonly specified for non-visible (infrared) LEDs.

Photoconductive Detector – Common name given to a photodiode operated in a reverse bias mode. This mode decreases junction capacitance, increases speed and linearity. It also increases noise current by introducing dark current to the circuit.

Photodiode – Generic name given to any diode used as a light detector. The device has no internal gain like a photodiode or photodarlington. Directly converts photons (light) into electrons (current). It is linear over at least 6 decades of light input. The average saturation point is 10mW/cm2. Used extensively where light must be accurately measured or higher speeds (greater than 30KHz) is required. The response is measured in Amps/Watt (A/W).

Photovoltaic Detector – common name given to a photodiode operated in a zero bias mode. This mode is commonly used in lower speed applications where rise time and junction capacitance are less important than minimizing dark current and thus reducing noise current.

Power Output – Value is expressed in Watts or milliwatts. A radiometric measurement of the total light energy radiating from an emitter regardless of wavelength. Measurement is made with an integrating sphere. This figure of merit is most commonly used with IRLEDs. It is the optical output measurement that is most easily correlated from one measurement facility to another.


Radiant Intensity Radiant measurement of on-axis intensity. This value must be known to calculate optical power incident on a detector that is greater than 6 inches from the LED. The angle of measurement is a critical component when comparing data sheets from one vendor to the next.

Response Time – Also known as rise or fall time. The period of time it takes an emitter or detector to go from the 10% – 90% point, emitting and detecting respectively, or the 90% – 10% point. RC time constant of the device is almost never the limiting factor. The speed of the device is almost always due to the transit time of the semiconductor material and the distance from the depletion region to edge of device.

Rise Time, Fall Time (tr, tf) Seconds  The measure of detector response time to pulses of radiation input. Rise time is the time required to rise from 10% to 90% of the maximum signal output and fall time is the time required to go from 90% to 10%.

RMS Noise (N) Volts rms or Amps rms – The electrical output of the detector with no incident signal radiation on it. It is primarily a function of detector area, background temperature, operating temperature, bias, noise equivalent bandwidth and field of view. 

RMS Signal (S) Volts RMS or Amps RMS – The electrical output of the detector due to incident signal radiation. It is primarily a function of chopping frequency, operating temperature, bias, and spectral content/temperature of the source. 


Short Circuit Current – Operation of a photodiode in a condition where a voltage bias is not allowed to generate across the diode. Usually accomplished by using a transimpedence amplifier circuit.

Shunt Resistance – The zero bias resistance of a photodiode. In practical measurements, most manufacturers put a 10mV reverse bias on the photodiode and measure current. The ratio between the bias voltage and current determine the shunt resistance value. This value must be known to determine noise current generated by the photodiode in a photovoltaic, short circuit current mode circuit.

Spectral Response – The conventional method for determining the sensitivity of photodiodes. The term is expressed in Amps/Watt (A/W). The monochromatic wavelength the measurement is done at must be specified as well.

Spectral Responsivity () Volts/Watts – The ratio of detector signal output to monochromatic incident radiation power. It is mainly a function of detector temperature, wavelength, chopping frequency and bias, and Rλ=S/W where the signal (S) is in volts or amps and W is the incident signal radiant power in watts. 


Peak Responsivity (pk) – The responsivity value at which the detector has a maximum spectral response, and is expressed, for example as R (pk, 1000Hz) where 1000Hz is the chopping frequency. 


Time Constant (τ) Seconds – A term usually used to describe the detector’s speed of response to a square wave pulse of radiation and 

* The Photonic glossary is a rendition from OptoDiode Inc. glossary sections (Glossary of Terms & Common Definitions for LEDs and Photodiodes)