Glossary of Terms

Glossary of Encoder Terms

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Accuracy
Related to the incremental encoding disk. It is the difference between the theoretical position of one increment or bit edge and the actual position of the edge.

Axial Loading
The force applied to a shaft end surface directed along the axis of rotation.

Axial Load (maximum)
Maximum axial load is the maximum force that may be applied to the shaft without reducing the rated operating life or causing deviation from the rated performance.

Bi-directional
Bi-directional refers to an encoder output code format from which direction of travel can be determined.

CE (European Compliance)
Sets essential electromagnetic compatibility, within the European markets, for all electrical and electronic equipment that may interfere with other equipment, or that may be interfered with other equipment.

Channel
Each channel is a unique incremental output of the encoder.

Current Sinking Output
A logic form that requires current flow out of the input of the PLC or counter and back to the output of the encoder. The encoder sink this current, which is sourced by the input circuitry. This is the most common output circuit configuration. It uses an NPN output transistor in the encoder.

Current Sourcing Output
A logic form that requires current flow from the output of the encoder to the input of the counter or PLC. The encoder sources the current and the input circuitry of the counter or PLC sinks this current. This output circuit is seldom used. It usually requires a PNP output transistor in the encoder.

Pulses Per Revolution/Cycles Per Revolution
The number of increments on the disk of an incremental encoder. A one thousand increment encoder has a PPR/CPR of 1000.

Differential Output
Differential output refers to the complementary outputs from a feedback device when the signalsare excited by a Line Driver. Optimum performance is achieved when the receiver input impedance is matched to the line receiver output and transmission line.

Disc
Typically made of glass, metal or plastic with precise position incremental lines. These lines are also known as increments. The number of increments determines the resolution or CPR of the encoder.

Encoder (shaft type)
An encoder is an electro-mechanical device that translates mechanical motion (such as position, velocity, acceleration, speed, direction) into electrical signals.

Frequency Response
The maximum frequency in cycles per second.

Index Reference/Marker
The Marker is a separate output generated by a single track which produces a single cycle/pulse (or transition change) at a unique position or positions such as centre, home, zero or reset point. Marker pulse is sometimes referred to as an Index.

IP 50
Protected against dust. Limited ingress (no harmful deposit).

IP 64
Totally protected against dust. Protected against water sprayed from all directions. Limited ingress permitted.

IP 65
Totally protected against dust. Protected against low pressure jets of water from all directions. Limited ingress permitted.

IP 66
Totally protected against dust. Protected against strong jets of water. Limited ingress permitted.

Line Driver
A circuit that provides error-free output pulses in electrically noisy environments or over long transmission lines when used with a line receiver.

Negative Going Pulse
When activated, the pulse goes low (logic 0) or in a negative direction. Do not be confused by negative going, meaning the pulse goes negative in relationship to the signal common or reference level. These statements are for positive logic only. All shaft encoders are based on positive logic.

NEMA 4
Enclosure rating intended for indoor or outdoor use primarily to provide a degree of protection against windblown dust and rain, splashing water, and hose directed water; undamaged by the formation of ice on the enclosure.

NEMA 13
Enclosures are intended for indoor use primarily to provide a degree of protection against dust, spraying of water, oil and noncorrosive coolants.

Open Collector
When the signal is taken directly off the collector element of the output transistor, no pull-up is used. This is the electronic equivalent of a mechanical switch closure to common. The input device of the PLC or counter is effectively placed in a series circuit that includes the output transistor and input device, which is often an optosolator and the positive voltage supply. When the output transistor turns on, the circuit is completed and current will flow. The output signal can not be observed unless the circuit is completed externally.

Positive Going Pulse
In the low or logic 0 state, it is in the quiescent state. It goes high or logic 1 when activated. This is a transition in the positive going direction.

Pulse Polarity
Either positive going or negative going. A pulse has two logic states: activated or inactivated. These two states are opposite. When the pulse is in its quiescent state (high or low), it is at one particular logic level (1 or 0). When the pulse hits or is in the activated state, this logic level reverses itself for the duration of the pulse.

Pulse Width
The actual real time between identical points on the leading or trailing edge of a pulse to the next successive leading or trailing pulse edge. The pulse width of the output signal of most encoders is a 50% duty cycle on the clock outputs. Some models utilize a timed or "one shot" output. This provides a constant pulse width irrespective of the pulse repetition rate or shaft speed. The factors to be considered when determining pulse width specifications are: (1.) What is the minimum pulse width requirement of the counter or PLC? This information is available in the counter or PLC specifications. (2.) Pulse repetition rate versus pulse width. With a constant pulse width, the individual pulses become closer together as the pulserepetition rate or shaft speed increases. At some point the pulses will overlap and the output signal as a series of well defined pulses ceases. The pulse repetition rate varies inversely with the pulse width and vice versa.

Pull-up Resistor
When added inside the encoder between the positive voltage and the collector element of the output transistor, it becomes a pull-up circuit. This is also know as a pulse output.

Push-Pull Output
An output circuit that will both sink and source current.

Quadrature
A dual output encoder used for bi-directional motion control. One channel leads the other by 90 electrical degrees. By monitoring the phase shift of both channel A and B, direction can be determined. Another benefit of a quadrature encoder is
count multiplication. With an appropriate counter, resolution can be multiplied up to four times. For instance, using this technique an encoder with CPR of 1000 can provide a resolution of up to 4000 pulses per shaft revolution.

Quadrature Error
Quadrature error is the phase error when the specified phase relationship between two channels is nominally 90 electrical degrees.

Radial Load
The force applied at a specific point to the encoder shaft perpendicular to the axis of rotation. Note: It is important to note where the force is being applied along the shaft, due to the cantilever effect.

Radial Load (maximum)
The maximum force that may be applied perpendicularly to the shaft without reducing the rated operating life or causing deviation from the rated performance.

Resolution
The number of increments on the encoder disk. For incremental encoders, resolution is defined as cycles per revolution.

Shaft Runout
Amount of shaft movement while spinning.

Single Channel
A single channel encoder produces one incremental output. They are often used for tachometry applications.

Torque (running)
Running torque is the rotary force required to keep an encoder shaft turning. It is typically expressed in ounce-inches.

Torque, Starting (breakaway)
Starting (breakaway) torque is the rotary force required to overcome static friction and cause the encoder shaft to begin rotating.

Unidirectional
Unidirectional refers to an encoder output code format from which direction of travel cannot be determined.

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