Quadrature Decoder

(available in Software Version 4.10 and newer)

Introduction

The Quadrature Decoder provides an interface between an external Incremental Encoder (Linear or Rotary) and the Sensor Trigger Control Block. The actual Incremental Encoder is connected to the Quadrature Decoder via the External Input.

Incremental Encoders provide motion information of a mechanical device in the form of 1 to 3 analogue or digital signals. These signals, once converted into digital format, are connected to the GPIO input pins of the iam. The motion information is conveyed via 1 or 2 signals (referred to as “Signal-A” and “Signal-B”) by providing a pulse each time the Encoder has travelled a particular distance. There is also an optional Reference signal (also called “Index” or “Home”) which is activated each time the Encoder reaches a certain starting position. The Decoder analyses these signals to determine what exact physical position the Encoder is currently at and generates a trigger signal to the sensor based on the Encoder position.

Physical Connection

The following pins are used as input for the quadrature decoder block.

Sensor Trigger Usage

To use the quadrature decoder trigger output as sensor trigger, set TriggerSelector remote device feature to FrameStart, set TriggerMode to On and set TriggerSource to Quadrature Encoder ("Encoder0").

(De)activating the Decoder

To activate the decoder, set SvEncoderMode ("Encoder Mode") to the desired operation mode (SinglePhase, DualPhase or Quadrature). To disable the decoder, set the feature to Off.

Also set the appropriate "Encoder Value Max".

Encoder Modes

The Encoder signals can be decoded using one of three different modes:

Open

Open

In addition to the motion signals, a third optional Encoder signal (Reference Signal) can be used to provide absolute position information of the Encoder. When a Signal-A/B change is detected the Reference Signal indicates that the Encoder has reached a defined position, i.e. top or left- or rightmost, or similar. This event can be used in the Decoder to re-align the internal Position-Counter and/or to detect if any errors may have occurred during the last Encoder rotation.

In the Decoder the reference signal will be updated, if Signal-A changes - in Quadrature Mode, if A/B-Signals changes. Therefore the length and position of the reference signal have to meet the requirement, that it can be sampled internaly. If the reference signal does happen close after a change of A/B-Signals, a timing window for determining the direction can be set via the SvEncoderSamplingDelay ("Encoder Sampling Delay") feature. Please note that the "Encoder Sampling Delay" contributes to the latency between the Decoder’s Signal-A/B inputs and the Sensor Trigger output.

Position and Rotation

The Position-Counter (EncoderValue) indicates the current position of the Encoder within a rotation (Rotary Encoders) or track (Linear Encoders). When the Direction Analyzer indicates that the Encoder has taken a step forward, the Position-Counter is incremented by ‘1’. Similarly, if the Encoder travels backward the Position-Counter is decremented by ‘1’. If the Reference signal is detected and the Encoder is travelling in forward direction, the Position-Counter is set to ‘0’. In backward direction the Counter is set to a value of (Counts-per-Rev – 1), which is stored in SvEncoderValueMax.

The Decoder uses either the rising or falling edge of the Reference signal as the synchronization point at which an encoder rotation is considered to be complete and a new cycle is started. The edge of the Reference signal can be selected with EncoderResetActivation. With EncoderResetSource the Reference signal can be enabled. The EncoderResetSource can be inverted with EncoderInvertResertSource.

The SvEncoderValueMax register is also used to detect “Position Errors” (SvEncoderPositionError) in the Decoder at the time the Reference signal is received. If the Encoder travels forward at the time the Reference signal is received, the Position-Counter is compared with SvEncoderValueMax. If the two values do not match, the Decoder may have lost synchronization with the Encoder at some point during the rotation. This condition is most likely due to phase error sequences received from the Encoder. The same error also occurs when the Encoder travels backwards through the Reference point, except in this case the expected Position-Counter value is ‘0’.

The Quadrature Decoder can also operate without the use of a Reference pin from the Incremental Encoder. In this mode (EncoderResetSource = Off) the Decoder relies on the Position-Counter to determine when a full rotation has been completed. All functions carried out by the Decoder are identical, except that (due to the missing Reference point) absolute Encoder position information is lost. Also, Position-Error checking is not possible.

The rotation refers to the number of reference signals received. When a reference signal is received, the SvEncoderRotationCount ("Encoder Rotation Count") value is incremented or decremented depending on the direction of movement. Since the Encoder can travel in either forward or backward direction, the Revolution-Counter is a signed (2’s compliment) register to indicate the total number of revolutions. The register can represent the number of revolutions within a range of -128 to +127. If the counter overflows (or underflows) an interrupt is generated to inform the Host of the limit condition. In either case, the Revolution-Counter is reset to ‘0’ when the over/underflow occurs.

Direction Analyzer

In Single or Dual Mode the Direction Analyzer receives Signal-A as indicator that another step has been taken by the Encoder. As an option Signal-B can be used as a directional input to allow the Decoder to operate in the same way as Quadrature Mode, albeit with a lower resolution. The table below shows how Signal-B is interpreted to determine direction:

In Quadrature Mode the Direction Analyzer receives both Encoder signals Signal-A and Signal-B and determines at what time a step is made by the Encoder and also in which direction it is travelling. The two Encoder signals are combined into a 2-bit Current-Phase (bit 0 = Signal-A, bit 1 = Signal-B) variable. When a phase change is detected, the Analyzer compares the previous phase value with the current (i.e. next) phase value and determines the direction as shown in the table below:

It is also possible that the Decoder receives erroneous phase sequences. For example, if the Encoder is travelling at excessively high speeds the Encoder output signals may get distorted, resulting in phases being skipped. Alternatively, electrical noise may interfere with the Encoder signals. Whatever the reason for the error, a wrong phase sequence is likely to result in the Decoder temporarily (or permanently) loosing track of the Encoder position. These error conditions are reported to the Host via a Decoder-Error-Interrupt-Request.

Decoder Start-up Behaviour

Trigger Mode

The SvEncoderTriggerMode feature allows to specify different ways of determining when triggers are generated based on position and rotation.

Trigger-Pulse-Counter

The Quadrature Decoder features a Trigger-Pulse-Counter which can be used to automatically stop image acquisition after a programmable number of Encoder pulses or Frame-Triggers have been detected.

The Trigger-Pulse-Counter (SvEncoderCurrentTriggerCounter) can be configured via the SvEncoderMaxTriggerCountMode and SvEncoderTriggerDeactivationCounterPosition.

When enabled (SvEncoderMaxTriggerCountMode unequal Off) the Trigger-Pulse-Counter is incremented or decremented each time a defined event is detected. When the Trigger-Pulse-Counter reaches a value equal to SvEncoderTriggerDeactivationCounterPosition all further Frame-Triggers to the Sensor are suppressed by setting a Trigger-Block, regardless of any external trigger events which may occur.

The table lists how the Trigger-Pulse-Counter can be configured:

The Trigger-Pulse-Counter is incremented in Modes “Encoder Forward Only” and “Output” but can be decremented in Mode “Encoder Backward Only”. It can be incremented and decremented in Mode “Encoder Forward and Backward”. In the last case a wrap-around of the counter value from the minimum value (-16777215) to the maximum value (16777215) may occur.  This needs to be considered when initializing SvEncoderMaxTriggerCount with an appropriate value. Regardless of the direction the Counter is counting, the Trigger-Block is always set when the Trigger-Pulse-Counter reaches the value equal to SvEncoderTriggerDeactivationCounterPosition.

The Trigger-Pulse-Counter can be enabled or disabled regardless of the operating mode of the Quadrature Decoder. When the Trigger-Pulse-Counter is disabled  (SvEncoderMaxTriggerCountMode equal Off) the Trigger-Block is inactive.