Encoder Considerations

The following table describes the supported encoder types based on the feedback card that is used and the physical terminal it is connected to. The Achievable System Safety Rating column shows the highest achievable safety rating for a system using the components listed. Other lower safety ratings may be achievable in specific situations. You must determine the actual system safety rating based on the encoder types used, the encoder diagnostics described in this chapter, the contents of this safety manual, and safety information provided by the encoder manufacturer. Contact the encoder manufacturer for further guidance.
Supported Feedback Cards and Encoder Types
Feedback Option
Primary Channel
Secondary Channel
Achievable System Safety Rating
Encoder Type
Encoder Motion Axis
Encoder Type
Encoder Motion Axis
20-750-UFB-1
Sine/Cosine
Motor Feedback
Not Used
Not Used
  • SIL 2/PLd with rated encoder OR
  • PLd with standard encoder
Digital AqB
Load Feedback
  • SIL 3/PLe with two rated encoders OR
  • PLe with two standard encoders
    1
20-750-DENC-1
Digital AqB
Not Used
Not Used
  • SIL 2/PLd with rated encoder
Digital AqB
Load Feedback
  • SIL 3/PLe with two rated encoders
When using a standard sine/cosine encoder, safety relevant data (MTTF) and safety diagnostic measures to achieve required diagnostic coverage need be considered. Encoder diagnostics for sine/cosine encoders provided by this product include: encoder voltage monitoring, sin2 + cos2 vector length monitoring, zero crossing detection, and signal offset (described in the section below). Additional (customer supplied) diagnostics may be required. You must determine the suitability of the encoder and the system safety rating.
To achieve a SIL 3 rating when using two encoders of the same type, consider whether the two encoders used have sufficient independence per IEC 61508-2 clauses 7.4.3.2 and 7.4.3.4
Depending on the encoder type, the module performs several diagnostic tests on encoder signals to detect faults in the encoder. You must determine if the combination of the selected encoder device type and the diagnostics satisfy the required safety function rating. The use of non-safety, standard encoders my require further analysis and assessment activities.

General Encoder Diagnostics

The following encoder diagnostics are available for all supported encoder types:
  • Encoder Voltage Monitoring (Configurable)
  • Maximum Speed Limit (Configurable)
  • Maximum Acceleration (Configurable)
  • Maximum Encoder Input Frequency
  • Dual Encoder Velocity and/or Position Discrepancy (Configurable)
IMPORTANT:
These diagnostics are based on the capability of the chosen encoder and its rated limits. They do not provide a safety-rated safety function.
Encoder Voltage Monitoring
The voltage monitoring diagnostic samples the voltage being supplied to the encoder to confirm that its level is within its configured range. If the voltage monitoring diagnostic detects a voltage that is out of the configured range, the safety feedback instance reports a voltage monitoring fault and causes the module to enter the safe state.
The following voltage monitoring ranges are supported:
  • 4.75…5.25V (Recommended setting when using 20-750-DENC-1 card with the 12V Jumper in the ‘Storage’ position)
  • 11.4…12.6V (Recommended setting when using 20-750-DENC-1 card with the 12V Jumper in the ‘Enabled’ position)
  • 7…12V (Recommended setting when using 20-750-UFB-1)
Maximum Speed Limit
The maximum speed limit diagnostic detects when encoder speed is above a configured limit. If the speed of the encoder is greater than the configured max speed limit, an exceeded max speed fault is reported by the safety feedback instance. This causes the module to enter the safe state.
If the encoder being used specifies a maximum speed, set the maximum speed limit configuration value to this value or lower. If the limit is configured as 0, this diagnostic is not performed.
Maximum Acceleration
The maximum acceleration diagnostic detects when encoder acceleration is above a configured limit. If the module detects that the acceleration of the encoder has exceeded the configured limit, a max acceleration fault is reported by the safety feedback instance. This causes the module to enter the safe state.
If the encoder being used specifies a maximum acceleration, set the maximum acceleration configuration value to this value or lower. If the maximum acceleration is configured as 0, this diagnostic is not performed.
Maximum Encoder Input Frequency
The maximum encoder input frequency diagnostic confirms that the safety feedback signals do not exceed the maximum frequency (encoder counts per second) supported by the module. This value is not configurable and has fixed values based on the encoder type.
Maximum Frequency of Encoder Types
Encoder Type
Max Frequency
Digital AqB
250 kHz
Sine/Cosine and Hiperface
163.8 kHz
If the module detects an encoder input frequency above the limit, a max frequency fault is reported in the safety feedback instance and the module enters the safe state.
Dual Encoder Velocity and/or Position Discrepancy
The dual encoder velocity and position discrepancy diagnostic confirms that the position and/ or velocity of the two encoders match within a configurable tolerance. The position and velocity discrepancy limits are individually configurable; setting the limit to a value of 0 disables the diagnostic check. If the module detects that the difference between the position and/or velocity of both encoders is outside the configured limit, a discrepancy error is reported in both safety feedback instances and the module enters the safe state. This diagnostic is only available when the module is configured in a dual feedback configuration.

Digital AqB Diagnostics

The following diagnostic functions are implemented in the module to perform diagnostics for digital AqB encoders:
  • Inverse Signal Monitoring
  • Quadrature Error Detection
Inverse Signal Monitoring
The inverse signal monitoring diagnostic confirms that the inverted and non-inverted signals are always at opposite signal levels. If the module detects a non-inverted signal, a feedback signal lost fault is reported in the safety feedback instance and the module enters the safe state. This diagnostic is meant to detect encoder wiring errors, such as open, short, or short to power.
Quadrature Error Detection
The quadrature error detection confirms that the A and B signals from the digital AqB encoder do not change simultaneously. This diagnostic is also referred to as an exclusive bit check. If the module detects a quadrature error, the safety feedback instance reports a quadrature error fault and enters the safe state. A simultaneous change indicates an error with the encoder wiring or an issue with the encoder itself.

Sine/Cosine and Hiperface Diagnostics

The following diagnostic functions are implemented in the module to perform diagnostics on Hiperface and or Sine/Cosine type encoders:
  • Sin
    2
     + Cos
    2
     Vector Length Monitoring
  • Zero-crossing Detection
  • Signal Offset (Sine/Cosine Encoder Type Only)
Sin
2
 + Cos
2
 Vector Length Monitoring
The Sin
2
 + Cos
2
 vector length monitoring diagnostic confirms that the sine and cosine signals are sinusoidal and 90° apart. This diagnostic is meant to detect errors in the wiring of the encoder and problems within the encoder itself.
If the module detects that the amplitude and or phase of the signals is out of range, the safety feedback instance reports a Sin
2
 + Cos
2
 fault and the module is placed in the safe state.
Sin
2
 + Cos
2
 Vector Length Monitoring Amplitude Range
Max
Min
1.3 V
pp
0.7 V
pp
Sin
2
 + Cos
2
 Vector Length Monitoring Phase Tolerance
Tolerance
90º ± 20°
Zero-crossing Detection
The zero-crossing detection diagnostic confirms that the sine and cosine signals have a similar offset to ground. The offset tripping point is ± 50 mV. If the offset of the sine and cosine signals is greater than the tripping point, the zero-crossing detection diagnostic will fail, a signal lost fault is reported in the safety feedback instance, and the module is placed in the safe state.
Signal Offset
The signal offset diagnostic confirms that a Sine/Cosine type encoder is producing the proper offset on the Sine and Cosine signals. This diagnostic is not performed when the feedback device type is configured as Hiperface.
If the offset of the Sine and or Cosine signals are outside the tolerance range, the safety feedback instance reports a signal offset fault and the module is placed in the safe state.
Signal Offset Tolerance
Max
Min
3.0V
2.0V
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