Safe Brake Control (SBC)

The Safe Brake Control (SBC) instruction:

Available Languages

Ladder Diagram

Function Block

This instruction is not available in function block.

Structured Text

This instruction is not available in structured text.

Safe Brake Control Application

Use Safe Brake Control with safety I/O, and safety contactors, to control the brake and brake timing for STO. The following figure illustrates an application with an external brake mounted to a motor controlled using SBC, a GuardLogix controller, safety I/O, and a safety contactor.

Operands

The following table provides operands used for configuring the instruction.

The following table explains the instruction inputs.

1

ISO 13849-1 stipulates instruction reset function must occur on falling edge signals. To comply with ISO 13849-1 requirements, add the logic immediately before this instruction. Rename ‘Reset Signal’ tag in this example to the reset signal tag name. Then use the OSF instruction Output Bit tag as the instruction’s reset source.

This table explains instruction outputs. The outputs are external tags (safety output modules) or internal tags used in other logic routines.

This table explains instructions outputs that are written to the user-specified tag.

Affects Math Status Flags

No

Major/Minor Faults

None specific to this instruction. See Index Through Arrays for array-indexing faults.

Execution

Ladder Diagram

Operation

The SBC instruction is used to control and monitor a mechanical brake. Timing between Safe Torque Off and brake operation is controlled by STO to SBC delay, which may be either positive or negative. Two timing cases showing STO to SBC delay > 0, and STO to SBC delay =<0 are illustrated in the sections that follow.

Pass-Through Tags

A Safe Motion Monitoring Drive has one or more motion axes that are controlled by a motion task. The Safe Motion Monitoring Drive also has one or more motion safety instances that support safety functions used in a safety task of a safety controller. Some of the tags associated with a drives motion safety instance are pass-through tags. The following table shows the pass-through tags and the corresponding axis tags for the SBC function:

1

module is the name for the drive module in Logix Designer I/O Configuration tree

2

instance is 1 or 2 for dual axis drives otherwise null

3

axis is the axis name in the Logix Designer Motion Group and is associated with module

When assigning the SBC Active, Brake Engaged and SBC Integrity instruction outputs to the motion safety instance pass-through tags, the corresponding axis tags automatically update in the motion controller. The motion control task of motion controller reads the axis tags to coordinate operation between the safety task and motion task.

Normal Operation, STO to SBC Delay > 0, Automatic Restart

When the STO to SBC delay is > 0 Category 0 stopping is typically used. With Category 0 stopping, torque is removed from the motor first and then, after the STO to SBC Delay, the brake is applied. In this case the motor coasts before the brake is applied. Typically Torque Off Request output is used within a safety application to initiate the STO function in a drive safety instance. The STO function in the drive will immediately remove torque from the motor without motion coordination. SBC Active and Brake Engaged are passed from the drive safety instance to the drive Axis Safety Status tag so that the motion controller responds accordingly.

SBC operation is described as follows. Assuming the SBC function has been reset, the SBC function becomes active when Brake Engage L input is cleared OFF(0). Upon Brake Engage L cleared OFF(0), SBC active and Torque Off Request are set ON(1). At the same time, the STO to SBC delay timer is started. The STO to SBC delay allows the motor to coast before Brake Output 1 and Brake Output 2 are cleared OFF(0). Whenever the Brake Outputs 1 and 2 change state, the feedback check delay timer is started. When the feedback check delay timer expires, the Brake Feedback 1 and 2 inputs are monitored and must be at and remain at the opposite state of the Brake Outputs. When the Brake Outputs are OFF(0) and the Brake feedback is ON(1), the Brake Engaged Signal is set ON(1) after Feedback Check Delay. With automatic restart, shown in the diagram below, the SBC function is restarted and ready for subsequent operation if it is not faulted and the Brake Engage L input returns to the inactive state ON(1).

Normal Operation, STO to SBC Delay

0, Automatic Restart

When the STO to SBC delay is ≤ 0 typically the application will be using category 2 stopping. With category 2 stopping the motor is brought to a controlled stop, then actively held at standstill. With the motor held at standstill, SBC is activated, applying the brake first then after the STO to SBC delay, torque is removed. Torque is removed by use of the Torque Off Request output to initiate STO in the safety instance of a drive according to the particular Logix safety application. The SBC Active, SBC Integrity, and Brake Engaged outputs of the SBC instruction are sent to the drive safety instance, and then the associated axis status tag is updated. The motion controller application then reads the updated axis status tag and takes any required actions for the application.

SBC operation with STO to SBC Delay 0 is described as follows. After the SBC function has been Reset, the SBC function begins when Brake Engage L is cleared OFF(0). Upon Brake Engage L OFF(0), the brake outputs, BO1 and BO2 are cleared OFF(0) and SBC active is set ON(1). At the same time the STO to SBC delay timer is started. The STO to SBC delay allows the brake to engage before the Torque Off Request is set ON(1). Whenever the Brake Outputs 1 and 2 change state, the feedback check delay timer is started. When the feedback check delay timer expires, the Brake Feedback 1 and 2 inputs are monitored and must be at and remain at the opposite state of the Brake Outputs. When the brake outputs are OFF(0) and the Brake feedback is ON(1), the Brake Engaged Signal is set ON(1) after Feedback Check Delay. With automatic restart, shown in the diagram below, the SBC function is reset and ready for subsequent operation if it is not faulted and the Brake Engage L input is set ON(1).

Manual Restart

When Manual Restart is used, the SBC function is begins operation with the ON(1) to OFF(0) transition of Brake Engage L. The function will be reset if it is not faulted, Brake Engage L is ON(1) and an OFF(0) to ON(1) transition of the Reset input occurs.

Cold Start

The SBC function requires manual cold start. When the controller run mode begins the SBC instruction starts with the Brake Outputs OFF(0) and waiting for Reset. The SBC function requires a successful reset in order to release the brake and allow subsequent operation similar to a Manual Restart. For a successful reset the conditions below must be satisfied before Reset makes an OFF(0) to ON(1) transition:

Brake Feedback 1: ON(1)

Brake Feedback 2: ON(1)

Input Status: ON(1)

Output Status: ON(1)

Brake Engage L: ON(1)

Faults and Fault Reset

The SBC instruction monitors the state of the brake feedback and the I/O module status bits continuously when the rung-condition-in is true. Faults are caused by invalid configuration, or by invalid inputs. Any condition that causes the SBC function to fault will clear Brake Output 1 and Brake Output 2 to OFF(0). The Brake Outputs will remain OFF(0) until the fault condition is corrected and the SBC instruction is reset. Torque Off Request Remains in its last state just before the fault.

Brake Feedback Faults

Anytime the Brake Outputs, BO1 and BO2 change state the Feedback Check Delay timer is started. When the timer is running, the Brake Feedback signals are ignored. When the timer is no longer running the Brake Feedback 1 and Brake Feedback 2 signals are continuously monitored. The Brake Feedback signals must be in the opposite state of the Brake Output signals, otherwise the SBC function will fault. In the figure below, the Brake Feedback signal did not change to the ON(1) state after the Brake Output 1 changes to the OFF(0) state and the Feedback Check delay expires. This results in a Fault and also clears OFF(0) the SBC Integrity bit. The figure shows an unsuccessful attempt to reset the instruction before the fault condition has been corrected followed by a successful reset after the fault has been corrected. The drive Axis tag, Safe Brake Integrity Status, is cleared when the fault condition is detected. Since the actual state of the brake cannot be determined when the fault is present, the SBC Torque Off Request will not be asserted. The motion task is then able to maintain control of the motor which may be necessary for some applications including those where gravity may cause motion.

If the brake feedback signal or I/O status signals are in the wrong state after the SBC function has been successfully reset then the SBC instruction will fault. The following figure shows the SBC function in Automatic Restart mode being reset initially by Brake Engage L set ON(1). Immediately upon reset the Brake Output 1 and 2 are set ON(1) and the Feedback check delay timer begins. After the timer expires, the Brake Feedback is in the incorrect state and SBC faults. In this case, the cause of the fault needs to be corrected in order for the instruction to not continue to fault after subsequent resets. In the figure, the fault is assumed to be corrected and does not fault again after the Reset OFF(0) to ON(1) transition and the final check delay time expires. For the case shown in the figure the fault indicates that the brake may have stayed engaged when it was supposed to be released. The SBC integrity is cleared to OFF(0) which will be reflected in the Axis tag, Safe Brake Integrity Status. This allows the motion task to keep the axis at standstill until the fault is corrected to avoid possible damage to system mechanical components.

Fault Codes and Corrective Actions

Diagnostic Codes and Corrective Actions

Example