SBT (Safe Brake Test)

Safe Brake Test description for drive option Functional Safety 2 or 3.

Description

The function SBT (Safe Brake Test) can be used for testing the brakes connected to X1, X2 or X4 of the AKD2G. SBT is carried out by applying a configured torque against the closed brake and supervising the motion on the connected axis. The test is successful if the axis motion does not exceed a parameterized tolerance. If the axis motion exceeds the tolerance, the test is failed and therefore the safety functions using the brake are lost. After a failed brake test, safe brake operation cannot be assumed anymore. Brake replacement or service will be necessary.

SBT is a diagnostic function for holding brakes. In order to operate brakes (SBC) that are part of a safety function (PLd / SIL2 or higher), frequent tests with SBT are required.
SBT must be triggered regularly by the end customer.
SBT considers the derating factor for brakes used in safety functions.
During SBT, 130% of the max. possible application torque will be applied to the brake. Additional forces caused by vertical loads will be taken into account automatically.
Brake slippage will be detected with a resolution of at least 1°.
Two brakes can be assigned to one axis. Each brake can be configured with an independent slippage tolerance.
SBT is only functional with SBC.
SBT does not lead to wear of the brakes.

Important Notes

The user is responsible for the trigger of the SBT function.
For correct operation of SBT the axis must be free to move when the drive releases the brake or releases both brakes as appropriate; if the machine has further motors and/or brakes mechanically coupled to the same axis, these devices must be set to states where they do not impede motion before SBT is carried out. In this case only one brake shall be tested at a time. Each brake must be tested independently.
The machine manufacturer has to determine the SIL / PL reached by SBC + SBT according to the configuration and parameterization.

When using external position command/PLC

At the end of SBT there is a mismatch between the set command value and the current position value of the axis. It is important that the master PLC executes an alignment of the set command position and the current axis position before SBT finishes to avoid a jump of the axis.

Execute a brake test

at least every 8 hours,
at the start of a machine or at the transition to operation mode using the brake,
after long standstill of the machine,
after drive reboot,
after brake service or replacement.

Depending on the target SIL / PL of the safety function that is performed by the brakes, it can be necessary to carry out SBT more often. In high safety demand applications (SIL3 / PLe, DC = high), it can be necessary to execute SBT every time prior to demand. For example if access to a dangerous area is prohibited by a safety interlock, SBT must be executed successfully before access can be granted.

Restrictions

The maximum motion for the brake during safe brake test (see “Maximum Motion” parameters (➜ # 1, Related Parameters)) cannot be set to a greater or equal value than 10800 arcminute. Otherwise, the SMM will return an error code once SBT function is started. The error code can be read by the keyword AXIS#.SAFE.SBT.ERROR, which will report “0x6 – SMM System Error”.

In general, it is expected to set the maximum motion for the “tested” brake as small as possible to prevent any mechanical hazards when the brake loses its holding function. However, an application might need a larger tolerance window because of a mechanical gearing between encoder and motor shaft.

The maximum slippage velocity in [RPM] is calculated by:

(10800 [arcmin] – “Max. Motion” [arcmin]) * 600 / 216

Example:

For “Max. Motion” = 300 arcmin (5°), the max. slippage velocity is
(10800-300) * 600/216 = 29167 rpm.
For “Max. Motion” = 7200 arcmin (120°) , the max. slippage velocity is
(10800-7200) * 600/216 = 10000 rpm.

The user has to consider for his application the worst case max. acceleration of motor shafts load including the additional "applied" test torque of the drive during SBT.

Preconditions for a brake test:

The motor is at standstill.
The tested brake is released.
The drive is enabled.
STO is not active.

Encoder requirements

The encoder verification is not required for safe encoders that can be configured on drives with option FS3.

The SBT function can be executed with any encoder type. However, the encoder shall have at least a resolution and accuracy of 1 degree.

Encoder verification:

The user shall verify that the encoder works properly by his application. As example the following types of machine motion can be used to verify the function of the encoder:

Perform a home move which includes a physical motion.
A position move of at least 1 mechanical revolution of the encoder.
A relative move to reach the start position of SBT.

After a long downtime of the machine, the encoder verification shall be repeated.

Current measurement verification

The verification check of drive current measurement should be done during machine commissioning and during re-commissioning due to changes such as replacement of the drive or firmware update.

The current measurement should verify expected current reported by the drive when holding a reference load. A small movement may be needed to overcome static friction.

Function Input / Output Variables

Inputs

SBT Activate can be activated by
EtherCAT, CANopen
Digital input (must be mapped)
WorkBench

Outputs

SBA State: logical status providing information on SBT. Can be mapped to a safe output and retrieved through FSoE status word.
0 = SBT execution necessary or failed in last attempt
1 = SBT was executed successfully within the parameterized period
SBT State: for status information on the actual brake condition see (➜ # 1, AXIS#.SAFE.SBT.STATE).

Number of Instances

One instance per axis.

Activation

By a non safe fieldbus: EtherCAT or CANopen SDO
By a non safe digital input (DIGITAL-IN1 to DIGITAL-IN12, (see "Digital Inputs")).
By WorkBench

Related Parameters

Safety parameters

Name	Variables	Default	Parameter
Function Activation	-	0 (Never active)	AXIS#.SAFEPARAM.SBT.FUNCTIONACTIVATION
Maximum Time Interval	-	480 min	AXIS#.SAFEPARAM.SBT.MAXTIMEINTERVAL
Max Application Load	-	1000 mA	AXIS#.SAFEPARAM.SBT.MAXAPPLOAD
Maximum Motion 1st Attached Brake	-	300 arcmin	AXIS#.SAFEPARAM.SBT.MAXMOTIONBRAKE#
Maximum Motion 2nd Attached Brake	-	300 arcmin	AXIS#.SAFEPARAM.SBT.MAXMOTIONBRAKE#
Torque Duration	-	500 ms	AXIS#.SAFEPARAM.SBT.TORQUEDURATION

Diagnostic parameters

Name	Variables	Default	Parameter
SBT Actual Status	-	-	AXIS#.SAFE.SBT.STATE
SBT Move	-	-	AXIS#.SAFE.SBT.MOVE
SBT SBA Active	-	-	AXIS#.SAFE.SBT.SBA.ACTIVE
SBT Error	-	-	AXIS#.SAFE.SBT.ERROR
Brake SBT Elapsed	-	-	BRAKE#.SAFE.SBT.ELAPSED
Brake SBT State	-	-	BRAKE#.SAFE.SBT.STATE

Timing

Timing	max	Remarks
t0		Start SBT
t0 to t1	200 ms	Measuring actual application load on motor
t1 to t2	300 ms	Brake applies
t2		Brake is applied. Position monitoring is started. Drive is switched to torque mode internally.
t2 to t3	1/3 * (t3 to t4)	Ramp-up to apply 130% torque on the brake
t3 to t4	Configurable	Torque duration
t4 to t5	1/3 * (t3 to t4)	Ramp-down torque on the brake
t5 to t6	300 ms	Release brake
t6 to t0	Configurable	Maximum time interval between tests

Setup in WorkBench

Select Safety Parameterization view (see "WorkBench Safety Function Setup").

Choose the axis (1), category (2) and SBT function (3). The instance number (4) is not applicable for SBT.

Fault Reaction / Failure Messages

If not all the preconditions are fulfilled and SBT is requested, the SBT button will be grayed out:

When a brake test fails, further operation shall be prohibited by means of an external safe PLC. SBA will remain 0 and the Safety Diagnostics will show a ”Brake Slippage Occurred”:

A controlled stop will be executed immediately when a brake slippage is detected during the brake test. The drive remains enabled.

Safety State / Status Signals

Status of each axis and the assigned brakes can be observed in the SBT diagnostics view.

The view gives information about:

Status, if the test is running or not
Test result (Not tested, Failed, Passed)
Time until next and in-between test
Status information on the test, if and which error occurred

If a brake test is not executed within the parameterized time interval, the Safe Brake Test Diagnostics will inform that the test validity is expired.

The warning 9008 “Brake Test Time Overflow” will be issued.