User Manual

Table Of Contents

Safe Torque-Off (STO)

Advanced Digital Motor Controller User Manual 39

The controller must have the Power Control input wired to the battery so that it can oper-

ate and communicate independently of the power stage. The controller’s processor will

then activate the contactor coil through a digital output conﬁgured to turn on when the

“No MOSFET Failure” condition is true. The controller will automatically deactivate the

coil if the output is expected to be off and the battery current is above 500mA to 2.5A (de-

pending on the controller model) for more than 0.5s.

The contactor must be rated high enough so that it can cut the full load current. For even

higher safety, additional precaution should be taken to prevent and to detect fused contac-

tor blades.

This contactor circuit will only detect and protect against damaged output stage condi-

tions. It will not protect against all other types of fault. Notice therefore, the presence of

an emergency switch in series with the contactor coil. This switch should be operated

manually or remotely, as discussed in the Manual Emergency Power Disconnect the Re-

mote Emergency Power Disconnect and the Protection using Supervisory Microcomputer

earlier in this section of the manual.

Using this contactor circuit, turning off the controller will normally deactivate the digital

output and this will cut the power to the controller’s output stage.

Important Warning

Fully autonomous and unsupervised systems cannot depend on electronics alone to

ensure absolute safety. While a number of techniques can be used to improve safety,

they will minimize but never totally eliminate risks. Such systems must be mechani-

cally designed so that no moving parts can ever cause harm in any circumstances.

Safe Torque-Off (STO)

Safe Torque Off is a safe method for switching controller in a state where no torque is

generated, regardless whether the controller is operating normally or is faulty. This func-

tion is a mechanism that prevents the drive from restarting unexpectedly. STO has the

immediate effect that the drive cannot supply any torque-generating energy. STO can be

used wherever the drive will be brought to a standstill in a sufﬁciently short time by the

load torque or friction or where coasting down of the drive is not relevant to safety. STO

enables safe working and has a wide range of use in motion control/ systems with moving

axes. The advantage of the integrated STO safety function compared with standard safety

technology using electromechanical switchgear is the elimination of separate components

and the effort that would be required to wire and service them. Because of the rapid elec-

tronic switching times, the function has a shorter switching time than the electromechani-

cal components in a conventional solution.

Speciﬁc motor controllers implement Safe Torque-Off (STO) circuitry, which is under cer-

tiﬁcation from TUV (T-version - Certiﬁcation No. M6A 104504 0001 Rev. 00). STO is the

most common safety function, meant for motor controllers, ensuring that upon trigger no

torque will be generated even after the controller power cycle. For controllers without the

speciﬁc circuit the STO is implemented in ﬁrmware alone and digital inputs 1 and 2 are

usually used (check controller’s datasheet).