What is Safe Torque Off (STO)?
Safe Torque Off or STO is a basic drive integrated safety feature available in most mid to large size digital servo drives AMC manufactures. STO is essential to making it safe to clear blockages, repair damage, or perform other maintenance in your servo motor driven system without fear of unexpected motor restart.
In the early development of servo motors and motion control systems, manufacturers struggled to create a safe ‘stop’ function that could act upon a motor without corrupting data or leaving the system liable to mechanical errors.
Safe Torque Off resolved many of these issues by allowing servo and stepper motors to remove power without causing damage to the drive or surrounding parts of the motor. That’s why STO can be found in industry applications ranging from conveyor systems to robotics.
How it Works
The STO function ensures that no torque can act upon the motor by blocking electrical signals from the power devices to the motor. As an emergency stop function, STO can reduce torque generating energy immediately and allow the motor to come to a dead stop using natural inertia and friction from the load. The motor in this case would be considered ‘free running.’
STO remains the trusted safety function of most servo manufacturers because it can prevent motor shaft rotation from restarting unexpectedly during emergency procedures. The only way to reactivate a servo motor after the use of the STO function would be to manually reactive it from the controller after STO is disabled.
Once STO is enabled, all drive gate pulses are immediately stopped, preventing the drive from feeding torque to the motor. Depending on friction and natural inertia, the motor may coast for a bit before coming to a complete stop.
In this state, equipment handlers can move and operate equipment by hand without interference from the motor. It’s important to note that STO will only occur if natural inertia permits. There are many ways to achieve STO from emergency stop functions to gate interlocking systems, but mostly your STO circuitry will contain their own proximity switches to assist in its function.
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STO Considerations
STO does not isolate the drive from electrical input. Its function is merely to assist in providing access to moving parts without fear of the motor restarting. Gaining access to electrical parts and circuitry in the drive would require locally isolating the drive manually from the main power supply.
As a final note, emergency stops are inherently volatile. This is why most manufacturers stress the need for a fail-safe braking system independent of STO.
STO vs. Inhibit
Some people might say STO is very similar to a servo drive's inhibit functionality, and those people wouldn't be entirely wrong. The end result for both is effectively the same, but there's a key difference in how it's done.
Inhibiting is done using the drive's logic. For an analog drive, this logic can often depend on certain pins staying grounded or ungrounded. For a digital drive, this is done at a software and firmware level. If a drive is inhibited, a glitch or loose connection can re-enable it, even if just for a second, but unwanted motion could occur.
Because STO is a safety feature, it must always be successful. There's no room for error. So in this case, it's not done at a software or firmware level, but a hardware level. STO causes a physical disconnect between the drive's power devices and the motor phase connections.
Again, once STO is enabled, the motor physically cannot start up again until STO is disabled and the controller commands the drive to re-enable motion.
Top Benefits for Servo Controls
STO is designed to be safe and reduce workplace injuries by preventing unexpected motor rotation. While not electrically isolated from the drive, once the STO function is activated, all electrical pulses from the drive will cease to create torque generating energy to the motor.
STO can be combined with other standard safety mechanisms. For example, proximity sensors can be used to activate the STO function and ensure that motor function will cease once a mechanic or contractor are within a certain proximity to moving parts.
By its very design, STO reduces the need for redundant wiring, multiple safety switch relays, and extra inputs. This not only offers safer alternatives to make your equipment safer, but it costs less.
Furthermore, by eliminating the need for multiple switch relays, STO functions can reduce electronic switching time, providing a faster solution than most conventional safety relays At ADVANCED Motion Controls our STO is suited for today’s modern-day safety requirements and can save you time and money while protecting your engineers and mechanics. Contact us today for a free quote and to get started on designing your servomechanism device.
