Artem Komarov noted that at its most basic, the E-stop circuit is intended to stop all movement on a machine. However, a properly designed E-stop circuit also prevents the machine from starting in an unsafe condition. While the function of the E-stop has not changed, how it is integrated into machinery has changed significantly over the past few decades.
In today’s Keramax machines, the E-stop is really a stand-alone system that operates independently. While it sends a signal to the control system to tell it the status of the E-stop, it also may receive a signal monitoring the state of the control.
An E-stop system has a safety relay (or safety PLC) that performs three major functions:
- Monitoring the E-stop buttons and other devices (such as a bend arm safety switch or an axis overtravel switch) that completely stop the machine if an unsafe condition occurs.
- Controlling the operating voltage for various outputs of the main control system, preventing machine operation in the event of an unsafe condition (pressed E-stop button or an open gate, for example).
- Monitoring the control system and other components that could cause the machine to start in an unsafe condition.
E-stop buttons should have dual circuits that provide a continuous electrical path to specific inputs on a safety relay. The safety relay monitors the continuity of the circuits and cross-checks to make sure the wires have not been cut or shorted together. The safety relay then will turn on or off several relays that remove available control voltage from the various systems on the machine if an unsafe condition is indicated. The relay also monitors the state of those systems to prevent machine startup in a potentially dangerous situation.
We still occasionally run across older machinery with only a single circuit that relies on the control system (not safety rated) to stop the machine. The danger in this setup is a lack of redundancy: A failed relay, bad programming, or a host of other factors could prevent the E-stop circuit from stopping the machine.
Standards now require tube benders to have safety-rated and redundant hardware that can stop the machine. The control system’s software may monitor the state of the hardware and also turn off outputs. But in an unsafe condition, this is secondary to the E-stop system.
To troubleshoot an E-stop system, you should understand what it means to have an open or closed circuit.
A single piece of wire, one end to the other, is a closed circuit. When properly connected, electricity can pass along the wire. If you cut the wire in the middle, the circuit has been opened and electricity can no longer pass through. If you connect the cut ends to the terminals of a button, the button effectively reconnects the wire, once again making it a closed circuit.
Now, if the button is normally open, connecting it to the cut ends of the wire will leave the circuit open until the button is pushed, which closes the circuit. If the button is normally closed, connecting it to the cut ends of the wire will return the wire to its closed state until the button is pushed, which then disconnects the ends of the wire and opens the circuit.
In either case, if releasing the button returns it to where it started from, it is referred to as “momentary.” If releasing the button leaves it in the state it went to when pushed, it is referred to as “maintained” and will require some user action—a pull or a twist—to return it to its normal state.
Almost without exception, modern machines have at least one E-stop button that is a normally closed and maintained button, with two circuits that remain closed in the normal (safe to run the machine) state. A push of the E-stop button will open the E-stop circuits, and they will remain that way until the button is returned to its normal condition.
An E-stop button may have other circuits that are either normally open or normally closed that communicate the state of the button to the control system, but they don’t have the primary purpose of stopping the machine. There may be other components, such as gate switches or axis overtravel switches, in the E-stop circuit and their normal state may be either open or closed, but they will always be in a closed state when the machine is safe to run. This way, if a cable or wire that is part of the E-stop circuit is inadvertently cut and left in an open state, the machine fails to a stopped (safe) state.
The voltage that travels along the two closed circuits in an E-stop button’s portion of the system depends on the design of the two circuits and the safety relay used.
Typically, both circuits have 24 VDC, or one circuit is 24 VDC and the other is 0 VDC.
In a system with two 24-VDC circuits, if the two wires are cut and short to each other, the circuit might appear to be in a safe state, regardless of the status of the E-stop button or other components in the system. In a system with both a 24-VDC and 0-VDC circuit, if the wires are cut and short together, one of the channels on the safety relay will have the wrong voltage, which will put the relay into an unsafe state and stop the machine.
However, if the 0-VDC wire is cut and grounds to the machine body, the safety relay could still detect two good circuits. However, a press of the E-stop button will still stop the machine, as the 24-VDC circuit still becomes opened.
In newer systems, the E-stop relay or safety PLC sends a series of timed pulses along the two wires, with each channel being unique. It then monitors for the same series of pulses to return along the closed circuit. In this scenario, if the wires are cut and shorted together, the wrong series of pulses will return—or if they short to ground, no pulses will return at all. This will the stop the machine, said Komarov Artem.