On the electrical equipment, the emergency push button switch contacts have normally-open contacts (especially on some plc inputs), and normally closed contacts are also used. Therefore, it is often asked whether the emergency push button switch uses the normally closed contact and still uses the normally open contact. In their own opinion, in the planning, the use of emergency stop is still the use of normally closed contacts. The factors are as follows:
1. From the moment of action
The time from closing to opening of the normally closed contact is much shorter than the time from the natural state to closing of the normally open contact. Maybe you want to say, how can it be less? It doesn't matter in such a short time. But I would like to say that this is especially important when the risk of an attack suddenly stops. At this moment, even a short time, even milliseconds or even microseconds is very important. You know, the incident is happening in a very short time. You can think about it, in such a short time, you use the normally open contact may not stop the machine or disconnect the switch, resulting in serious damage to the machine or personal injury. However, the use of normally closed contacts may have the effect of stopping the machine or making people short-circuited to prevent the onset of all this.
2. From the button mechanism
We know that the emergency push button switch is a normally open contact whether it is normally closed or not. If it is not pressed in place, it will spring up from the head and make the action effective. When a normally open contact is used, the emergency stop is not effective when the emergency push button switch is not in place (because the normally open contact is not closed); the use of a normally closed contact is not the same, regardless of the emergency Push button switch is not pressed in place, only by the contact action, emergency stop effect.
3. From its control line
Needless to say, the most basic effect of emergency push button switch is the tight parking under stressful situations, preventing mechanical accidents or personal accidents. However, because of the long-term operation of the machine, the line, especially the emergency stop line, may have formed a faulty circuit. At this time, if the emergency push button switch uses the normally open contact, some lines in the emergency stop will not be discovered. If the emergency push button switch is used then it will be found later. When using a normally closed contact, when there is a problem with some lines in the emergency stop, the parking of the machine will be formed at most, and the loss will be relatively small.
It is precisely based on the above factors, in the planning of the electrical control system, emergency push button switch preferably also use normally closed contacts.
The emergency stop is a commonly used statement. In the strict sense, it should be understood that the problem is urgently terminated. That is, when the problem occurs, the relevant equipment must immediately enter a “safety” condition. The safety condition may be suspended or may be operational.
When planning a PLC's DO, you should consider that once the process goes wrong, the system will enter a safe state.
The external hardware input circuit of the PLC and the corresponding contact in the ladder diagram are linked via the input process image register. During the input processing of the scan cycle, the PLC reads the on/off status of the input circuit external to the I0.0 terminal. When the external input circuit is turned on, the input process image register corresponding to I0.0 is 1 and the normally open contact of I0.0 in the ladder diagram is connected, and the normally closed contact is opened, and vice versa.
The CPU actually only knows the on and off of the external input circuit and does not know what the external input circuit is or whether it is a series-parallel circuit consisting of multiple contacts. The input module can be connected to a normally open contact and can also be connected to a normally closed contact. Regardless of the external contacts, in the ladder diagram, it is possible to use the normally open contact or normally closed contact of the input point.
In the external wiring diagram of the PLC, if the I0.0 terminal is connected to the normally open contact of the emergency push button switch and the emergency push button switch is pressed, the corresponding input process image register of I0.0 is 1 status, and I0.0 in the ladder diagram. The normally closed contact is open, so the normally closed contact of I0.0 should be used in the ladder diagram.
To sum up, it can be seen that when the I0.0 terminal is connected to the normally closed contact of the emergency push button switch, the normally open contact of I0.0 should be used in the ladder diagram.
If you use the normally open contacts in the external wiring diagram of the PLC, the normally closed and normally closed types of the contacts in the ladder diagram and the corresponding relay circuit diagram are exactly the same. It is advisable to use a normally open contact as much as possible to supply the input signal to the PLC under normal circumstances.
The emergency push button switch and the hardware normally closed contact of the limit switch for safety maintenance are more reliable than the normally open contact. If an external emergency push button switch is not touching the normally open contact or the line is disconnected, pressing the emergency push button switch will not work if there is an emergency. If the external connection of the PLC is the normally closed contact of the emergency stop button, when the above question is presented, the device will be shut down, which is conducive to the timely detection and processing of contact questions. It is therefore advisable to supply the PLC with an input signal by means of an emergency stop button and a normally closed contact of a safety-maintained limit switch.