In many industrial control processes, there are many situations where the sequence of work requires that one part work in sequence with another, and each can be timed independently. For example: in industry, work is desired in sequence with the following steps:
- Load 1: operate the motor rotation for the belt for 5 seconds, to move the workpiece into position.
- Load 2: spray paint on the object for 2 seconds.
- Load 3: heat the workpiece for 10 seconds. Then stop or return to the initial process.
The circuit above can do all of that, meaning that this circuit can do several types of work alternately. Alternating in this case means that the existing loads will be active one by one, and the time can be determined individually.
The way this circuit works is as follows:
From Figure 7.21, blocks can be made for each component use as in Figure 7.22. Where for the switching circuit, SCR components are used, for the trigger circuit, transistors are used and the timer circuit uses UJT with delay settings from the multiplication between C and R.
In the circuit when the start button is pressed, there will be a voltage entering the gate of SCR 1, which functions as a switch to activate load 1 (in this case represented by lamp 1 which is on). After the voltage at the anode of SCR 1 rises to approximately 0.7 Volts, the SCR will make the transistor Tr1 forward biased. The transistor will charge the capacitor C1 which will be used as a comparator to determine the delay together with the resistors R4 and Rv1.
Figure 7.21: Sequential Control Unit Circuit
When the voltage through the capacitor is the same as the emitter trigger voltage on UJT 1, the UJT will work and will provide voltage to the gate of SCR 2. With the voltage on the gate of SCR 2, the second load (Lamp 2) will be active and at the same time the capacitor C4 will provide a reverse current that can turn off SCR 1 so that load 1 will turn off. The working principle above continues continuously until the last load. If you want load 1 to work again, then place SW2 in position A, which is the position to provide voltage feedback on the gate of SCR 1, so that load 1 will be active again. If switch SW 2 is in position B, after load 3 (lamp 3) turns off the system will stop working, and will work again if the start button is pressed. The working time of each load is set by RV 1 to RV 3.
Figure 7.22 
Diagram of the Sequential Control Unit System Block
There are three common cases that often occur:
- When the start button is pressed, load 1 and load 2 work normally, but when load 2 is finished working, the circuit immediately stops working, meaning that load 3 never works, wherever SW2 is positioned. Case answer: from the way it works above, it is certainly very easy to guess which damage is in the circuit that is not working. Namely block 3, which is related to the timing circuit. So the damaged components are Rv3 open, C3 short circuit, transistor 3 open base emitter or UJT 3 open.
- Load 2 and load 3 will be on at the same time when load 1 ends working, and will also stop working simultaneously as soon as possible (working for a while). Case answer: here it will be difficult to estimate the damage if you have not really mastered the circuit above. But if you understand it properly, then this second problem is very easy to detect which component is damaged, because there is only one possible damage that causes something like the above, namely C5 short circuit. Because C5 is short circuited, the conduction or off of Tr2 and Tr3 must always be at the same time.
- When the power supply is turned on, load 3 immediately works continuously, the others are off. Case answer: for this one damage, because load 1 and load 2 do not work, then the problem is not in block 3 but rather in block 2. Namely damage to SCR 2 anode and cathode short circuit or UJT 2 between B1 and B2 is short circuit. Of course, damage to this component will cause load 3 to continue working because SCR 3 will continue to be on.
Summary
- Industrial control equipment has three important blocks, namely input which is usually a sensor, control and output which is usually an actuator.
- Servo systems (systems whose input and output are connected by a control function) consist of two systems, namely: open-loop systems and closed-loop systems.
- Damage checks by checking input and output signals must know the correct sensor used and the actuator used so that the correct signal results are known.
- The function symptom technique is used by an experienced repair technician.
- Signal-tracing techniques are more suitable for open-loop systems.
- Voltage resistance techniques are rarely used in industrial control system repair because of the loading effects of measuring equipment that can reduce the circuit load.
- The most frequently occurring problems in industrial control must be fully mastered, both in the sensor section (strain gauge and mechanics) which are prone to failure and the solenoid actuator which fails more quickly than the motor.
- It would be wise to note down all the symptoms present and study the manual for the control.
Exercise
- Make a block diagram of the industrial control equipment that you know and explain the function of each block.
- In your opinion, which motor is more suitable for use for: a. Fan, b. Conveyor belt, c. Heavy machinery.
- Name the types of sensors that you know!
- Which actuator is related to wind?
- When can we use signal-tracing techniques to find control circuit failures in industry? Why?
- What is the danger of using voltage resistance techniques when we are looking for damage to industrial controls?
- Mention the main problems that are often found in industrial control!
Group task
In groups of 4 people each, work on the damage cases below:
- By mentioning which components are damaged, the type of damage and the reasons. Look at the circuit diagram 7.21 on page 7-21, if SW2 is in position A and when SW1 is started, the lights will turn on sequentially from 1, 2 and 3 but then stop working as if SW2 was in position B.
- Now analyze what will happen if SCR2 between anode and cathode is short-circuited when turned on and SW1 is pressed?