This section will touch very briefly on the topic of circuit design. The idea is to give you a starting point from which you can further improve your skills
As with any machine, the control of it needs to be defined before you can start designing a circuit. This is once again where we need to make sure we do not try and follow shortcuts as in the end, they will make you lose time
Before you start with the circuit diagram, you need to have a full understanding of what the machine needs to do. Start by writing a description of the entire system
As an example, we will look at a design where we have four conveyors that need to be semi-automated. So let’s start with the description of the system
1Â Â Â Â Â Â Â Machine components
Conveyor 1
Conveyor 2
Conveyor 3
Conveyor 4
2Â Â Â Â Â Â Â Component Function
Conveyor 1
An empty container is loaded onto this conveyor and the operator pushes the “LOAD BUTTON” for the conveyor to move the container to the end of the conveyor. Once the container gets to the end, it is detected by a limit switch and the conveyor stops. This conveyor will be named: LOAD CONVEYOR
Conveyor 2
If there is no container on this conveyor, it will detect the container on the Load Conveyor and start automatically. Once this conveyor is running, a signal will be provided to the Load Conveyor to start and transfer the container to conveyor two. Once the container has been transferred, the Load Conveyor will detect that it has no longer got a container on it and stop
With the container now moving forward on conveyor two, it will move the container to the end of the conveyor where it will be detected and the conveyor stopped. This conveyor will be named: ACCUMULATION CONVEYOR
Conveyor 3
The same will apply as with the previous two conveyors. If there is a container present, the system will wait until it is moved. Once there is no container detected, the Accumulation Conveyor as well as conveyor three will start. The container will be transferred to conveyor three which will stop the Accumulation Conveyor. Conveyor three will run until the container is detected at the end of the conveyor where it will be stopped.
Once it is in position, the container is filled by an operator. When the container is full, the operator will push a button to move the container to the last section. This button will provide a signal to conveyor four as well as conveyor three to start. This may only happen if there is no container present on conveyor four. This conveyor will be named: FILL CONVEYOR
Conveyor 4
This conveyor will receive it’s start instruction from the operator pushbutton. Providing there is no container present, it will start at the same time as the Fill Conveyor and move the container to the end of conveyor four where it will be detected and the conveyor stopped. Once the container has moved into position on this conveyor, it needs to provide and audible Alarm for a time of 10 seconds to warn the unloading operator. This conveyor will be named: UNLOAD CONVEYOR
Once you have this basic description, proceed to draw a sketch of how the systems will fit together. On this sketch you can then start adding the switches you may need to perform the relevant tasks
Notice how we have not drawn any part of the circuit yet, but already we have created a typical instruction set in the description?
Diagram
 
From this diagram, you can see what hardware you will need for the circuit. There is not a lot in terms of the field devices, 4 limit switches and two pushbuttons. The other items you need to consider here are the four motors and that means four disconnecting devices or as it is commonly referred to, isolating switches. In the next post we will go into the items we have not shown and you will understand why we do this exercise before we draw any circuit!
In the meantime, why don’t you have a go at the circuit before the next post?
Untill then, think safe, work safe and you get to go home safe