The goal of this project is to show how can be solved one of very often production line problems. The problem is- how to do periodic maintenance of one machine on the line without stopping the rest of machines. In this particular case, product is traveling over transporting line, and it is necessary that decoration machine is being cleaned every 30 minutes. Naturally, all the preceding machines (upstream machines) cannot be stopped because of that.
This problem is usually solved by installing one machine called compensator. It compensates this lack of time. If there is stop after this machine (downstream), compensator is being filled. If the opposite case, if products are traveling over the line with significant distances or products are not arriving at all, compensator is being discharged.
Machine has to be equipped with safety devices (mushroom, steel rope, etc.), and has to have two working modes- automatic and manual.
This project is going to be realized by Siemens controller and additional modules. Besides, we will have some other tasters, lights and switches.
Description of machine functioning
Ceramic tiles are arriving by transporting line. On this place they should have spaces between them, in order to avoid arrival of two or more tiles in the same time.
When photocell F2, which registers passing of spaced tiles, registers forming of line of tiles (or stoppage on the line behind-downstream the compensator), loading of compensator begins:
- Checking of existence of tile under the F1;
- When tile releases the F1, it is being moved up (into the compensator), until sensor PX1 becomes asserted.
With continuously activated sensor F2, loading cycle continues. After releasing sensor F2 starts the cycle of discharging of compensator, synchronized with tiles arrival on its entrance; actually, it can happen only if some time is elapsed after it was a last time when F1 was registered tile.
If during execution of step down (discharging of compensator) tile arrives to photocell F3, started discharge step will be executed, but discharging will be stopped, in order to allow passing of new tiles without compensator movement.
Purpose of photocell F3 is to register arrival of the tile before F1 does it, so that way we are avoiding collision between new tile that arrives and discharging cycle of compensator.
- Purpose of FC3 is to signalize that compensator is almost full and that it has to stop charging cycle, or that some other compensator before the one we are analyzing (upstream) has to begin loading cycle (etc. in the case of several compensators installed on the same line).
- Purpose of FC1 and FC2 is to immediately stop compensator movement up or down. In particular, FC1 stops movement to up, because compensator is full.
- Purpose of FC4 is to immediately stop automatic work of transporting belts, since collision of tiles occurred. It immediately gives the signal ‘’STOP’’ to the operator.
- In the case that M1 is overloaded, safety switch is being switched off and transporting line stops.
Machine is designed to start loading after receiving of external signal, which has the same purpose as F2 during loading.
As it is mentioned, this project is realized with Siemens PLC (CPU unit and several I/O modules). One proposal of solution is downloadable on Downloads page of this site for free.
 In this project we are not going to analyze parallel work of two compensators, but we are going to foresee one input on PLC for connecting FC3 and one output for sending signal to eventual compensator upstream; this means that in the case of connecting our PLC to a network with some other PLCs, it will be possible to have information if compensator is almost full.
One of the basic actions, or better let’s say controls, that an PLC (Programmable Logic Controller) is capable for is to switch his output port ON and OFF, based on the value of the signal on its input port(s). To demonstrate this fact, let us assume that we need to design one PLC based project, which has the purpose to control street lighting.
This project maybe seems to be too simple, but actually it is not, or at least doesn’t have to be. It depends on our requirements. So, let us specify our requirements for this PLC project:
- 1. It is needed to design Street Lighting Controller, which switches one of its output ports ON when one of his input ports is ON; consequently, it switches the same output OFF when mentioned input goes OFF.
- 2. Input signal has to be ON for at least 30 sec without interrupt, and it also has to be OFF for at least 30 sec without interrupt, before output value toggles ON/OFF.
- 3. In the case that sensor connected to the input port of the PLC becomes broken, street lighting must be switched ON independently of the input state of PLC.
More detailed explanation of the requirement number 1 is the following- we have one photo sensor (photo cell) and its task is to monitor the level of lighting from Sun. If this level goes too low, street lighting should be ON and vice versa.
The complexity of this project is hidden in the requirements 2 and 3. Requirement number 2 is present because it is possible that during the night some light temporally shows up (e.g. because of the firework, or maybe some vehicle with its lighting), but in this situation street lighting shouldn’t switch OFF, but to remain ON. In reality, very short power cutoff produces longer loss of light on the street, because usually street luminaries need some time to be energized before they start shining. Analogously, during the day some situations can cause temporal darkness of photo cell, leading to switching ON of street lighting, although these are harder to imagine. There is a lot of photo detectors today on the market with embedded possibility to adjust the value of necessary ‘’accumulation’’ of some value, but it should be foreseen in our PLC project that regular photo detector will be used. Both of these unusual situations are potential problem, and both of them can be solved by the use of timers.
Requirement number 3 says that designer has to foresee the situation in which photo cell is broken. Why this is important? Since regular maintenance of this system is not needed, after the start up of the system technician will come again when the problem is detected. There are two possibilities of malfunction. First, the less tragic case- photo cell is staying asserted, meaning that our PLC is getting information that it is dark outside. What will happen? Street light will be ON all the time. In this case, damage is made only in the sense of power efficiency.
The other case, which is more dangerous, is that photo cell is remaining OFF, even during the evening. This scenario is more tragic, since it affects safety of human lives, as well as property. It is crucial that light is ON in the evening!
These two possible scenarios are solved in this case by timers. If light is too long OFF (in proposed solution it means that difference between day duration from yesterday and today is not bigger than 900s), timer will switch it ON for safety reasons and trigger the alarm; if the light is too long ON(in proposed solution it means that difference between night duration from yesterday and today is not bigger than 900s), timer will trigger the alarm, but light is going to remain ON. Of course, in this case it is obvious that system will waste some energy, but since the safety has always to be a top priority- this way of working is more than acceptable.
Malfunction of main contactor and its auxiliary contacts is also foreseen- if they are not working properly light will be switched OFF and alarm will be triggered.
No matter how small your PLC is, even if it doesn’t have real time clock, it is possible to answer to requirements of the project by using regular timers. One of the possible solutions is going to be available for download for free from Downloads page on this website.