when to use a plc? whats the rigth way to contol a machine?

hi

im here again in a beginners question

im currently doing my last semester an that involves doing some real work in a company.

in the company im doing my service they buy old machines and replace the control.

this week they brought this electric oven and so we replace the old control with a timer and a temperature controller tzn4m.

the way it work is that we set a time interval that activates a relay that goes to the temperature control and when the temperature hits the set point it activates another relay that allows the internal resistor to turn on.

once the timer reach the interval time it cuts the power and the resistors turn off


so basically the timer relay is connected in series with the relay of the temperature control


is this kind of control ok?

what would it be the best way to control the oven?

in advance thank you

This is a very good question!

Sometimes the complexity of an application makes a PLC an obvious choice. At other times (a simple timing or simple control of one temperature) makes a dedicated control the choice.

At other times the simplicity of the control for troubleshooting is the major consideration.

Unfortunately, no one can give you a simple equation to plug in control variables to arrive at an answer whether a PLC or discrete controls are better. Usually only time and experience can provide enough background.

At the level of a student you must depend on the knowledge of experienced operators and maintenance technicians to help make a decision in the borderline cases. Over a great amount of time you will gain the experience to evaluate easily yourself.

In this particular case - simple control to a temperature then simple time control - it sound like discrete controls are the easier implementation. But do your work and ask those who would have to deal with this control arrangement.

yes the oven is working so solution is good.
however you will have to think about better solutions like how to catch the PID when it is overshooting, etc.

Control solutions vary from specialty designed electronics that you will find in almost any high volume product to custom software on standard hardware, which you will find in lower volume & one of a kind applications. There are standard microcontroller based products that you can program in 'C' to do the job. PLCs are standard hardware devices that you can program graphically to provide control. The 'best' solution depends on the application requirements and your capabilities. PLCs are the common choice of one of a kind applications.

There are a large variety of PLCs available. They vary in size, capability, development environment and interfaces available. A good stating point in looking for the right PLC is http://PLCcompare.com

Thanks for your comments and the very useful link

about the topic for example instead of using a timer and a temperature control.

would it be better if we use a smart relay for example or a micro controller?

in your opinion which option would be more reliable,easier to fix or easy for the operator to understand


i ask this because the next project is to make the control of an old autoclave and i believe a plc would be the best option. But the person in charge says is better to use again a timer, a temperature control, and a bunch of relays.

what do you think?

papejo said:

Thanks for your comments and the very useful link

about the topic for example instead of using a timer and a temperature control.

would it be better if we use a smart relay for example or a micro controller?

in your opinion which option would be more reliable,easier to fix or easy for the operator to understand


i ask this because the next project is to make the control of an old autoclave and i believe a plc would be the best option. But the person in charge says is better to use again a timer, a temperature control, and a bunch of relays.

what do you think?

Click to expand...

Anything that involves "a bunch of relays" is always easier with a PLC.