plc coursework help

shakerbess

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Nov 2014
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hi I was hoping to get some help with some college work the question I am on is as follows:

A machine starter motor is only allowed to run if a start signal has been received and emergency stop signal has not been received. Also there must be a signal detecting that the machines safety guard is closed and a safety feature in front of the machines working area has not detected any human presence

I was hoping to get some help with how this would be set up and the equipment used

I hope some one has the time to point me the right direction thankyou
 
Welcome to the Forum !

Is this meant to be a hardwired circuit, or a software program in Relay Ladder Logic format ?

Or do you think that part of the question is to find out if you're aware of the required separation between safety and general-purpose logic ?

If you can provide an example of your thinking on the matter, or show some work you've already done, Forum members will trip over themselves to help.

But if you just pose the question without showing some of your own effort, you usually meet with skepticism and derision.
 
Getting answers to questions is simply but you don’t get you much after that. Helping find the answers is harder but provides not only the answers but an education as well.
 
Ken Roach said:
Welcome to the Forum !

Is this meant to be a hardwired circuit, or a software program in Relay Ladder Logic format ?

Or do you think that part of the question is to find out if you're aware of the required separation between safety and general-purpose logic ?

If you can provide an example of your thinking on the matter, or show some work you've already done, Forum members will trip over themselves to help.

But if you just pose the question without showing some of your own effort, you usually meet with skepticism and derision.
Click to expand...

I was looking at the hardwired circuit I am not going to lie I don't have much plc background trying to change direction I am reading up and looking at the industrial environment application to the question I need to look at the question as to what I would implement for the simple industrial process and how I would cater for future expansion so I agree with all the comments made did not even expect reply so fairly shocked like to say thankyou to all those who took time out to reply to this novice lol I was hoping to know hard wired what control gear and the plc would be a modular for expansion for future

what input devices could possibly be used with this plc please
 
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what input devices would apply to this this plc please
Click to expand...
You listed most of them in the problem statement:

1. Machine Start Pushbutton, normally open contact
2. Machine Stop Pushbutton, normally closed contact
3. Emergency Stop Input from the Emergency Stop relay, normally closed
4. Machine Safety Guard, closed = safe, open = unsafe
5. Machine Working Area Light Array, closed = safe, open = human in unsafe area

It seems you would need at least 5 digital Input terminals on your PLC. Now the importatant part: What PLC Output terminals (to do the work) do you need?
 
Last edited:
Lancie1 said:
You listed most of them in the problem statement:

1. Machine Start Pushbutton, normally open contact
2. Machine Stop Pushbutton, normally closed contact
3. Emergency Stop Input from the Emergency Stop relay, normally closed
4. Machine Safety Guard, closed = safe, open = unsafe
5. Machine Working Area Light Array, closed = safe, open = human in unsafe area

It seems you would need at least 5 digital Input terminals on your PLC. Now the importatant part: What PLC Output terminals (to do the work) do you need?
Click to expand...

firstly thankyou for your time


what would you suggest?
I would expect output to be a sounder, beacon led light. a shut of for the machine.
I would also expect that when all the safety is in place that the machine plc output would indicate a green light to let the operator know its safe to work
hope im talking sense lol
I would also be looking at safety screen, interlocks,acuators and infra red beams.
I have never seen the stuff that's why I am struggling is there any good books I could read?
 
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Those PLC Outputs are all good to have. Don't forget the most important, a PLC Output for the machine motor. The machine motor starter coil should be energized when the PLC Output is ON, and de-energized when that same PLC output is OFF.
"A machine starter motor is only allowed to run if a start signal has been received..."
Click to expand...
(I think there is a small error in your assignment. It should read:
A machine motor starter is only allowed to be energized (to start and run the motor) if a start signal has been received...blah blah blah".

A motor starter is a special-purpose electrical contactor with provisions for handling the motor full-load current and usually has overload protection devices.

On the other hand, maybe there are a few machines that actually have a "starter motor", such as automobiles with electric starter motors. I don't think that applies here to your machine.
 
Last edited:
Lancie1 said:
Those PLC Outputs are all good to have. Don't forget the most important, a PLC Output for the machine motor. The machine motor starter coil should be energized when the PLC Output is ON, and de-energized when that same PLC output is OFF.

(I think there is a small error in your assignment. It should read:
A machine motor starter is only allowed to be energized (to run the motor) if a start signal has been received...blah blah blah".

A motor starter is a special-purpose electrical contactor with provisions for handling the motor full-load current and usually has overload protection devices.
Click to expand...

mate thankyou for you kind advice

I need to do more reading lol
im now with your advice hoping to look up read more
in terms of the PLC type am I write to think I would need a safety modular plc type in case the production line in the future was subjected to more machines?
 
A modular PLC would be a smart choice. A safety-rated PLC could be too, if there will be a lot of dangerous machines that need emergency stop controls. Otherwise the emergency stop relays will have to be separate external devices, which is not so bad if there are only a few (4 or 5).

Remember, in the real world, cost is always a consideration (unless it is a government project, then the taxpayers are assumed to be willing to pay infinitely-high taxes, forever). Saftey-type PLCs are much more expensive than the standard non-saftey-rated types.
 
Last edited:
Lancie1 said:
A modular PLC would be a smart choice. A safety-rated PLC could be too, if there will be a lot of dangerous machines that need emergency stop controls. Otherwise the emergency stop relays will have to be separate external devices, which is not so bad if there are only a few (4 or 5).

Remember, in the real world, cost is always a consideration (unless it is a government project, then the taxpayers are assumed to be willing to pay infinitely-high taxes, forever). Saftey-type PLCs are much more expensive than the standard non-saftey-rated types.
Click to expand...
thankyou

if at some later stage I wanted to integrate all the other machines and process in the production line in to the PLC
Please do you have any suggestions how I could do this?
 
Please do you have any suggestions how I could do this?
Click to expand...
If you know which machines could be added in the future to the PLC, then you can make a list, with all the input sensors and switches, and the motors, heaters, solenoids, and other devices that require outputs. Then simply add them up, and buy I/O modules and rack spaces for that many, plus some spares.

For a typical project (there is no such thing, they all turn out to be more difficult and longer than anyone guessed!), you would leave about 20% spare capacity for Inputs and 10% for Outputs. Depending on the budget constraints, you can buy a rack with spare slots for future Input/Output modules, and you can even provide a few unused Inputs and Outputs in case the project gets modified or some equipment has been overlooked. All this depends on the situation, and on your years of experience doing similar projects.

In many cases, it may be smart to have one large PLC rack in the electrical room, with Ethernet cables connecting to Remote I/O racks as machinery areas throughout the plant. That saves running a lot of wires back to the main electrical room, and avoids the need for a large number of spare module slots for future use.
 
Last edited:
Lancie1 said:
If you know which machines could be added in the future to the PLC, then you can make a list, with all the input sensors and switches, and the motors, heaters, solenoids, and other devices that require outputs. Then simply add them up, and buy I/O modules and rack spaces for that many, plus some spares.

For a typical project (there is no such thing, they all turn out to be more difficult and longer than anyone guessed!), you would leave about 20% spare capacity for Inputs and 10% for Outputs. Depending on the budget constraints, you can buy a rack with spare slots for future Input/Output modules, and you can even provide a few unused Inputs and Outputs in case the project gets modified or some equipment has been overlooked. All this depends on the situation, and on your years of experience doing similar projects.

In many cases, it may be smart to have one large PLC rack in the electrical room, with Ethernet cables connecting to Remote I/O racks as machinery areas throughout the plant. That saves running a lot of wires back to the main electrical room, and avoids the need for a large number of spare module slots for future use.
Click to expand...

thankyou so much its given me a lot of information thankyou so much mate for your time
 

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