[moved] Safety circuit design - truth table with logic gates

I need to design a truth table and circuit for a powered guillotine.

A powered guillotine uses a motor to position the cutter. The cutter must not operate while the motor is running. There is a guard which must be closed and locked before the cutter can operate but there is a maintenance facility which will allow the cutter to operate with the motor stopped but the guard open. A traffic light system must be incorporated to show when the Guillotine is unsafe (red), safe (green) and when in maintenance mode (amber). The guillotine can only operate once when in maintenance mode.

Draw the Karno map and design the safety circuit using two input NOR gates (use as many as you require).

If each gate has a MTTF 1526173, calculate the MTBF for your circuit (in hours).

How could you redesign the circuit for an improved MTBF?

How could you achieve a MTBF of 1 × 109 hours?

What will the reliability be after 5 years?

That is the question. I have drawn a truth table and a circuit using AND gates however I dont know how to put the requirement of the cutter operating only once when in maintanence mode into a truth table or how to convert my circuit to NOR gates. I have tried substituting the AND gate for three NOR gates but this effects my truth table and I am unsure as to where the inputs would go with three NOR gates and how I could improve the circuit for better MTBF. I think I could try to reduce the number of gates but I would be unsure as to where my inputs would go again.

Any help is much appreciated

re: Safety circuit design - truth table with logic gates

I dont know how to put the requirement of the cutter operating only once when in maintanence mode

Click to expand...

This might be the place for an RS flip flop. It triggers once in response to a 'set' signal. It does not respond to subsequent 'set' signals.

When you exit maintenance mode, give a 'reset' to the flip flop.

I don't know whether this idea fits your overall specs or not.

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The RS flip flop changes state and stays there, after a Set signal. Naturally you would not necessarily drive the cutter from the RS output.

The RS output by itself is a memory unit. The output can go to your other logic circuits, to determine whether the cutter does or doesn't activate.

re: Safety circuit design - truth table with logic gates

makel12 said:

If each gate has a MTTF 1526173, calculate the MTBF for your circuit (in hours).

Click to expand...

Could you be more clear? What is MTTF? What is 1526173 represents? How was this determined? How the MTBF defined? What are the assumptions made?

re: Safety circuit design - truth table with logic gates

makel12 said:

Draw the Karno map and design the safety circuit using two input NOR gates (use as many as you require).

Click to expand...

The steps to draw whatever circuit using only NOR gates are as follow:
1)Take the truth table and simplify it in "maxiterms" (products of sums). To implement that you would have to use OR-gates on the first level and AND-gates on the second level.

2)Here 2 ways could be used(choose the one you like, it does not matter):
METHOD 1: Take your simplified function in 1) and deny it 2 times(invert it 2 times... I do not know how is that called in English but I will show an example). Use de Morgan rule and there is the result in NOR gates.

METHOD 2: At the output of the first level OR-gates place on them the inverting point and at the input of the 2nd level gates (AND-gates) as well, now the 2nd level gates are NOR gates. There is the result in NOR gates.



EXAMPLE: Suppose we have F=(x+y)(x+z) => see picture



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The RS idea Brad pointed out is a good one if you are allowed to use flip flops.

Re: Safety circuit design - truth table with logic gates

CataM said:

The RS idea Brad pointed out is a good one if you are allowed to use flip flops.

Click to expand...

I do not know why did I say that... Of course RS fits well because RS internally is made of NOR gates. (Asynchronous RS)