Circuit help needed

[Machine Doctor 13]

I installed the attached circuit on the old Mazak turret lathe and it works 80% of the time. Here is the sequence:

1. The machine has a relay shown on page 1 bottom left labeled INDF which sends a +24 V signal to a voltage divider to produce a 12V clock pulse every time a tool change is called for. There are 8 tools on a turret.

2. Every time there is a clock pulse the 4022 outputs sequentially go high to + 5V from 0 to 7 for the tool being selected.

3. The corresponding outputs from the 2981 go high to + 24V and turn on the relay for that particular tool.

4. The output from the relay that is energized goes to the machine CNC control and is displayed on the operator monitor as the tool number that is currently active. That is the sole purpose of this circuit. To provide feedback to the operator as to what tool is currently

The circuit works fine 80% of the time, but occasionally when the clock signal goes from + 24V to 0V it acts like another clock pulse. So if you are using tool 2 and wish to go to tool 3 instead the relay for tool 4 turns on and the operator display shows tool 4 is active even though on the machine tool 3 is active. The ascending slope of the clock pulse is working good but sometimes the descending pulse is triggering the 4022 again.

To verify that this is what is occurring I installed a push button to simulate a clock pulse (shown lower right pg. 1). If I hold the push button in for 10 seconds or so the next relay led turns on as expected, but occasionally when I release the push button the next relay led lights up. So I know it is activated by the descending slope of the clock pulse.

What I am wondering about is the fact that the 4022 is using a + 5VDC power supply with VSS (pin 8) connected to the negative terminal of the power supply, not ground, whereas the 2981 is connected to GND for pin 10. Is this a problem? If not, any ideas on why there is an occasional extra clock pulse?

 

[eTech]

Hi

You need to debounce the relay contact connected to pin 14 of the 4022.

eT

 

[Pommie]

Your schematic doesn't show any decoupling capacitors.

Mike.

 

[rjenkinsgb]

As etech says, you need a debounce circuit between the relay contact and the counter input.

The counter can work with signals up to the megahertz range and the slightest imperfection in the relay contact opening or closing will be seen as extra clock pulses.

It's normal practice to use an R-C filter and schmitt trigger input gate (eg. 4093 or 40106) so signals less than 1mS or even 10mS are ignored and definite, stable transitions are needed to change the state of the logic inputs.

 

[Machine Doctor 13]

Ok, I understand the idea of a debounce circuit. Just not sure how to incorporate it.

 

[rjenkinsgb]

Looking at the 4022 data sheet, the clock input has a schmitt trigger input already.

Just add a 0.1uF cap at the junction of the two divider resistors in your existing circuit, plus eg. a 10K from that to the 4022 clock input (for current limiting if the CMOS circuit loses power while the cap is charged).
That's not absolutely essential, but good practice.

Personally, I'd also use a much lower input resistance for the 24V signal, for noise immunity. eg. Add a 4k7 or 10K resistor to ground from the 24V relay contact.

 

[Machine Doctor 13]

Is that an electrolytic 0.1 uf cap?

 

[rjenkinsgb]

I'd use a polyester or similar type.
Plastic film caps (or ceramic) are commoner for that type of value and are better for high frequencies than electrolytic caps.

 

[Machine Doctor 13]

Thanks for your help.

 

[Pommie]

Did you add decoupling capacitors?

Mike.

 

[Machine Doctor 13]

I put a .01uf capacitor from the junction of the two resistors to ground. Also, I replaced the 100k resistor connected to ground with a 67k resistor reducing the clock pulse from 12V to 6V. And finally, I connected the 5V power supply neg. terminal to ground.

The end result is that the machine ran an entire 8 hour shift without fail. I want to thank you all for your help. Couldn't have done it without you.