Safety Programming

S

Thread Starter

Sebastian LaBella

Hello, my name is Sebastian LaBella. I am new to the control world and have a task in programming a Micro-logic1200 PLC. My designer designed a gage that measures parts for dimensional characteristics. When he purchased the pneumatic solenoid vavles for part up/down, slide in/out he bought detented solenoids. Pulse one side and the part moves down, pulse the other side and the
part moves up. I am starting to write the ladder logic for the manual mode and I cannot think of a way to get started. For safety reasons, to lower the part the operator is required to press a palm switch and the part lower button. His hands must stay on the switches untill the part is lowered into the nest. If his hands are removed before this operation is complete the part must come up.
Do you know of a circuit that I can use to program this type of feature?
I can be e-mailed at [email protected].
Thanks for your help.
Regards
 
Hello,

If I understood you right this logic should help
---| |--| |-O Latch a Bit with Inputs (switches)
---|\|--|\|---O Unlatch same bit with same Inputs

---| |--|\|----O If bit is latched and if Part isnt in its down postion then
run move pulse solenoid down.
---|\|--|\|----O If Bit is unlatched and if part isnt in up postion then
pulse solenoid UP.
 
G

Guy H. Looney

Sebastian,

No fancy circuitry needed for this. I'm not familiar with the phrase "detented solenoids", but it sounds like you've got two-position valves and I hope that at least the vertical one is fixed position (as opposed to center exhaust). For my explanation, I'm gonna assume that you're using 110VAC valves and the PLC outputs are switching voltage.

I've never used the Micro-logic 1200, but I'll assume the output card has a single common for multiple outputs. That being said, wire 110VAC to the common. Wire one side of all the pneumatic valves to neutral (doubt you'll see a polarity indicated, but if you do be sure to follow it). Wire the PLC output for upward motion directly to the other side of the valve. That's got the upward move taken care of.

Now for the downward move. If you've got an E-Stop button (which you should), wire the palm buttons in series with the E-Stop button & any other safety devices to form your E-Stop circuit. Your E-Stop circuit should be
actuating a safety relay or MCR. To the valve that moves downward, wire the PLC output for downward motion in series with a set of normally open contacts off the safety relay or MCR. Now you've got the downward move taken care of. When the E-Stop circuit fails, the voltage is removed from the valve regardless of what the PLC is doing and the downward motion is stopped immediately.

The palm buttons should also have an auxiliary set of contacts for monitoring purposes. Wire those contacts into the PLC to be used as a
programmable input. Finally, you'll need a set of proxes to sense fully lowered & fully raised positions; wire those in as programmable inputs to
the PLC as well. Now you're ready to go.

1) PLC waits for palm switch input to be on.
2) PLC turns on valve for downward motion & turns off valve for upward motion.
3) PLC waits for fully lowered input switch to be on OR palm switch input to be off.
4) If fully lowered input switch is on, PLC turns off valve for downward motion
5) If palm switch input is off,
a) Motion will automatically stop due to wiring
b) PLC turns off valve for downward motion
c) PLC turns on valve for upward motion
d) PLC waits until fully raised prox is turned on
e) PLC turns off valve for upward motion

Hope this helps,
Guy


Guy H. Looney
Motion Control Engineer
 
E
A few comments on Guy's reply, and the original post as well...

<clip>
> No fancy circuitry needed for this. I'm not familiar with the phrase
> "detented solenoids", but it sounds like you've got two-position valves
and
> I hope that at least the vertical one is fixed position (as opposed to
> center exhaust). For my explanation, I'm gonna assume that you're using
> 110VAC valves and the PLC outputs are switching voltage.
<end clip>


I think Sebastian's "detented solenoids" are referring to 2-position double solenoid valves. I also think that what you described is really a
THREE-position valve, which normally sits in the center position, and moves temporarily to either of the other two positions when the corresponding
solenoid is energized. Yes, these are available in closed, open, pressure, or exhaust-center, and I agree that Sebastian would NOT want to use an
exhaust-center valve. He could use the valve he has if he doesn't require that the cylinder STOP when the palm buttons are released. IOW, if it's safe to allow the cylinder to go back up. If so, he could even use a 2-position SINGLE solenoid valve. Loss of air pressure to the cylinder has to be taken into account though, as the cylinder would drop if a 2-position valve is used.

The concern with using rod-type cylinders in any vertical application is that even when both ports are blocked (or have pressure on them), the
cylinder will "creep" due to the different pressures generated on either side of the piston (remember that the cylinder rod uses up some surface area). In most applications, the rod end faces downward, and this differential pressure works against you. In an installation where operator safety is a concern, you always have to keep these things in mind.

One thing to add to the system (no matter how he winds up doing it) would be a pilot-operated check valve to the bottom port of the cylinder.
This check valve only allows air to exhaust from the bottom port when pilot pressure is present. Pressure for the pilot should be from the top port. This way the cylinder can only move down when commanded. Cylinders are also available that use a mechanical clamp to provide a stop if air pressure is removed.

<clip>
> I've never used the Micro-logic 1200, but I'll assume the output card has
a
> single common for multiple outputs. That being said, wire 110VAC to the
> common. Wire one side of all the pneumatic valves to neutral (doubt
you'll
> see a polarity indicated, but if you do be sure to follow it). Wire the
PLC
> output for upward motion directly to the other side of the valve. That's
> got the upward move taken care of.
----------------
> 1) PLC waits for palm switch input to be on.
> 2) PLC turns on valve for downward motion & turns off valve for upward
> motion.
> 3) PLC waits for fully lowered input switch to be on OR palm switch input
> to be off.
> 4) If fully lowered input switch is on, PLC turns off valve for downward
> motion
> 5) If palm switch input is off,
> a) Motion will automatically stop due to wiring
> b) PLC turns off valve for downward motion
> c) PLC turns on valve for upward motion
> d) PLC waits until fully raised prox is turned on
> e) PLC turns off valve for upward motion
<end clip>

This will work fine, and will help Sebastian greatly on the programming end, but I feel that the palm buttons (or "Part Lower" and palm switches in Sebastian's case) should be connected to the system through a "Two-Hand Anti-Tiedown Relay". This prevents the operator from keeping one of the switches "held" with a piece of tape, large brick, etc., thereby freeing up one hand to put in the gage! This could be done with logic, but I don't like relying on a PLC.

By the way, pneumatic-only versions of the Anti-Tiedown systems are available as well.

Just my added 2¢ worth when it comes to safety! ;o)

- Eric Nelson
[email protected]
Controls/Software
Packaging Associates Automation Inc. [email protected]
Rockaway, NJ, USA
 
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