Safety Switch/Circuit Design?

Hi all,

I wonder if someone with a bit of electrical know how can help. I have my lathe working using a 36v DC motor, which connects to our central solar system. My usual system for any machine in the workshop is as follows:

Central workshop switch panel: Switch that activates the relay, providing power to the machine

Then at the machine itself:

On/off switch on the speed controller

Foot switch on the speed controller

Both of these are on the power feed going to the potentiometer

In other words, once it's on at the wall, you need to have the switch on on the machine and then press the foot switch for the machine to come on (likewise to turn it off just take your foot off the pedal).

So far, so good. However, I noticed today that for one reason or another there is less room for the foot switch at the lathe and, despite my best intentions, every time I stopped and fiddled with the lathe, I forgot to turn it off at the switch. This means that if I accidentally stood on the foot switch the lathe would come on. No I have it set to accelerate very slowly, so not the end of the world, but that's not the point.

So I would like to add a circuit/relay into the system that allows the following:

Turn on the on/off switch - activate potentiometer circuit

Turn on the foot switch - turn on lathe (by completing potentiometer circuit)

Turn off the foot switch - turn off lathe and deactivate potentiometer circuit

The circuit should not then turn on again until the on/off switch has been set to Off and then On (i.e. pressing foot switch shouldn't bring on the lathe)

I realise many will see such a system as being OTT, but I guess we are each responsible for our own safety and being a little absent minded I would rather have one feature too many, than one too less.

Has anyone got any suggestions on how to do the above??

Many thanks,

James

I would rethink the foot switch as accidental restart is a problem, having to remember to hit a stop to ensure you are safe is not a safe process. If the foot pedal is a feature you would like to retain I would change it to a shrouded type so you need to insert your foot deliberately into the pedal and accidental operation will be much more difficult, in industry these type of foot switches are used to operate presses and spot welders. Any device that can start a machine should be shrouded from being accidentally operated, start buttons should have a shroud so that they can only be operated by a deliberate press with a finger and falling against it will not operate it. Normal operation of a lathe should not require isolating while taking measurements and setting up. A means of local isolation is a good idea for cleaning and maintenance.

Mike

Is the foot switch just on/off or a speed potentiometer? If just an on/off wire it to a no-volt release.

Hi Bazyle,

That's exactly what I'm looking for, but I just want the relay, so I can build it into the system myself. I don't seem to be able to find one. Obviously it would have to be DC low voltage.

Jamss

You did not answer Bazyle's question, is the foot control ON/OFF or a variable speed control?

Also you imply that the power is going to the "potentiometer" I assume this is a manual control knob fir the speed controller and not an actually carrying power to the motor?

Normal arrangement as said is a momentary start/stop button but this does not directly tie into a pedal control. Having to press a start button each time defeats the purpose of a pedal control (other than as a secondary safety function which is not your case). My solution would be to have an "ARM" button. that enables the footswitch for a limited time. Functionally, pressing the arm button would start a time delay (relay) enabling the foot switch. if the foot switch is pressed within the pre-set time the machine runs and the time delay is reset and starts again when you release the foot switch. If you don't press the foot switch (again) within the pre-set time the foot switch is disabled until you press the arm switch again.

The timer could be an electronic circuit made for the job or a general purpose industrial timer like those made by Omron IMO etc. It could even be a microprocessor like a PIC. An "ARMED" light would be a good idea.

Robert G8RPI.

Hi Robert,

Apologies, the foot switch is just on/off the potentiometer is a rotorary one on the control panel. The pedal control's real function is for stopping the lathe, sort of a deadmans handle. The system you describe would work, although the ideal, rather than a timer setting, would be that it reset on the pedal being pressed.

I did think, if I had a non contact relay, which had a 36v input and activated the potentiometer circuit, a momentary switch could activate the relay, then be replaced by a feed off the motor power (once the foot switch on the potentiometer circuit is pressed). Once the motor turned is turned off the potentiometer circuit wouldn't be activated again until the end momentary switch is turned on.

Any thoughts?

James

Hmmmmmm guess I would need some sort off diode in there or else the motor would come on when the momentary switch was pressed.

James

That would be very straight forward to wire up but you would have to press the button every time as well as using the foot switch. The other problem is you could press the switch but not use the lathe leaving it "armed" and then comeback later an start it inadvertently with the foot switch. A cover on the foot switch would be a good start. Maybe even with a door or bar guard you have to lift or push with the toe of you shoe to get your foot under the cover.

Robert G8RPI.

I would arrange it so the foot switch needs to be depressed for a separate start switch to work and when released operates a stop, so you can't do a start without operating both controls.

Ironic that James fits a foot switch as a dead man's handle only to find that accidentally stepping on it starts the lathe. Easy to go wrong with this stuff so I offer this circuit with trepidation! Be gentle with me if it's wrong.

Operation.

Press the momentary switch and foot switch at the same time causing power to flow to the relay coil and the speed controller.

The relay contacts bypass the momentary switch which can now be released, returning to its OFF state. However, power is maintained to the relay coil and speed controller so long as the foot switch stays closed. When the foot switch is released, the relay powers off, its contacts open, and the circuit is broken.  Power is only restored to the motor when the momentary switch and foot switch are operated together.

Dave

Edited By SillyOldDuffer on 23/10/2018 18:17:03

That is exactly how I would do it Dave!

Edited By Phil Whitley on 23/10/2018 19:17:59

Thanks so much for this - I will play around, but this looks like it should work well.

Thanks again - really appreciated and I will let you know how I get on.

James

I would go with the idea of a covered switch that you have to put your foot into to operate the switch, this might also help to keep swarf an other crap out of the switch.

Ian S C