Resetting a relay? 1

[red300zx99x]

I'm developing an application for a friend, mech auto guy here, but know alittle about circuits, so don't go easy on me, I don't mind. I need to be able to push a button and have a motor turn on, forward direction, till this motor moves a device that later comes in contact with a switch, when this switch is activated the motor reverses and brings the device back to another switch which shuts the whole thing down. Problem is the relay to switch to reverse mode stays in reverse mode once switched, I need to beable to reset this so that the cycle can start again.

 

[Skogsgurra]

red300zx99x,

You have selected a rather tricky task. These things are normally done in a little micro-processor nowadays. The technique used is called "Finite State Machine" and that describes what it is all about. You have a number of states. In your case, you have the HOME case, the FWD case and the BWD case. If you want to do this with relays it is good to have one relay for each state, but you will find it smarter to have a /HOME (not home) relay that pulls in as soon as the device leaves home position (home limit switch has a NO contact). OK.

The cycle starts with FWD, BWD and /HOME off and the device in home position.

Press START and FWD pulls in. A holding contact keeps FWD energised. /HOME pulls in when START is pressed and is kept energised by the home limit switch when the device leaves home position. It is important that you pull /HOME in when you start. There can be a short circuit between FWD and BWD if you do not do that.

When the end switch is reached, it opens the circuit (NC) for FWD relay so that it drops out and the motor stops. When the FWD relay drops a NO contact in series with /HOME NC contact pulls the BWD relay in and the device starts moving home again.

If your motor is a high-current one, you could need some delay here to avoid short-circuiting when you change direction.

When the device reaches home position, the /HOME relay drops out and the /HOME NC contact opens so that the BWD relay drops out.

You will probably also need an emergency stop that stops all motions and then another push-button to take the device back home without having to complete the cycle.

There should also be a thermal motor protection to avoid burning out the motor if it gets stuck. Put at least a fuse in series with the motor.

I leave no guarantee that this is the exact function you need, but it will start you in the right direction.

 

[felixc]

It looks like the switch that reverses the motor keeps its state latched after the system turns off. Why? Is it a mechanical switch, or a relay activated by a switch? The switch that shuts the whole thing down has to reset the state of this switch too. But not knowing how it's been implemented makes it hard to give a solution.

 

[nbucska]

Window wiper ?

<nbucska@pcperipherals DOT com> subj: eng-tips

 

[red300zx99x]

Nope, doorbell. Yes the problem is that the reverse switch is still latched, mechnical switch. But the microcontroller solution is sounding better and better as time goes by, easier to house and probably more cost effective. Any thoughts would be greatly appreciated

 

[nbucska]

Wouldn't a dual timer simpler ?
___-----------------_______ Current on
___-------_________________ + forward/-reverse

REverse longer, so the mechanical STOP assures
home position




<nbucska@pcperipherals DOT com> subj: eng-tips

 

[buzzp]

I think there is a solution without a micro but need to know more details.
1. What resets the reversing switch? Is this manually reset? Is it possible to change this to a standard NO or NC switch?
2. Does the forward motion switch latch in as well?
3. I assume you have another relay or switch to switch the motor load direction (contactor)? Is this DC or AC motor?
4. You only want one cycle (go forward then back then stop) to occur on pressing one start button correct? Do you want to have a reset button that needs to be pressed as well after one cycle?
5. This is for a doorbell? What in the world?

Thanks

 

[CarlPugh]

Permanent split capacitor motor-reversible while running, upper and lower limit switchs and relay will perform this function. If someone knows how to place a schematic on this forum, let me know and will post schematic. If no one knows how to post schematic, contact me at carl@pughmagnetics.com and will Email you schematic using Microsoft Word.

 

[red300zx99x]

Please send schematic to purelax99@yahoo.com

 

[BrianR]

This would be simply done with a little PLC.

 

[peebee]

Relays still get used for this stuff all the time in the 480-volt world.

Here's a schematic for a reversing motor starter that would suit your application nicely -- the control logic is identical to what you need, just ignore the 3-phase power and replace with 1-pole 12v contactors accordingly:

http://www.ab.com/en/epub/catalogs/12768/229240/229248/3005006/3178786/229359/tab5.html

This also has the advantage that it never ever needs to be rebooted!

Another thought -- what you want to do sounds a whole lot like the automatic reverse on a garage door opener.

 

[red300zx99x]

Yep, when my friend first explained it to me he said like a garage door opener. Any thoughts on using a PLC design, little knowledge in the subject here

 

[BrianR]

red300zx99x (what a mouthfull)

If peebee's diagram does what you want then that's a straightforward way to go, however just for interest, a PLC is programmed with a ladder diagram on a PC and uses logical relays to perform the logic. These relays have N.O. and N.C. contacts like physical relays. The outputs to the real world of will be 2 physical relays, an ON/OFF relay and a REVERSING relay for the motor. The inputs will the limit switches. PLC's usually have logical TIMERS and other goodies like COUNTERS that let you do some really powerful things. Read up on ladder diagrams as they are a very powerful tool.