Polarity Reversal in a DC Circuit

This would be really easy to do if you just wanted it to swing back and forth, a reversing switch at east end of the travel (which was 'wiped' by the arm..with suitable back up switches..room for over-run, bump stops etc) The problem is ..presumably you need it to pause while the operation takes place?

In this case you need the switching to be controlled by the 'operation'.

We need more detail to provide a sensible solution....
Does the boom weigh 200g or 20kg?
Is it 30mm long or 30m ?
Does the 'operation' take 30mS or 30seconds?
Does the boom actually perform the operation or is the operation controlled from something else..if so, what and how?
... What is your shoe size?

Del

Del,

When the boom reaches the 90deg mark, a protrusion from the base of the 'tower' triggers the 'stop boom' switch, holding the boom at 90deg. The 'operator' does trigger the 'boom start (reversed)' switch when it has completed its task.

- Dimensions of the boom are roughly 25cm long by 5cm wide by 5cm high.
- The boom weighs about 500g, including 'operator'.
- The 'operation' takes about 20 seconds.
- A second device is located at the end of the boom which completes the operation. Its circuit has already been designed and modeled, it is powered by a different source, and uses a different motor so will not interfere.
- The entire operation (boom swinging, operation completion, return to initial position) is completed autonomously, using only the switches to proceed from one task to the next.

The system's entire process is as follows; (1) system activated by human operator by connecting power supply, (2) boom rotates out to 90deg position which triggers a 'boom stop' switch and an 'operation start' switch. (3) At the other end of the boom, the operation is proceeds and when completed triggers the 'operation stop' switch and 'boom start (reversed)' switch to (4) return the boom to its initial position. No other user input is provided, and one complete cycle takes less than 60 sec.

Thank you.

You should be able to do this with:

2-12vdc relays

1 for on/off

1for forward/reverse

2-limit switches

1 for +90⁰

1 for home

2-start switches

1-forward

1-reverse

The only items added are the relays & the 0⁰ limit switch.

the forward start pulls in the run relay, wired to latch, +90⁰ limit switch unlatches.

The reverse start pulls in both relays, wired to latch, 0^o limit unlatches.

need further details?

Garthh,

Thanks for the great parts list and suggestions. From the parts list, this is the circuit I came up with; is this what you had in mind?

Thanks to all for great suggestions and analogies. The light, compact apparatus wouldn't make full use of the features of the garage door, but definitely could be useful in a larger application.

Perfect! Thanks again Garthh, great help.

great help..

Well click on the 'Rate' box in the bottom left of his answer and score it as good...(I have already done so)... without our 'good answers' we all _{fade away...}

_Del

Is there a need for an emergency stop or reverse? This may make things a little more complicated, but it may save damaging something/someone.

The garage door opener has one button, you press it once to open, the power shuts off when it is fully open and sets itself up to close next time the button is pushed. If during opening (or for that matter closing) you press it a second time it stops, press it a third time it reverses (closes), a 4th it stops, a 5th it continues to open.

If there is anything in the way of the door whilst closing, the resistance felt by the stalling motor reverses the door.

Whom ever suggested the garage door closer, it is a good idea, and could be adapted to a boom gate

Look at a garage door opener. It has all you need even the push button and its a remote.

If it must be 12 volt DC look at the wiring on a standard electric pallet truck.

The mechanics of the door opener will still work. May be fit the DC motor in one.

Isn't a windshield wiper doing exactly what you want?

A windshield wiper if I remember correctly performs it's reversing function mechanically. At least the ones I have seen do it that way, there is no reverse function involved, it makes a revolution and the wipers move out and back.

Because I haven't opened any such device (no need) I will trust you. Still, there is a motion, about 90°, of a boom. If this little boom is comparable in size with the one in the problem, it is not difficult to add two limit switches. They are necessary, anyways.

Wipers for some large trucks do it inside the wiper motor but he's got a motor just needs a disconnect.

Well I have done this many times for microphone booms. All you need for a low voltage DC circuit is a 3 pole (or more) Relay of the appropriate voltage and a couple of cheap microswitches.
The relay has 2 poles wired as a normal polarity reversing switch and the 3rd pole acts as a "latching contact" to permit the reverse sweep. If left to itself of course this will keep on repeating but the use of a simple counter circuit and a mosfet can make it operate just as many times as you wish before stopping.

You may use adequately rated power contactors one for each direction of rotation of the motor - one contactor for forward rotation of the motor and the other contactor for the reverse rotation of the motor - with the respective contactors energized and de-energized via push buttons and limit switches. However, the motor speed also needs to be controlled to prevent overshoot (due to inertia) towards the end of the motor's rotation.

Hey, as long as we're making analogies, how about a phonograph arm? Doesn't it move in a similar fashion? Just thinking that might be another place to look.

My son wants to know what a Phonograph is....

This same requirement(reversing DC Polarity) is used on some Industrial Damper Vane Control. The one I have used, utilizes two adjustable cams on the motor shaft. Each opens a switch at the point when the motor has travelled the distance required in that direction. The pos. & neg. are connected opposite polarity on each switch, thus causing the motor to run the opposite direction to the first location. The start and stop (2 different cams) positions are locked in the position you choose with small Allen Set Screw, making position adjustment very easy.

Thanks everyone for the great tips and suggestions. Good luck with future endeavors. I'm hooked on CR4 now, thanks!

you do not need a seperate power source to bring back the boom. All you need is a to create a unique control system for yor machine.