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Reversing dc motor w/ stop Circuit

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Aardwolf

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I need to build a control circuit to run a cutter unit to notch some styrofoam over and over and over.

My control experience is about 25 years out of date, but I think I still do have a decent understanding??.

I know it should be basically a simple circuit to do what I need, but I just can't get this sorted out in my head.

Im using a small variable speed dc motor with a chain drive on it to push/pull a rolling table back and forth about 16 inches. The table has some arms on it with shaped hot wire cutters to notch the foam.

The table would usually start out at the near end in a stopped position. The table is on wheels, runs in a straight line and will follow a vertical profile. The table will always have the same end points and will always follow the same profile.

I have not built this yet, until I get the control worked out.

I am planing on using a DC variable speed motor, geared down to get into the speed range I need which is 15" per min + or - 25%. To fine tune the cut, I have to be able to adjust the speed from one end of the range to the other, but basically once I have the speed working right, it is not going to vary except rarely.
.
For the control:

I need to press a button to start the motor in one direction, let it run forward until it hits a limit switch at the far end where it would stop.

It would stay stopped until I press a button again (same one or different one). At which time it would run back until it hits a limit switch at the near end, where it would again stop.

... and then repeat

Also only while the unit is moving I need to turn on another power circuit for the cutting head.

I know that this should be easily doable with relays and switches.

Any help would be appreciated

Thanks
 
I would use a DPDT toggle switch (which has six terminals) mounted such that the handle position shows which way the table moves, i.e. move the handle back, and the table moves back; move the handle forward, and the table moves forward.

Assuming the motor runs both directions depending on the polarity of the applied voltage, you also need two Normally-Closed limit switches, one to stop the motor when the table reaches the full-forward position, the other to stop the motor when it reaches the full-aft position.

Wire the switches as an H-bridge so as to reverse the current in the motor. See below:
 

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Here's a diagram for the circuit similar to what MikeMi mentioned. (Look down the page for "Alternative Motor Control With Automatic Limit Stops").

If you want to use push button control, then you will need to use latching relays or standard relays in a latch connection. In that case you also would want a panic (a big, red emergency stop) switch to halt everything.
 
Thanks for the above, Both are a great help.

I would prefer the push button(s) with the latching relays (& the big red emergency stop).

Does anyone have a circuit diagram available for that type of setup?

Thanks
 
Here's my take on a solution using relays. There are two relays, K1 & K2. When one is one, the motor travels in one direction, when the other is on, the motor travels in the opposite direction.

The logic for the relays is
CodeCogsEqn (1).gif

Which can be realised as
toggle start motor bidirectional.png

Note the start button has two contacts.
 
...
The table is on wheels, runs in a straight line and will follow a vertical profile. The table will always have the same end points and will always follow the same profile.
...

There is a possible problem there if the table must move vertically.

If you use an adjustable regulated voltage to set the motor speed, that will work pretty well on a horizontal table, in both directions.

But if you have a vertical table, unless it is perfectly counterbalanced the motor will work a lot harder (ie slower) moving upward compared to downward. So you will have issues with getting the same speed in both directions. (And you said the speed must be adjusted right, to set the cut parameters).
 
Here is a second cut of dougy's approach, but it adds a couple of things. First, Em Stop button cuts power to the motor. Second, simplifies the interlock so only single pole Up and Down buttons are used. Third, addresses the speed difference with the addition of a balancing resistor if needed.
 

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A level wind mechanism to guide cable onto a cable-receiving spool utilizing an endless chain with a drive pin to shuttle a cable-guiding carriage back and forth along the length of the spool. The drive mechanism powering the shuttle chain includes two chain sprocket assemblies

https://www.freepatentsonline.com/3583653.pdf
 
WOW!

This was my first post here and I appreciate all the fast responses and ideas and suggestions.

I am not worried about the difference in speed up vs down, its only a 2" drop over 10" run and I am using a DC motor that is geared down so has lots of torque.

I think I have my head wrapped around it now and I am going to go ahead with it at this point.

I will let you know how it turns out

Thanks all.
 
...I will let you know how it turns out...

Feedback always appreciated.

Something else occurred to me. When the moving platform trips either limit switch, as the relay resets, that puts a short on the motor. It may be hard on the relay contacts, or it may stop the motor too abruptly. Putting a small resistor in series with the motor might reduce the current pulse and the abrupt stop.
 
I have been working on a very similar project, I chose not to use toggle switches, but a sr- latch cd4043 and 2 proximity sensors. my circuit is much more complex, because I am driving stepper motors, and have delays, varaible oscillators, etc. If you are interested in going more high tech. the circuit is fairly simple if you are looking for an electronics project more than just function.
 
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