DC Motor Limit Switches for Oscillation 5

[truseal]

Hello,

I have a low profile ball screw linear actuator. The shaft is connected to a 90VDC motor. I need the wiper plate on the acutator to oscillate. The stroke is approximately 15".The limit switches will be mounted on this wiper plate and have contact at each end of stroke. I have (2) NO and NC (2)wire plunger limit switches and several solid state relays.I would like to get a schematic and/or instructions how to wire this for constant oscillation back and forth.

Thanks much.

 

[Barry1961]

One way would be.

NC FWD LS---NC REV RELAY----FWD RELAY


NC REV LS--------FWD LS-----------REV RELAY
I I
-REV RELAY-


Not sure how this will post so here it is in Boolean.

NOT FWD LS—AND—NOT REV RELAY--OUT--FWD RELAY


NOT REV LS—AND—FWD LS—OR—REV RELAY—OUT—REV RELAY

This is probably going to post really ugly.

With this logic it will always start in the FWD direction first unless it is already on the FWD LS.

Barry1961

 

[sreid]

Could I suggest that you control this using a low end PLC. Between the limit switches, some kind of memory is required to keep the moror running. It would also probably be wise to let the motor coast to a stop before reversing it. Changing the motor direction while running is called "Plugging" (Reversing the plug) and almost doubles the motor current.

Two mechanical relays could be used, motor run and motor reverse.

 

[aolalde]

Yes, instant reversing of the motor will overheat and increase the mechanical stress of the windings, leading to a premature failure.

 

[waross]

Try this on. Use two electromechanical relays cross connected.
Each relay is energised through the normally closed contacts on the other relay.
Now when either relay is energised the other relay is inhibited. When the appropriate limit switch is operated it will de-energise the relay. The circuit to the other relay will be completed and the motor can be reversed. Now we have a circuit with two relays and two limit switches that will alternate between relays when the appropriate limit switch is actuated.
We still have to consider initial energisation when both relays may complete enough of their stroke to make contact before they interupt each others circuit.
We still have to consider motor coasting time. There are several ways to insert a time delay into each relay circuit to retard the energisation of the relays to allow coasting time. If the time settings are slightly different that will take care of the circuit race on energization.
I am looking at a two wire series connected "Delay on Make Timer". It is common in the commercial refrigeration field. Possibly under $10.US.
Ratings, 19 to 250 Volts, AC or DC,
1 Amp max.
Time o.1 Minutes to 8 minutes.
The only issue I have had with these is with very small relays when I had to add a couple of resistors in parallel with the relay coil to increase the current. The units are made by a number of manufactures so the current issue may be specific to only one manufacturer.
If you use relays large enough to switch the motor directly, the little timers should work without resistors.
yours

 

[Compositepro]

The best solution is a self reversing ball screw:

http://www.motiontech.com.au/assets/pdf/Ball Reverser.pdf#search='Flennor Ball Reverser'

Or, for adjustable speed and stoke the rolling ring drive:

http://www.amacoil.com/html/page3d.htm

Plugging works okay if you stay below full motor speed. Otherwise, use a dynamic braking resistor to stop the motor before reversing it. Control of the velocity profile during reverse will not be great but it will work.

 

[nbucska]

Simplest solution is to use a single relay with three
double throw contacts. 2 of the contancts select the
polarity of the motor (e.g.inverted if actuated ).

One limit switch is NO and the other one is NC, both
momentary. Snap-action or debouncing is not necessary.

The NC switch is in series with the relay's coil
and the NO one energizes it. The remaining third
contact is parallel to the NO limit switch, so
when the NO switch is actuated, the relay pulls in
and latches itself.

To limit the current and the force at turn around
the simplest would be to insert a resistor in series
with the motor.




<nbucska@pc33peripherals.com> omit 33 Use subj: ENG-TIPS
Plesae read FAQ240-1032

 

[waross]

hi nbucska
That would work but there may be a problem with the relay ratings. If you intend to use the relay to operate the motor directly the relay contacts should not be wired in what used to be called "Opposite polarity". Most double throw control devices that I have seen are not rated for opposite polarity.
You may supply the power to the movable contacts of a double pole device, but it is not within the ratings of most control devices to connect opposite polarity sources to both fixed contacts of a double throw device.
I understand that truseal has some solid state relays in hand.
He could use your circuit to control the solid state relays and get away with a 2 pole relay, or even a single pole relay if the limit switch has enough capacity to handle the mechanical relay and the solid state relay both. He will also then have control lines in which to add time delays.
Good suggestion nbucska

 

[nbucska]

Since I don't know the currect of the motor, I can't supply
an actual design. The 3P2T relay can be realized by combination of other relays.

I wouldn't use solid state relays for polarity switch
-- they tend to fail closed . Other problemis that
you need to add timing circuits to assure that they
don't short the supply.

 

[truseal]

Hello,

I want to thank everyone for there comments.