Scissor Lift

Hi All,

I am sure this has been done and is really straight forward but I could do with some guidance,.

I have built a large scissor lift machine that is powered by a basic winch motor, currently with basic up and down buttons. Mechanically its fine but I now want to add some sensors to make the lift stop at the same point each time (both going up and down) to stop it over extending or contracting and destroying itself.

I have attached a basic diagram of what I am thinking (very basic, please dont judge :slight_smile: :o )… but I would like. Ideas, suggestions, diagrams, Codes… anything and everything you wonderful people can provide to help would be massively appreciated

Thanks

P.s In case it helps anyone this is the proposed motor

http://vi.raptor.ebaydesc.com/ws/eBayISAPI.dll?ViewItemDescV4&item=254199910395&category=184199&pm=1&ds=0&t=1555268473000&ver=0

If the top and bottom limit switches were normally closed switches, and pressing them opened the circuit, cutting power to the motor, no microcontroller would be needed.

PaulS: If the top and bottom limit switches were normally closed switches, and pressing them opened the circuit, cutting power to the motor, no microcontroller would be needed.

...except most switches can't cope with the current of anything but a small motor. The schematic with relays is a good start but doesn't really show the wiring; just the concept.

If the top and bottom limit switches were normally closed switches, and pressing them opened the circuit, cutting power to the motor, no microcontroller would be needed.

the problem with that is, when the switch opens, no juice to the motor, how do you reverse the direction?

I am thinking big hydraulic scissor lift here: one “almost there” switch to downshift the motor, so it creeps up to the top limit switch, so you don’t overshoot. top limit switch does not disconnect power, top limit switch disables motor power at the H bridge. when you need to reverse direction, reverse the polarity at the H bridge, then enable power

OP, have you researched H bridges?

Fundamentally, you need DPDT relays to control the motor power/direction. Then you need to use the switches to operate the relays. The limit switches need to open the circuit to the switches, so only the opposite switch can be used until the limit switch is closed, again.

Paul

Please post the details of the "basic winch motor", or a link to the data sheet or product page.

To know how far you are, add an encoder to the motor. That way you can start slowing down shortly before you expect to hit the limit switch.

How does the scissor lift carry out the lifting ?

Does it have an hydraulic cylinder with a motor controller or is it a one speed lift ?

You could use limit switches but not knowing how you control the lift or how the lift works or how it goes up and down.

If you could give more details I may be able to help as I did a similar thing some years ago where I had soft start & stop using limit switches.

I had to add input filters and a small current draw through the limit switches to make them work correctly.

Like I mentioned you could have lift up/down buttons and then add limit switches mounted on the scissor pack and have the controller monitor it on an input then once the limit switch has been activated you turn the relay off and the same for down.

I'd be intrigued on how your scissor lift works, I'm taking a guess here but you power the motor to lift and lower the pack ?

Paul_KD7HB: Fundamentally, you need DPDT relays to control the motor power/direction.

No you don't You need two SPDT relays which are much more robust in general than DPDT. Think about it.

wvmarle: That way you can start slowing down shortly before you expect to hit the limit switch.

Not really necessary if the limit switches are correctly positioned.

I used two DPDT relays wired so the UP and the DOWN switches could not be operated simultaneously.

Paul

If you use two SPDT relays, it does not matter if both UP and DOWN switches are operated simultaneously.

|490x500

Much more simple and reliable. The (NC) limit switches are in the relay coil circuits, but could be in the relay NO contact connections if that were more appropriate.

Paul__B:
If you use two SPDT relays, it does not matter if both UP and DOWN switches are operated simultaneously.

Much more simple and reliable. The (NC) limit switches are in the relay coil circuits, but could be in the relay NO contact connections if that were more appropriate.

You can’t stop the lift anywhere in between full up and full down with this circuit.
Tom… :slight_smile:

TomGeorge: You can't stop the lift anywhere in between full up and full down with this circuit. Tom.... :)

the 2 SPDT relays are for direction. the unshown relay is for On/OFF the unshown logic for direction dictates which way the direction will be. I would prefer to use a single DPDT latching relay in order to control direction. then use the end stops to change direction, and have a manual MOM-O-MOM center off to be able to manually set direction if you are half way up, but want to go down.

TomGeorge: You can't stop the lift anywhere in between full up and full down with this circuit.

What on earth do you mean? :astonished:

Paul__B:
What on earth do you mean? :astonished:

Sorry, just the circuit is short of coil control gear… my fail…
Tom… :slight_smile:
To much blood in the caffeine stream.

TomGeorge: Sorry, just the circuit is short of coil control gear...

You take what you get dredging images off Google. It demonstrates the principle of using the simplest, most robust relays to control the high current parts of the equipment. Each relay will then have the respective limit switch in series with its coil and be controlled by the corresponding button. I could draw all these details in "CircuitLab.com" but don't feel it should be necessary. :grinning:

The interesting consideration that may be asked is what happens if both buttons are simultaneously pressed and both relays therefore actuated. What is most elegant is - precisely nothing! It is a "fail safe" situation. :sunglasses:

I mentioned in #8 that the limit switches need to be set so that they actuate sufficient travel before the mechanical "hard limit" in order to protect the motor and gearing. Nothing startling there, but it is fascinating to note that if the mechanism is already at one limit of travel when both switches are pressed, it will in fact start up and move away from the limit - and then stop as soon as the limit switch clears.

Paul__B:
I could draw all these details in “CircuitLab.com” but don’t feel it should be necessary.

Because there are scissor lift schematics available online to use as examples/guidance.

Is this a shop-floor mounted thing or one that travels with a man in it?

Paul__B: If you use two SPDT relays, it does not matter if both UP and DOWN switches are operated simultaneously.

|490x500

Much more simple and reliable. The (NC) limit switches are in the relay coil circuits, but could be in the relay NO contact connections if that were more appropriate.

Thank you for this Paul, It 100% gives me a more detailed Idea of what I need to do. I may well need to implement your design,

jremington: Please post the details of the "basic winch motor", or a link to the data sheet or product page.

Hi,

The motor is the same as in the one in the below link

http://vi.raptor.ebaydesc.com/ws/eBayISAPI.dll?ViewItemDescV4&item=254199910395&category=184199&pm=1&ds=0&t=1555268473000&ver=0

dougp: Because there are scissor lift schematics available online to use as examples/guidance.

Is this a shop-floor mounted thing or one that travels with a man in it?

Hi it is a static mounted device that is actually mounted under the floor in a pit and its design is to basically lift a table top out of the floor