Force required to pull open door?


I have a door on a spring that automatically closes. I have a dog that has figured out how to push the door open, but much to his surprise it closes behind him. He hasn't figured out how to get out. This makes him sad.

I had some crap laying around so i decided to make a automatic door opener using a nema17 stepper motor that can take 1.3amps max. The stepper driver I got can only output about half that, and even though i am using a geared stepper motor (bondtec extruded i am no longer using) it ain't got enough power. CLICK CLICK CLICK and now i am sad. :frowning:

So, before i buy more things i figured the first step would be to determine how much torque i need.

How can i determine that?

Right now, the stepper turns a lever that rotates a 1ft aluminum bar. Any ideas? Not sure where to put this since it's not an arduino problem.

Use a fish scale and hook it to the long end of your 1ft aluminum bar.

do you think the force required at both ends of the lever are the same?

do you think the force required at both ends of the lever are the same?

Yeah, but without pictures, 'spanations, and other learning aids, I don't know exactly what your dog door opener works like. But to determine torque in general, measure the pounds needed to turn a one foot lever.

Right now, the stepper turns a lever that rotates a 1ft aluminum bar.

That is not clear.

In general, torque equals distance times force, provided that the force is perpendicular to the distance.

The torque reqd to hold a door open can be measured by attaching a fish scale to the knob (holding the scale perp to the plane of the door) and multiplying the force times the width of the door (to the knob).

You'll need more torque than that to overcome friction, the inertia of the door, wind pressure, stack effect, and aerodynamic effects.

You can estimate the reqd total torque by pulling the door open with the scale, at the speed you want the door to open. Then double it.

here are some pics

When the door is open (motion detected), the stepper motor pulls the rope back till it "homes" against the metal plate. When the door is closed, the stepper moves 90* and the slack in the rope allows the door to close.

i don't want to out anything on the outside of the door.

I'm not 100 percent sure, but just from reading a little bit after posting this question, i doubt my current set up will be practical because i am going to need an extremely large amount of force to move the lever because the arc-moment is essentially zero.

i might try to attach the rope to nuts on a leadscrew and take advantage of the ole incline plane.

maybe a torque wrench and a nut installed right above the stepper would give me a proper result…

Where you have the cord between the lever and the door handle put the spring balance suggested by ~ChrisTenone. Then open the door by pulling the lever by hand and measure the maximum force needed.

You then need to multipy that force by the distance from where the cord attaches to the lever to where the lever attaches to the the stepper. That will give you the torque the motor needs to have.

The way things are setup the lever is giving mechanical advantage to the door, it is working against the motor being able to move the door. Try putting one finger on the lever far from the stepper and push the door open. Now do the same with your finger half way down the lever. Then try it with your finger near the stepper. more and more force is required the closer you get to the pivot point.

The idea of a pulley suggested by ~Travis sounds like a good one to me.

The way things are setup the lever is...working against the motor being able to move the door.

As the following shows, the opposite is true (or else I misunderstood you): the shorter the "lever," the smaller the required motor torque (but the door won't open as far with a smaller lever, either).

Belt and pulley sounds like it is a little more complicated than i wanted to get into. Id have to get a pulley, belt, ect...

Would a large gear and small gear do the trick butted up against each other, I assume is the same principle as a belt?

Thinking something like this

I assume you don't have to pull the door open very far to let the dog in/out. I'd try attaching your string to the "lever" as close to the motor as possible (while opening the door only as far as needed) before trying anything else (unless you cooked the motor with your previous attempt).

I got it to work at a minimum acceptable level.

I had this put together for a coreXY machine I a building... this was made on the drill press but now that I have a milling machine I'm going to redo the x-axis (with a beefier linear rail as well).

No gear slippage but the door doesn't open all the way because the bar is in the way. I tried tying the string to the plate without the bar but it doesn't open enough.

I think it might be the belt though, it's sort of stretchy and I think a lot of the movement is getting lost in belt-stretch.

Thanks for the responses so far and still open to suggestions :slight_smile:

I like your solution. At my house the dog barks until one of us notices, then we get up and open it. I suppose a robot with sound sensors, mobility and a grabber arm could be used too. But I like the simplicity of your system compared to that! :o

I got it to work at a minimum acceptable level.

Glad your system works, but I don't see a dog-system interface. IOW, how does the dog get the door to open?
Also, how does the system know when it's safe to close the door, without squashing the dog?
You say that the door doesn't open all the way. Does it open far enough to let a human through?

I will post a vid once it is finished.

It doesn't force the door closed, it forces it open. The spring that forces it closed is a separate system and isn't that strong (the dog forces it open easily)

I am using a motion sensor to signal it to open

In consideration of the task at hand, I would have assumed a spool of cord, a pulley, and the stepper )or other DC motor, along with a limit sensor- would be the simplest solution, mounted near the top of the door (as most door opening and closing systems are). If you wanted to get funky, force-wise, you could always create a block-and-tackle.

Cord draws door towards wall, pinioned through a pulley (or even an eyebolt). Stepper feeds the cord in and out.

the problem with using a spool is that there is no guarantee it would wind back up, especially if you consider that if you have the door closed, then manually open it, there would be slack on the line that would become tangled/fall off the spool. I suppose you could put a preload on the spool, but that adds to the complexity.

the shorter the "lever," the smaller the required motor torque (but the door won't open as far with a smaller lever, either)

I think we are agreeing. At the moment the lever is working against the motor, so the shorter the lever the less torque the motor needs.

The last set of photos seems workable.

Would it not be simpler though to tie a good thick piece of cord to the handle and just train the dog to pull it?

It is far easier to train pixies than it is to train puppies.

This is turning into a major pain in the ass... i am having issues with the limit switches...

for some reason, on the esp8266 using input pull up, the pin will read low before it hits the limit switch.... I also tried rewiring so it would read high when it hits the limit switch, high before it hits the limit switch... which is strange since i have a 2.2k ohm resistor pulling it down... arg....

If i use a dmm to read the voltage on the metal pieces i am using as "endstops" it works as expected. Not sure why.