Window Motor question
 

We are using one window motor to power the winch that pulls up our ladder. The ladder assembly being pulled up is about 10 lbs, more if you count the lever being our grabber assembly at the end. I don't know how to calculate that, but it's 7 inches long, and the grabber weighs ~6 lbs, making the total 10+grabber. If someone could explain how to calculate this to me, I would love it!

Anyways, we were testing it today, and it randomly stops when going up. The motor is getting very hot, and when we cool it down (with snow of course!), it starts working again. However, when we measure the current going through, it's only 10 amps. I thought that the stall current of a window motor was 21, or 18 (I dont remember if we're using right or left here).

What is causing this problem? Is it that we are simply pulling too much with a window motor? Or is it the thermal breaker in the motor breaking? Is something causing the motor to overheat? Or is it the infamous issue with Jags and Window motors? Would that cause it to overheat?

We are running it on a Black Jaguar, on the CAN bus, wired to the 40 amp breakers.

Any help would be helpful!

Picture of device, imagine that there is cable going off of those spools to pulleys:

Re: Window Motor question
 

Grim,

The window motor has a positive coefficient temperature (PTC) resistor built in. When it gets hot, the resistance increases causing less current and voltage being delivered to the motor windings.

You are asking the motor to do too much. Unless you can identify frictional losses which can be minimized, a redesign is warranted...

Regards,

Mike

Re: Window Motor question
 

Mike B. is 100% accurate. This motor is not very strong relative to the CIM and FP. The thermal device will kick out on severe load and current. A 40 amp circuit is a bit much for this motor as the protection will cut out well before this level. Use a 30 amp max.

Re: Window Motor question
 

Do you guys think that putting another window motor on the other side, as the winch was designed for would work? Or do we need to completely redesign it?

Will an FP with a gearbox be enough?

EDIT: The only other motor we have available is the RS550 with a 26:1 gearbox, and from what i've heard, banebots gearboxes arent meant to be used in high torque applications.

Re: Window Motor question
 

Without knowing exactly what the load; it's hard to tell what will be enough.

A second window motor will help and might do it. A Fisher price is usually good for most arms and if it doesn't do it then a redesign maybe necessary or a higher reduction in the FP.

Edit: Guessing at the size of the drum on the winch and using the other weight it looks like adding a second window motor would do the trick. The 10 to 12 current you were measuring is not too far off from the torque curve with the given weight. You could put the RSS 550 onto a fisher price gearbox if you can get a pinion on it.

Re: Window Motor question
 

Does anyone know how to calculate the force exuded down by:

A 6 lb object, cantilevered out ~8 inches.


What is the gear ratio of the KoP FP gearbox? I don't have one on hand.

To respond to an earlier comment, the motor is only drawing 10 amps at stall, which is is a bit weird. This happens ever after the motor is starting from a cool stop.

Re: Window Motor question
 

You might try lubricating any sliding surfaces. If you reduce the diameter of the drum used to take up the cable you will effectively increase your gear ratio. Also look into different ways to string your cable to give you mechanical advantage. You can ease the load on your motor by providing counterweight that works to pull the lift up at all times. This may be an actual weight, or preferably springs, penumatics, or other springy like device to help your motor out.

Also look into loosing some of the weight of the part that moves up and down, weight up high is nearly always a bad thing!

Re: Window Motor question
 

I think we need to know where the force is being applied, otherwise you can just calculate the torque.

And I'm not 100% sure but I think the FP gearbox is a 117 reduction. If I'm not right, I'm close.

Re: Window Motor question
 

Also, in this application a 6lb weight will always exert 6lb force in the down direction. However the added force you need to lift this thing is due to the friction of whatever is holding it out away from the upright portion. you calculate that by summing the torque on the mechanism at a convenient point and finding the resulting force at the point of contact. Then you can approximate the actual force if you have some values for coefficient of friction for the materials. For more info look into a subject called "Statics"

Also as an aside thought, If you dont already, try to place a bearing at each place that contact is made between sliding surfaces, or at least some slippery plastic, that may help your motor some as well

Re: Window Motor question
 

how are you measuring the current?

power down the robot, disconnect the motor, and measure its resistance with an ohmmeter... post the answer here

Re: Window Motor question
 

Though we have lubricated everything already, the surgical tubing counterweight is a good idea. We already considered making our spindles smaller, but we have no easy way to do so, considering how they were made.

In fact, we currently have a surgical tubing counterweight pulling it down


This might be another contributer to why this isn't working.

Like I said, unless we can design a brand new type grabber in the next three days, I think its going to stay that way. Any suggestions for quick, simple, light grabbers? Keep in mind that we would like to keep floor pickup.

EDIT: We are measuring current with a clamp on multimeter. I can't do anything to the robot right now because it's in the school machine shop, and im in my house.

DOUBLE EDIT: We already have delron sliders at most contact points, and nylon washers at some others.

Re: Window Motor question
 

since you only have the RS 550 to work with... Is it possible you can press the pinion gear from a fisher price motor and drop the RS 550 into the Fisher price gearbox? The added power should be enough.

Re: Window Motor question
 

No, no, we have an FP, from the KoP, I meant that the only additional motor to that was the RS550.

In relation to a (or two) window motors, how fast and/or powerful is an FP on the gearbox? What is the output shaft of said gearbox? It would be a major plus not to have to take our entire winch assembly apart to lengthen the shaft on it.

Actually, does anyone have a spec sheet, or CAD (inventor) of the FP gearbox?

Re: Window Motor question
 

Try looking at this document, which explains how to calculate forces and how to pick the right motors and gear ratios for your mechanism.

In short, if you're pulling 10 amps, you're running at about 20% efficiency (based on the motor curves), which means 10 A * 12 V * 80% = 96 Watts getting turned into heat with nowhere to go. Generally, you should run motors close to their maximum efficiency, which for the Denso window motor would be a torque of about 2 N-m and a current of 5A.

Re: Window Motor question
 

The fisher price is about 12 times more powerful than a single window motor, or 6 times more powerful than two window motors in tandem. With a max torque approximately 3500 oz inches at the output of the gearbox at 40 Amps There is no output shaft at the end of the gearbox. It has a weird drum that it drives. The off the gearbox free speed is faster than the window motor.

The JVN calculator has the actual FP gearbox reduction. While

http://www.firstcadlibrary.com/ Also had a Cad models of the FP gearbox.

Re: Window Motor question
 

Question, How long before it randomly stops? If it is not too often, could you possibly cool the motor with the muffin fans?

Re: Window Motor question
 

We are able to go up and down twice before it stops.

I wonder if the fans would do enough?

It's unlikely that we would ever be jumping around like that at competition, but you never know. Maybe with two motors, and a fan, it would be better.

Re: Window Motor question
 

Oh, and measure the voltage at the motor terminals when it is drawing the 10 amps.

Re: Window Motor question
 

So another thing (I'm also on this team). The arm works fine for a few times, but after some amount of time it stops once an extra load is added (the second stage starts lifting too). The normal raising runs at 7-8 amps, and the stall is 10-11. This makes me think heat. I wonder if this has nothing to do with the load, and more with the duty cycle. The jump in the load only after a few runs makes me think that it is heat and doesn't have to do with the added weight. Is this the case, or could there be another factor I'm missing?

Re: Window Motor question
 

The motor has a PTC the stops the motor when it gets too hot.

Running a drum off the motor will cause a higher torque needed to move the load as the drum gets larger.

I think a second motor and some fans might get you by the match. If you are drawing 10-12 amps a second motor would drop you near or below peak efficiency for each individual motors. Coupled by fan or two and you may find that it runs well.

Re: Window Motor question
 

Sorry for the poor wording of that question the first time, What I mean to ask is: a second motor wouldn't decrease the duty cycle, and I wonder if the difference in load would be enough to decrease the heat generated. From previous discussion the 7-8 amps isn't an enormous load indication, therefore even half that with the long duty cycles might not make much of a difference?

Re: Window Motor question
 

The torque is the force x distance So 8 lb x 6 inches = 48 inlb
If you take the Banebots 550 motor and look to adapt it to the window motor gearbox I think you will solve your problem. FIRST take the window motor off--by removing 3 screws. On the end of the window motor is a plastic driver--slowly remove it.Look at the window motor shaft, there is 2 flats back to back.
Now take the 550 motor and mask/tape up all the openings. With a belt sander carefully sand the 550 shaft to match the window motor shaft. Note you will have to shorten the shaft slightly. Next make an adapter plate out of 1/8 alum bar stk. The banebots will bolt to the adapter plate and then bolt the adapter plate to the window motor gearbox. Worth a try?

Re: Window Motor question
 

The heat generated in total would be cut down as you are running the motors more efficiently. With two motors ( and based on your current measurements) you'd be running a at close to 80% efficiency, with a single motor you'd be running closer to 60% efficiency. You'd also have twice the surface area to dissipate less than half the excess heat.

I don't have too much experience running these window motors to know under what loads the PTC likes to trip.

Edit: I've been reading the RPMs as %efficiency... so my figures for efficiency are off...

Re: Window Motor question
 

I wonder if that counts as modifying the window motor, and is therefore against the rules?

Re: Window Motor question
 

I think that depends if there is a vendor for those window motors. If there is you can make modifications to the gearbox of the window motor. Otherwise...

Re: Window Motor question
 

They are from Denso, and are OEM, and you cant get any availible. So I guess not?

Re: Window Motor question
 

Since it's stalled it would be running at exactly 0% efficiency.

Re: Window Motor question
 

I don't understand how this works, but a 20% increase in efficiency, what does that gain us in terms of performance? Say now we can raise it twice, and it fails half way up the third time. Would this give us a real boost in a match? Would it complete a third trip up? Four?

Re: Window Motor question
 

I apologize, but I was reading the RPMS as the efficiency...

But this would definitely give you a boost. It should keep the PTC from tripping as often as it does now. By adding a second motor it should at least operate twice as long before the PTCs trip. Instead of 12 amps across 1 motor, you'd have 12 amps total across two motors or 6 amps each.

However, I would expect even better, you will dissipate the same amount of heat faster with twice the surface area.

Perhaps someone who has used these motors can comment on where the Denso window motor can operate continuously?

Re: Window Motor question
 

Ah, explains some of my confusion ;)

Anyhow, if one motor is slightly faster than another, does that nullify a large portion of the problems, or even make them worse for the faster motor?

Re: Window Motor question
 

Neither, The faster motor would simply slow down, if it is running in tandem with the other motor their speeds would have to be the same.

Re: Window Motor question
 

So, we did some math, correct me if any of this is wrong:

Our cylinders are 4 in diameter, so piD=4pi=~12

Therefore, the "gear ratio" of the twine winding is 12:1.

A Window Motor, at stall torque is 1501 oz/in. Change that to 125, since we are "gearing" it up by 12.

Now we are at only 125 oz of torque. That is roughly 8 pounds. Consider the grabber weighs six, and the other stage 4 lbs. Thats 10, and that is why our motor is stalling.

If we are running at 10 amps, this is about halved.

Like I said, correct me if any of this math is incorrect.

With this info, we now know that two motors would provide roughly double the torque, bringing us to 20 lbs, easily enough. If only we had done this math earlier :(

On the other hand, we still have the question of why is the motor running at 10 amps, and stalling, when it shouldn't stall until later. If we put two motors on, will they just stall at 10, and we only get 10 lbs of lifting force?

Re: Window Motor question
 

There is no gear ratio involved in the drum. Simply a smaller drum will apply more force and a larger drum less force.

Your torque is in oz-inches. Now correct me if my math is wrong...

If your arm is 10 lbs then for torque you need 10*16 160 ozs. across a 4 inch drum it would be 160*4 = 640 oz-inch.

Re: Window Motor question
 

I think it would be 160*12, since you want the circumference not the diameter.

Re: Window Motor question
 

You want the radius because of the lever arm.

And to your earlier question about the stalling motor.

Your current and weight numbers match the torque/ force output of the curves. Are you sure the Thermal breaker inside the motor isn't being tripped which would cause your arm to stop moving.


Edit: So with a 2 inch drum. You're motors are actually producing 320 oz-inches of torque to pull a 10 lb load.

Re: Window Motor question
 

Wait, im not sure either of us are understanding each other:

For each rotation of the winch, the grabber rises 12 inches on the ladder. When it reaches the next stage, it pulls that up with it.


We are not using it as a lever, as in a traditional arm, but in a system of pullies, which eventually makes it raise up.

I drew you a picture! Excuse the poor visual.


Consider something with a 1 in shaft pulling in a 10 lb weight. It needs to turn 12 times to get a foot. Every turn has an amount of torque.

Consider the same motor with a 12 in shaft, pulling the same weight. It now needs to turn once, and that one turn has 1/12 the torque.

Re: Window Motor question
 

The amount of force applied to your ladder/winch system is dependent on the size of the drum. A larger drum will pull the ladder up quicker but require more force. A smaller drum will require less force but it will be slower.

Torque is a measurement that includes Force and a distance. If I am spinning the drum at 320 oz-inch it will take 320 oz at 1 inch to stop the drum from rotating or 160 oz at 2 inches since it is only applying a force of 160 ozs at 2 inches.

You're throwing pi in there which is ok as long as you take it out again...

And I'm assuming there are no reductions in the pulleys.

Re: Window Motor question
 

I see!

I guess I wasn't properly going to torque, I was just going directly to how many pounds of force the motor needs to exert to pull our assembly up.

BTW, thanks for you help tonight, its really been...helpful.

You are correct, there are no reductions in the pulleys.

Re: Window Motor question
 

An additional thought here.
If you have surgical tubeing pulling your lift down... take it off, the motor will have to work harder and harder the farther your arm goes up. The much better way is to have the winch work as a capstan winch, where the cable only loops around the drum a few times, then attach the cable to the bottom of the lift, in that manner the winch pulls the lift up, then pulls it down if needed. In either case you have full winch power to pull with.

The quickest fix would be a smaller drum, that may mean making a whole new drum, but it would likely help you a lot, and be less weight.

For example, if you change from a 4" diameter drum to a 2" diameter drum you double your pulling power on your winch cable. with no additional weight!!!

you should be careful of minimum bend radii on your cable though.