Zinger power add-on

[JoeC]

Just saw this video- http://www.youtube.com/watch?v=5MtzU4ZAd_o

I don't have a lot of details, other than that it's a small friction drive wheel pushing the wheelchair wheel, and a lithium battery pack.

Thoughts?

[JoeC]

I just saw the website for the product- http://www.patentproducts.org/Site_PPC/Inventions/Entries/2008/1/15_Motochair_powered_wheelchair.html

Apparently it's been in its present state for a couple of years now, so I have some doubts about if it will go anywhere.

[Burgerman]

as power add ons go its not bad. But I never understand the point of trying to motorise a manual wheelchair. It is still impossible to carry a bowl of soup, as you need both hands. Pulling levers to press motors onto tyres means accuracy of control is difficult. Coordinating a throttle and two levers is a pain. And you still have tiny casters, skinny wheels and all the discomfort you get in a manual powerchair.

Use the same lithium batteries and brushless motors and build proper better lighter powerchairs instead. Then theres real benefits.

[JoeC]

An ultra-light power wheelchair would be useful for us- it doesn't have to be a converted manual chair, but if the whole thing could weigh about 40 pounds with batteries, that would be great. Even two easily separated 40 pound chunks that were compact enough to easily fit in the trunk of a car would be a big help.

We're about to fly away to a wedding in the northern midwest (great lakes area), and we're taking the manual chair instead of the power chair. The church has a flight of stairs and no wheelchair access, and nobody in the area has a wheelchair accessible van. It won't surprise me if the bars and restaurants we go to are also similarly lacking in access, so I'll be carrying my wife up and down flights of stairs while somebody else drags the manual chair along. We could bring the P200, but as my wife says- it's too much trouble. I offer, and don't mind the five minutes of assembly and disassembly every time we need to get into a car or through a non-accessible area, but it's a lot of trouble. You know all the rest of the risks of flying with a powerchair.

It would be NICE to have a way to giver her self-propulsion in the kind of places our families live. I guess that's another project that I'll just have to do myself...

[JoeC]

Maybe some continued development on something like this would help us get by with the tiny brushless outrunners without burning them up on ramps: http://www.rehab.research.va.gov/jour/85/22/3/pdf/reswick.pdf

[Burgerman]

Tiny outrunners or inrunners dont really get hot. 2HP motors of 28mm diameter and 40mm lomh are sat on my bench. A cheap pair of chinese lipos is 10Ah and 22.2v nominal... You could build your own with a joystick using a small 50 amp brushless controller and a cheap servo tester to drive it. Actually you would need 3 of them, and 2 speed controllers. The whole lot would be about 60 dollars. You need bigger low rpm per k motors obviously. Possibly only 2 cell batteries for 8.4v to keep current down. Cheap in line epicyclic gearboxes are available too. Have a look at the better motors here at HET http://www.highendrc.com/

http://cgi.ebay.com/RC-Model-480-Outrun ... 3caf6ca4ce COUPLE OF THESE WOULD BE OVERKILL!

2 OF THESE connected to a joystick would give you control. Just connect to where the rotary pot is.

http://cgi.ebay.com/Turnigy-760LV-Dual- ... 2eb0524493

[Burgerman]

There you go! Project! Knurled wheel against tyre. Sprung loaded and releasable. About 3 lb in weight inc lithium battery.

Added... You cant use those speed controllers no reverse! You need boat or car ones. But you get the idea.

[JoeC]

I'm really not crazy about the knurled wheel against a tire, but it DOES get around finding a 60:1 gearbox...

[JoeC]

How do the controllers and motors react to sitting there for multiple seconds at the full rated current and zero (or near zero) speed?

[Burgerman]

Dunno. They will buzz for a second while the controller figures out the timing, then go. Some of the better ones are programmable and allow current limits, ramp time adjustment and braking control etc. You will need to research controllers. Some car ones are sensor type and have matching controllers. If you want reverse then you need a boat or car controller anyway. But sont wotty about heat. But you might worry about too many amps. Hence the low gearing and low rpm motor and 2 cells.

I think its a case of testing and watching amps so the controller and batteries do not get exeeded. A max amp setting on the controller helps! Some dont have one... They have to be able to rotate a bit to work out timing. But with low gearing that should be ok if not too refined.

[Burgerman]

The sprung loaded contact will work like a clutch at ultra low revs where max torque is..

[JoeC]

I have had the chance to do just a very small amount of research so far, but it appears that a lot of the brushless ESCs have a default behavior of immediately shutting down if the current limit is reached. In airplanes it appears that the idea is to ensure that the prop and gears and batteries come together in such a way that the current limit is never reached. Maybe I need to read more manuals...

Right now I think that using parts from cordless drills would be a good start. They have an adjustable clutch built in, and a gearbox that normally accepts an input in the 20,000 rpm range. The mechanical clutch is good because a friction wheel slipping on a tire is somewhat unpredictable. A more efficient brushless motor attached to the input might be just the thing.

[Burgerman]

Dunno about that.I have a fast high reving brushless motor that is on a plane. Its doing 52 amps on the battery cable at 38,000 rpm. The speed controller is a cheap chinese 30 amp one and it gets a bit hot. In the air it revs much harder as the prop unloads due to speed. So draws less. Nothing shuts down? It just does as it says but gets hot. So hot that it unsoldered a connection on one occasion... That speed controller was seven uk pounds delivered... And about 10 grammes and 2 inches long, less than an inch wide, and 1/4 inch thick.

I think if you just use a big one and gear it with the drive on tyre directly it will be fine. Even the motors dont mind huge currents intermittently. Like when laid on the floor! Prop cant turn... But they never seem to be hurt. Dont think the size or weight or lack of efficiency is a problem. Anyway some speed controllers can be programmed with a max current limit. But a correctly sized motor will do that too. Remember KISS that is my mantra. Simple is better and more elegant! Too many amps? Less cells, lower rpm motor, lower resistance motor, programmable controller. Not enough torque? Do the opposite. Not enough speed? Taller gearing, more volts, higher rpm/v motor. Too much heat? bigger motor/controller/battery. Its just a balance.

[Burgerman]

http://hyperion-world.com/products/type/1

Scroll down to the bigger motors near the bottom. All with data and specs. And the same site has speed controllers and are max current limitable...

Same servo tester electronics will drive the speed controllers via a joystick. A couple of tiny 5 ah ebay lipo batteries will power a manual chair miles with your wife in it. Easy to organise. Hyperion stuff isnt cheap though but similar cloned stuff is, on ebay... But this will give you the tech specs.

[JoeC]

Just to get my friction wheels, manual chair, clamping, etc worked out, I should also be able to get a pair of Harbor Freight 18 volt cordless drills for $20 each, and run it off of one of the many working powerchair controllers I already have on the shelf, and power with a lithium pack. There are also issues of balance and stability (my wife never uses a manual chair unaided), so we might need to lower the chair a bit and add anti-tips to make her feel safe. This is starting to look like a very cheap and easy demo project, with a nice clear path to all-day shopping mall usability.

[JoeC]

The clear path to greater practicality would of course be the much higher efficiency brushless motors.

[Burgerman]

The only obvious problem with that that I can see will be too low gearing and it will sound like you have two kitchen fitters allong for the ride! I think the noise will stop your wife wanting to do this.

The outrunner low rpm direct drive will be almost silent. And easier to mount as well as cheap too if you use chinese ebay motors. The only problem is that you dont have a brushless sensorless controller.

[JoeC]

Might I modify an outrunner to add sensors? Would that make it easier to find a suitable controller? I have access to a CNC if it's just a matter of making a couple of precision spacers to orient sensors or magnets. I guess I have more research ahead of me... But I'm at a wedding for the weekend, so there won't be a lot of time for that!

[Burgerman]

http://www.bestrcdeals.com/category/Ele ... ors/4.html

Sensored ones are already available eg

[Burgerman]

The reason people use outrunners is cheapness and lightness for planes. The reason they use in runners is power potential and sometimes better high rpm efficiency. So I use them for those daft pylon racers.

Most sensored motors are rev monsters and for cars. So they have good starting torque and smoothness. You want the biggest lowest rpm sensored inrunner. And with the correct sized "wheel" to give say 3 mph roadspeed at half rpm. So if you need say 6ooo rpm for 3 mph then choose a motor that will work at 2 cells for eg and be at a 12k revs when looking at k rpm per v Or whatever the real rpm you need is. But sensored motors less cheap...

I would be tempted to try the cheap outrunner with cheap speed controllers and see what happens first...

[JoeC]

I know that I want the tangential speed of the friction wheel to be somewhere between 80 and 120 inches per second at full no-load speed, corresponding to a chair in the 5 to 7 mph range. I have read that the diameter of the friction wheel can have a very serious effect on the efficiency of that interface, so I would like to start with about two inches diameter as a ballpark guess on what's a compromise between excessive gear ratios and excessive losses at the friction wheel.

I'm going to say that I want each side to push with at the very least 50 pounds of tractive force- probably enough for most of what she'll encounter, and it sets the bar low enough to be achievable. With a two inch drive wheel, that means that I want a gearing and motor that will do about 1,000 rpms and provide at least 50 inch pounds of torque with some 'reasonable' current draw.

I'm still tempted by the cordless drills as a first step, just because they are so extraordinarily cheap and will let me figure out if the friction wheel is a big problem or not. I'm also starting more intense research on the brushless hobby motors, looking for something that can meet the performance goals. It looks like a Hyperion charger may be in my future for various reasons- both for managing Lipos and for getting a decent estimate of energy used for driving a given path.

[Burgerman]

I think 50 lb may be a bit much as it would be able to propel her vertically straight up a wall!

I would have aimed at 1/4 to 1/3rd of that maximum. Remember that slope diagram? A 30 degree slope is more than double my ramp. That requires only a 25lb per side actual push...

I also think after years of hands on engineering that even 1 inch diameter should not be a problem. Agreed its less efficient. But with high tyre pressures and a nice "coated" grippy wheel (say fine grit and epoxy as used on brake testing drums or even heavy diamond knurling) it shouldnt be an issue with a lower torque level?

And that means many more cheap motors and batteries become practical.

[JoeC]

Yes, fifty pounds does sound like a lot- but I had it in mind more as a single-wheel figure for climbing over obstacles on just one side, or turning around on a slope, based on my experience of pushing her around in the chair. It's not the ramps that cause me to exert high forces, but the turns in place on them and going over bumps. I think I said already that she'll generally not be out alone with this chair, so torque is definitely something that we can compromise on- I will be there to give a push if she can't get over something. I'll look for a motor that can be geared for my speed and more like 25 inch pounds of torque. I like having more than I need so that I can avoid running into the ESC's current limit, and so that I have some flexibility in friction wheel and gearing as I learn more.

[Burgerman]

http://www.robotmarketplace.com/products/RL-SRVTST.html

Replace the potentiometer here with a cheap rc style joystick from this or similar sites and you have a device to control your cheap rc motor controllers.

[JoeC]

Thanks, this looks great!

But-sorry... I think I'm missing this- how do you achieve mixing for steering? Rotate the joystick 45 degrees? Is there another circuit that takes the forward/reverse and left/right voltages and mixes them to a proper output for the two independent single channel ESCs?

[Burgerman]

http://www.robotmarketplace.com/products/RL-IMX1.html

This will do it but its ott. There are much cheaper and smaller "elevon" mixers or V tail mixers on ebay... They all do the same thing. The one on the robitics site is reversable but your wife wont be running upside down hopefully!

[Burgerman]

http://cgi.ebay.com/Hobbywing-V-tail-Mi ... 3efb45a914

Cheap tiny single chip version.

There are lots. You realise you can fit 2 gyros too? one for direction, one experimental for you... Wheelies! I already tries that. Rate gyros work better than heading lock usually here... They are also stupidly cheap...

[JoeC]

Thanks! I'm still getting my bearings in the hobby RC world, so your help is really appreciated. I think I may just need to build a plane or helicopter to improve my learning before I put it into a chair!

[Burgerman]

When I say gyros are cheap I mean cheap ones are cheap! From a few dollars. Good rock solid reliable programable low drift switchable heading lock and rate gyros are stupidly expensive... Or can be,.

[JoeC]

She's even more anti-wheelie than before, since I was out driving her chair and landed a wheelie wrong and broke a caster fork. That was rather embatrassing- I had an accident in a wheelchair and it left me walking home!