Building of the Mountain Wheelchair has Commenced

At long last, I finally decided to go ahead and start building the mountain wheelchair frame.

For a long time I’ve been using a computer to create simulations of the wheelchair overcoming obstacles and different types of terrain. Every time I do, I notice something new which is going to create a problem. For this reason, I’ve been extremely reluctant to actually start the build process (in case I decide to revise the design).

Something changed the other day, I guess I felt like I was wasting time and it made more sense to get the thing built and see what happens. So with much trepidation, I cut into the forks:

Having finally cut them, this spurred on a wave of activity and by the end of the day I had built something that was starting to look like a Mountain Wheelchair Rocker Bogie:

Wow! It’s actually coming together! And what’s more, I’m really pleased with how straight everything is and how well all of the angles have turned out. I don’t know what’s going to happen once it’s under load, but so far building the mountain wheelchair is going according to plan.

All Six Motors for the Mountain Wheelchair are Working

There were a couple of tasks that had been causing me a bit of a mental block and preventing progress with the mountain wheelchair so it’s really pleasing to have them ‘done and dusted’ so that I can get back to it.

One task was to disassemble all of the wheels, make some modifications and then reassemble them all. Having done this, all of the spokes in the wheel rims are sitting far better than they were before.

The other task was to change and re-wire the connectors on all of the controllers and motors. Having done this, I could finally go ahead and get all six motors wired up:

As Ada said in the video, there are many things which need doing in terms of wiring for the wheelchair, but it is an enormous relief to see all six motors turning and relieve myself of the concerns I’d been having. It shouldn’t be long now before I start building the rocker-bogie for the wheelchair frame…

Working Radio Controls for the Mountain Wheelchair

At long last I managed to get the radio controls for the Mountain Wheelchair to work!

It’s taken so much and time and effort to get this to work. Not wanting to get too technical, I basically needed a way to convert an RC Receiver 1-2milisecond pulse to an analogue 0-5volts. I tried all sorts! In the end I found a very elegant, simple and cost effective solution from an American company called Astro Flight. There were some issues with the shipping but once the item arrived it was a very simple plug-and-play and at last, the mountain wheelchair has a working radio control system.

As I’ve said before, it was extremely important to get this working because it will allow me to drive the wheelchair into the mountains without needing to be concerned for the safety of the driver.

The Mountain Wheelchair Braking System

Most of my time of late has been spent working on the electronics for the mountain wheelchair and more recently building the wheels, however, I have, for some time now, been thinking about the brakes…

When testing the smaller prototypes for the mountain wheelchair, I discovered that:

“When [the mountain wheelchair is] overcoming obstacles such as large steps, the front wheels are pushed into the step and with the traction created with all six wheels, the front wheels ‘drive’ up the step. The problem is that the front wheels don’t always get traction [and sometimes just wheel-spin, or even worse, get stuck]”.

To prevent this from happening, I had been thinking about using hydraulics or linear actuators to lift the front wheels off the ground before approaching the obstacle. A far simpler method is just to use the motors and brakes which are already on the wheelchair therefore requiring very few additional parts and reducing the overall weight of the wheelchair.

To demonstrate this; if you were to apply power to the front and middle wheels, whilst at the same time applying the brakes to the rear wheels, then the front wheels would lift off the ground resulting in an increased possibility to overcome large steps. Like so:

This seems incredibly easy to achieve from an engineering point of view, the problem is making it easy for the driver of the wheelchair to operate. If you only had rear brakes, then the wheelchair would skid under braking, so you need brakes on all wheels.

As I’m planning to use hydraulic disc brakes off a mountain bike, this does present some problems. These brakes usually have one brake lever for each brake caliper. For a six-wheel-drive wheelchair requiring six brakes, having six brake levers, I think, would be difficult to operate.

I have however found this hydraulic hose splitter made by Hope Tech. If I were to use these, then I could theoretically operate three brakes from one lever (one lever for the left, and one for the right). This way you could skid-steer the wheelchair down a mountain just by operating the left or right brakes.

If you have one lever to operate the three brakes on one side of the wheelchair, and another lever for the three brakes on the opposite side, how then do you apply the brakes to the rear wheels only? With a solenoid valve on the hydraulic lines which feed the front brakes perhaps?

At the flick of a switch, maybe a thumb operated switch on the top of the joystick, a solenoid valve would close, thus prevent the hydraulic fluid from reaching the front brakes. To help illustrate this, I made this (very) rough sketch:

As you can see in the sketch, I plan to have the brake lever upside down and mounted to the joystick levers (You can see this more clearly here). For testing purposes, I made this:

As an aside, the picture above shows a disc brake on one of the motors. This is a very cheap disc brake that I found in the back of my shed, it’s about 20 years old and the pads are worn down. It does at least allow me to do some testing; if I squeeze the brake lever as hard as I can, the motor still has sufficient torque to keep turning. I’ve run the motor like this for five minutes trying to get it to overheat in order to test the thermal protection mechanism which I installed. After five minutes the disc brake was almost glowing hot and smoking, but the motor showed no signs of warming up. A positive outcome.

Anyway, back to the topic at hand…

Hydraulic brakes on a mountain bike have a reservoir built into the lever to hold the hydraulic fluid. When you squeeze the brake lever, the pressure moves the fluid from the reservoir to the brake calipers, thus forcing them to close. This is great, except that the levers are designed to operate one caliper only, not three. Actually, and this is where the gaps in my experience of engineering start to appear again, I’m not sure that it’s the reservoir that causes the problem, but rather the levers don’t create enough pressure to operate multiple calipers?

I spoke to Hope Tech and they’ve reassured me that their “Tech 3 Lever” will operate two of their “X2 calipers”, but not three. This is great for two reasons..

First of all, they both come in purple, so this will make Ada happy:

Perhaps more importantly though, I think it might be better not to have brakes on the front wheels and I’ve made this video to help illustrate why. The video represents the wheelchair rolling down a hill and then applying the front brakes. As you watch the video, imagine what would happen next.

Just like on a mountain bike, it would want to “throw you over the handle bars”. On the mountain wheelchair this effect is amplified because of the rocker-bogie suspension mechanism. In the video, it’s easy to imagine how the centre of gravity moves to the front of the wheelchair and would result in quite a nasty accident with the full weight of the wheelchair, its heavy motors, batteries and frame coming down on top of you.

Ultimately, I think this is going to be a trial and error thing during which time there are some other ideas which are worth exploring.

It would still be possible to have brakes on all six wheels, it would just have to be setup so that there wasn’t much braking force at the front wheels.

As well as this, the motors in the wheels have some resistance anyway; they act as generators which recharge the batteries. I plan to have a “knob” near the joysticks which lets the rider adjust how much regeneration the motors are doing. In effect, this  dial would adjust the rolling resistance of the wheels. In this regard, perhaps brakes aren’t needed on the front wheels?

Perhaps the best solution though would be to replicate what happens on a bicycle. i.e. have one lever for the front brakes and one for the rear. This would give the rider complete control of the braking balance between front and rear wheels. In fact, this would then remove the need for the solenoid valve. To lift the front wheels off the ground, you would just have to squeeze the left brake lever (which would operate the rear brakes) and drive forwards. Far simpler than having to operate switches as well as brake and drive at the same time and for people who previously were passionate cyclists, this setup might be more familiar to them. The only problem with this is that it removes the ability to steer the wheelchair by operating the brakes. As I say, I think this is largely going to be trial and error.

Mountain Wheelchair Bogie Mockup

It’s taken me the best part of the day to filing away so that the motors fit onto the forks, but it’s done, and all six motors are now sitting in the forks. Shoulder aching, I decided to push on and managed to get another wheel laced, then much to Ada’s delight, mocked up part of the frame (a bogie) for the actual Mountain Wheelchair!

The Mountain Wheelchair has its first Wheel

Woohoo! It’s been a busy day in the shop. The spokes FINALLY arrived from China today and I managed to get the first wheel made.

It’s a motorised mountain wheelchair wheel!

Most of the the time was spent with a drill and file trying to get the separate parts to fit together, so although it’s a little bodged at the moment, it’s taken such a long time to get to this point that it feels like an important milestone.

I got Ada to control the throttle for a moment whilst I let the wheel roll on the floor. It’s much faster than it needs to be, but I think that will add to the fun of driving the wheelchair once it’s finished.

As well as the spokes, the “C Washers” which I had laser cut arrived today too and the guys at Microkerf did an excellent job. They’ve been extremely helpful and I’d definitely recommend them:

As I said in a previous post, I can’t be the only person who needs these washers so have listed them on eBay to try and raise some funds for the wheelchair.

Also currently on its way from the USA is a part which I hope will enable me to make the mountain wheelchair radio controlled – useful for testing purposes and driving it on/off trailers etc.

It’s also worth mentioning that Google doesn’t like us at the moment and we’re currently on page 3 for the search terms “mountain wheelchair”, however, on Bing at least, we are the very first result at the top of page 1.

All in all, a successful day! :D

Wiring for the Mountain Wheelchair is Coming Along

Some more parts arrived from China today which have enabled me to make some progress with the wiring for the mountain wheelchair:

Put Simply

In simple terms, these components add some additional protection into the circuitry and much to Ada’s delight, the wheelchair now has a key ignition switch.

Detailed Version

In more detail, each controller should draw a maximum of 15 amps. With 6 controllers, that’s a peak of 90a (15 * 6). So the first component on the positive wire is a heavy duty 200a fuse.

The next component is the big red emergency stop key switch. This acts as a master switch and completely disconnects the battery.

Next is a keyed ignition switch which goes to the power switch on the controller. Although this is still operating at 48v, it only draws a maximum of 40ma so it is much easier to route cable so that the key is somewhere accessible. This also has an inline 2a fuse.

After this is a 200a main contactor. The key switch closes the contactor at which point the large capacitors in the controller draw a substantial amount of current. To prevent the contactor from welding closed as a result of arcing, there is a 1k Ohm 10w resistor across the main contacts. Because this resistor is in place, it meant that the controller was always connected to the battery (regardless of whether the contactor was closed or not), thus draining the battery. Hence the need for the big red eliminator.

The plan is to have one key ignition and one contactor to supply power to all six controllers.

Although not shown here, I’ve also successfully wired a thermistor inside the motor housing. The manufacturers have suggested a theoretical max operating temp of 120℃ for the motor. To allow some wiggle room, the controller will cut power to the motors if they reach 90℃.

What’s Next?

The next job in terms of wiring is to include a watt meter. To this end, I’ve purchased a really cool Eagle Tree E-Logger. Not only does it monitor and record power consumption, temperature, speed and altitude, but also plots all of this into graphs and with an additional GPS unit, can display all of this data alongside a satellite map of the route you’ve taken. This sounds extremely useful because it will allow me to see where spikes in temperature and power consumption have occurred, as well as providing the tools I need to make far more accurate estimations of battery capacity needs. Although it’s intended for radio controlled aeroplanes, it’s good for up to 80v and 150a so should be perfect for the mountain wheelchair.

First Outsourced Parts for the Mountain Wheelchair Arrived

I finally bit the bullet last week and outsourced the manufacturing of parts for the first time.

The mountain wheelchair frame is in part being made using BMX bicycle forks. The parts at the end of the forks, where the motor axles sit, are called dropouts. Looking at the image below, you can see that there is a circular indentation in the dropout where the motor axle sits.

This indentation is intended for quick release wheels. If the cyclist doesn’t tighten up their quick release wheels properly, then this raised edge will help to prevent the wheel from falling off. The common term given to the raised edge is a “Lawyer’s Lip”. I believe this is because they were added for legal reasons.

For the mountain wheelchair a quick release mechanism isn’t needed. In fact, it creates a problem; because the axle isn’t perfectly centred in the dropout, this means I don’t have a washer to fit, and therefore the nuts which hold the wheel in place will only press against the raised edge. The front forks on BMX bikes were designed for any amount of torque; they just hold the front wheel in place whilst all the drive comes from the back wheel. Considering the torque produced by the wheelchair motors, this becomes quite a large problem. If the nuts holding the wheel in place only press against this raised edge, then it’s quite likely that the wheels would fall off whilst driving the wheelchair in the mountains.

To help prevent this from happening, I decided to design a washer which would fit the void where the quick release mechanism is supposed to sit. To begin with, I 3D printed the part to ensure a good fit:

Safe in the knowledge that my measurements were correct, I started looking for quotes to laser cut the washers from stainless steel. The first quote for 20 washers was £150! Eek! That’s a lot of money just for a few washers.

The following day, another quote arrived for £80. Looking through the pricing structure, I realised that the £80 was a minimum fee, and that my parts were actually only costing £12. After discussing this with them, I discovered that I can order 150 washers for the same price as 20, so this is what I decided to do. Hopefully I can sell these on eBay and recover the costs. After all, I can’t be the only person having to overcome this problem.

The company have sent me a sample before placing the order and they seem perfect:

As I say, this is important for the mountain wheelchair because it will go some way to preventing the wheels from falling off, and it’s my first experience of outsourcing some of the work.

As said though, the forks aren’t designed for high torque motors, so whilst this will help to stop the wheels from slipping off, it does nothing to prevent the forks from snapping. I do have an idea for this, but this will have to wait for the moment as I have plenty of other things to be getting on with.

This page was last updated on April 18th, 2018 by .
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