electric wheel chair

i am making a electric wheel chair,i am using 24 volts dc motor,but i am unable to determine which parts to buy to drive motor, the motor specifications are 24 volts voltage and 8 amp current. but how much is the battery i have to buy,how can i control the speed of motor, which convertor should i buy to convert voltage drops if had a more voltage coming from baterry source?

You actually need to do quite a bit of research. Starting with what is the average current draw from the batteries under normal conditions. Then from that, you can calculate the size of the batteries needed based on how far you need it to go. For controlling the motor, you can use something like an Arduino that senses say a joystick position (lots of the chairs/scooters have steering as well using 2 drive motors). To control the power / speed to the motors, you would use PWM for best efficiency using a driver assembly between the Arduino and the motors. You also need to monitor battery voltage and warn the user when they are getting down as well as shutting them down if they get too low to prevent damage. Part of your trade study on battery capacity is physical size vs range. You could have two full size car batteries in there, but it is heavy and big (but should give good range). Smaller batteries will not give the range but will fit better. You also need to consider a charger for the batteries. My dad used to have an electric scooter with 2 - 12v 31AH that did fairly well (downside was the stupid charger would not start charging if the battery voltage was too low - if you ran them down too far or let it sit too long, the built in charger would not function). If you really need the distance/endurance, lithium batteries would be better, but expensive and you have to be very careful charging them if you want to avoid NASTY fires !!.

Other option that I saw last year - neighbor lady has CP so she has one of those electric scooters, but she had a little trailer behind it with a honda generator on it - her range was limited by how much fuel it had!! (she was zooming around the neighborhood - had pretty good speed too!!)

8 amps of load and say 8 hours of use comes to 64AH. But a wheelchair generally never runs at 100% load for 100% of the time, say is only runs for 20% on full load so AH is now 64 x 20/100 = 12.8AH If you are using a lead-acid battery you should never discharge it greater than 50% DOD (depth of discharge) so at worst you might get by using a battery rated at only 12.8/0.5 = 25.6AH. Preferably your battery should be rated greater than this, say 36AH Since lead-acid batteries are generally 12 volt, you will need 2 of these connected in series, both rated at say 36AH to produce the desired 24 volts.

You need a PWM type controller for speed control and if the unit is properly designed it will vary "apparent" motor voltage from 0 to 24v.

If you use PWM you can "over-volt" the motor. For example, say you are using 4 batteries to produce 48 volts, you can set your PWM to limit at 50% so the motor never "sees" an effective voltage greater than 24 volts. Again it's a case of designing the PWM to accept a supply voltage higher than the motor rating and setting the PWM range to suit the motor voltage.

To steer the chair you ideally need 2 drive motors, one to each main wheel. Steering is then accomplished by variation of speed between the two motors. So a 2-channel PWM with a slight degree of differential control to accomplish steering

You will also get comment that wheelchair design is a complex matter and shouldn't be attempted by an amateur but I appreciate it may be a case of needs-must.

Edit : Previous post came in as I was composing ! With respect to lithium batteries, if you go this route go for lithium iron phosphate type LiFePo4, they are much safer and not prone to combustion.

dhiraj421: i am making a electric wheel chair,i am using 24 volts dc motor,but i am unable to determine which parts to buy to drive motor, the motor specifications are 24 volts voltage and 8 amp current. but how much is the battery i have to buy,how can i control the speed of motor, which convertor should i buy to convert voltage drops if had a more voltage coming from baterry source?

First thing to measure is the stall-current of the motor (or equivalently the winding resistance between the terminals). Any driver circuit has to be able to tolerate that as a peak rating, and its probably at least 100A for a motor of that size and class (assuming 8A is a continuous rating).

Then you have to consider safety - you must be able to stop the thing, which means either using a certified controller designed for wheelchairs / mobility scooters (the requirements on these are stringent), or fit an emergency-stop contactor for the power. The contactor must survive stall currents too of course, you don't want to find its contacts have welded shut in an emergency situation.

So if you thought you were going to "cook up" your own controller, best forget it!

Hello! Depending on what you are trying to do exactly, you'll probably find this to be a very interesting project! Locally, I assist a friend of mine in electric wheelchair and scooter repair (we take in donated chairs, repair or modify them to people's needs, then re-donate them out to those who can't afford them otherwise). As such, I know a bit about such real chairs. So I think I can help you a bit. See my responses below:

dhiraj421: i am making a electric wheel chair,i am using 24 volts dc motor,but i am unable to determine which parts to buy to drive motor,

This can actually vary a lot, but will ultimately depend on the actual specifications of your motor(s).

dhiraj421: the motor specifications are 24 volts voltage and 8 amp current.

Where did you get this information? What motor(s) are you using? Can you link us to these specifications? The voltage sounds fairly standard (most electric chairs and scooters use 24 volts DC), but the current is at best ambiguous: The current value you want to know most is the stall current. This is because a motor, at rest (that is, stopped) is "stalled". When a motor starts up from this "stalled" position, there is usually a huge demand for current by the motor. To me, for a motor on a wheelchair, 8 amps seems low (I would expect a much higher value). But I won't believe anything until I see the actual specs (or you take some measurements in lieu of those specs). Regardless, you need this information FIRST.

dhiraj421: but how much is the battery i have to buy,

Most electric wheelchairs (those that use two motors and differential/skid-steer directional control) use two 12 volt DC, 31-35 Ah (amp-hour) batteries wired in series - that is, a pair of these:

https://www.batterystuff.com/batteries/rv-marine/agm/8AU1H.html

As you can tell, they aren't cheap - nor are they lightweight.

If you are in the USA, Harbor Freight sells similar batteries for their solar power systems; we have tried these, and they work ok for a while, but after a few months they die. With a coupon, though, you can pick them up for around $65.00 USD each (so you get what you pay for).

Anyhow - these are wired in series, and then a large fuse is placed inline on the positive cable within 6 inches of the positive terminal. This is a must in any chair design, because if a short happens, or something similar, these batteries can easily start a fire, explode, both, etc (we've played around with welding using a few of these batteries in parallel - fairly easy to do, but not advised).

So - make sure you fuse you main power line before your controller - the fuse should be rated to the maximum current you will expect to see from the motors, controller, auxiliary devices, etc - under normal usage. Use a slo-blow fuse (if you can get one in that size).

dhiraj421: how can i control the speed of motor,

If you are building your own chair from mostly scratch, then I would stay away from standard wheelchair controllers (that is, controllers off other chairs). Why? First off, they are highly proprietary. Secondly, they are basically "impossible" (I'm sure a dedicated hacker could figure it out, though) to program and configure on your own without using the proper hand-held programming terminal for that particular controller.

Such terminals are only available for sale to authorized repair centers for these chairs (and are extremely expensive brand new). Sometimes you can find such programming terminals on Ebay - but they will still be expensive (but much less comparatively to the new price!). That said, unless you get the exact terminal for the controller at hand, it may or may not work for that controller. In some cases, you'll only be able to program a subset of functions, in other cases you'll just get an error condition or nothing - it just won't work to program the controller. When you can program the controller, though, you can set all kinds of various parameters for control of the motors, etc (acceleration and deceleration curves, min/max current settings, min/max speeds, etc).

The reason for this level of secrecy and proprietary-ness is due to liability: If users were able to easily change the settings of their chair to make it run "high speed" with "small inputs" - the chairs can easily go out of control and crash (ask me how I know this information - yes, you can do donuts and "burn outs" with a chair, properly programmed). They instead certify and sell them as "medical devices" so they restrict access to this information and technology (well, here in the United States).

Even if you opened up the controllers, you would find them heavily potted in conformal coating. In some cases, the chips and transistors won't even have ID numbers on them, or they will all be "house numbers". That said, I have seen in the joystick controller units of some chairs (those where the joystick was separate from the motor controller itself) that ATMega uC's were being used (I'm sure all kinds of different microcontrollers are used, actually).

So they are trying to keep people from hacking their chairs - as well as keep their controller designs secret from competitors. Generally, the controllers are not made by the manufacturer of the chair, either - but by a third party that specializes in these medical-grade motor controllers.

Part 2:

What we have used in the past when we create robots from chair parts are Vantec motor controllers:

https://www.vantec.com/acatalog/spdcat.html

As you can see, these controllers aren't cheap (we generally use one of the RDFR21 or 22 series) - but they are easy to hook up and interface with (they just need servo signals) - one thing we have found, though, is that the older models don't like the signals as-is from the Arduino Servo library (the pulse width isn't right), but it can be fixed in code, so it isn't a big deal.

There are also controllers by a company called Roboteq that I have heard are good controllers as well - but they aren't any cheaper.

I'm actually planning on trying to control some wheelchair motors using cheap BTS7960 h-bridges off ebay. If you are planning on building your own controller, I would start with this (again, though, you need to know what your stall current value is - don't just buy some of these without getting that info, first).

Beyond this, the only way you can keep things cheap would be to use a reversing 24VDC contactor (which aren't easy to find - and aren't very cheap when you can find them, either - not like similar 12 VDC contactors, which are usually used for off-road winches), and PWM the speed of the motor(s) on the low-side with logic-level n-channel mosfets (multiples in parallel for about 100 amps or so, most likely).

As you might be able to tell - there's a good reason why the Vantec and Roboteq controllers are so darn expensive - designing and building such controllers isn't easy (and if you attempt to go at it yourself - there's a good likelyhood that you will end up spending more blowing mosfets than you would had you bought a pre-built controller).

dhiraj421: which convertor should i buy to convert voltage drops if had a more voltage coming from baterry source?

If you mean something to drive the controller electronics (the Arduino, the logic side of the h-bridge or controller, etc) - then look for a DC-to-DC switch-mode converter that can take 24 volts as an input, and convert it to the voltage you need (5 - 12 volts). Also - don't be tempted to just tie into one of the 12 volt levels direct from the battery; while this will work, it can result in uneven battery levels (depending on length and level of current draw), which can lead to charge/discharge issues (where one battery is trying to "recharge" the other). You might also experience motor noise leading to resets of the electronics being powered.

What you might do is get a 24 to 12 volt DC-DC converter, and run that off the batteries. Then, put a fuse on the output side, after that fuse tie a 12 to 5 volt DC-DC converter. Put a fuse on the output side of that as well. Run the electronics off the 12 and 5 volt outputs (make sure the converters can put out decent current levels for your needs). The "dual tap" and fuses will be useful and fairly safe.

Lastly: As you are developing your chair, and doing programming on it, put the frame/motors/wheels up "on blocks" - don't try to develop with the wheels on the ground. If you don't do this, and your code fails (or something else fails) - you could easily have a chair that weighs a few hundred pounds go out of control. This is not something you want to happen, because such a chair can cause serious damage and injury. Don't put the wheels on the ground until you are fairly certain that doing so won't be an issue.

In fact - if you can - buy or build a small table-top platform (with wheels, motors, small h-bridges, etc) - to develop your controller code first. If it goes out of control, at worst it will fall off the desk and break, but it likely won't hurt you in a grave manner.

Finally - make sure as you develop your code that you design in - from the beginning - security features to monitor the system and shut down for any particular reason. In short, you want to develop a "fail safe" system, so that should anything fail (overcurrent, lack of brakes, movement when there isn't any inputs, movement without a person in the seat, etc) - all current is shut off from getting to the motors. Commercial chairs do this by monitoring inputs, outputs, voltage and current levels, etc. There's also built in mechanical and electromechanical braking systems involved, too. However - there isn't any form of "cutoff contactor" used generally - I've never seen one on any of the many chairs we have messed with. That said, if you feel that would be the safest way to cut the power, then do so - make it so that as long as power is available, it works, but if the power fails, or the microcontroller tells it to "shut off", the contactor disengages and removes main power to the chair and electronics entirely.

I hope this helps - and good luck with your project. Oh - and again - find out that amperage for the motor(s) - post it here, too!

What part of the world are you in? The legal requirements for the use of electric wheelchairs, mobility scooters and the like vary from country to country.

Russell.