SITWAY




You are never to old to learn and try new things. I think one of the best days in my life was the day I discovered the Instructables web site. It opened up a whole new world to me. This is my third instructable . I really enjoy building anything that I can ride on or get in to.
I bought an Arduino Uno and was planning on building a balancing Robot. I was really impressed with the Balancing Skate Board that was published by Xenon John. It had most of the code that I would need to build a balancing something. That something evolved from a Robot to a Sit Down Segway clone, which I named the SITWAY. I want to at this time thank John for all the help and patience he showed me in building and testing this ible.

This is my second project involving a discarded electric wheel chair. The motors have great torque and are very reliable. They use 24 volts and have great range using two U1 type garden tractor batteries. You can’t go any cheaper than that.

SITWAY




The build went pretty smoothly. Thankfully Xenon John pitched in and helped me modify his code to work with my wheel chair motors.
After running all the tests I felt were needed I elected to have a young neighbor take the first ride. It turned out to be a real blast. So far eight or ten people have ridden it, the youngest being 12, and the oldest 81 (me). The training wheels limit the speed by limiting the forward tilt. I plan to keep the rear training wheels on permanently because I don’t need a lot o speed going backwards.

The SITWAY appears to be pretty safe, but it does not have all the built in backup systems that a real Segway has, I have only tested it on my smooth driveway at this time. I have driven over small objects, and it still stayed stable..Any one can learn to drive it in about 5 or10 minutes. With all the testing and driving we have done I have yet had to charge the batteries. The original wheel chair had a published range of 20 miles.. HAVE FUN!!!

Step 1: MATERIALS AND COSTS

Item                                                                        Source                                                                                         Cost

1. Donor electric wheelchair                                  Various places                                                   $50 to $200 Depending on condition
2. Arduino Uno                                                      Maker Store                                                       $30
3. Arduino Uno Proto-shield                                  Spark Fun                                                         $15
4. 5 degrees of freedom  IMU                                Spark Fun                                                         $49
5. Sabertooth 2X25                                               Dimension Engineering                                     $129
6. Two surface mount LED’s                                  Radio Shack                                                     $1.29
7. Two Momentary contact switch’s  (trim)             Radio Shack                                                      $1.29
(normally open)
8. Two micro switches for Steering                        Radio Shack                                                      $3
(normally open)
9. Single pole single throw  power switch             Radio Shack                                                      $1
10 13X20X1/4″ plate for base                                Local                                                                 $10
11. 1/2″ steel tubing for seat frame                        Local                                                                 $10
12. Vinyl and foam for seat                                    Local                                                                 $4
13. Asst hook up wire                                             Local                                                                 $3
14. One can spray paint                                         Local                                                                 $3
Total                                                                                                                                                 $309 to $459

(note) I actually bought my used wheelchair for $35 at a yard sale
It was pretty beat up but the motors were good and even included
A 24 volt charger.My project cost less than $300.

Step 2: Salvaging parts from the donor wheelchair

Save the motors and drive wheels, they are usually one piece. Also salvage the connectors and wiring from the motors to the battery., leave the leads as long as possible. Keep the electronics if you want them for future use. Mine were trash. I never throw away wheels, they always come in handy. Most wheelchairs have two castoring wheels for steering, and two for stability when getting on the chair. Save the two stability wheels for training wheels on the project. Save the battery box and battery cover, you will use these. Dis-card the rest of the chair unless you think you will have use for it in the future. I threw most of it out to reduce clutter.

Step 3: Build the frame and mount the wheels and motors

I guess you could make this frame out of plywood, but I like to use steel. it’s a lot stronger and welding is a lot of fun. The base is a 1/2″ plate measuring 12X20″. The uprights and seat frame is made of 1″ steel tubing. Don’t forget the 45 deg. braces in the corners. the four holes in the seat braces are for mounting the plywood seat support. The holes for mounting the motors are slotted about 6″ long . At this point you do not exactly know where the C.G. will be. The motors can be adjusted fore and aft to adjust the C.G.The small wheels are used as training wheels and to keep the machine from falling over when not in use. Now is a good time to paint the frame You can make the frame any size you want. I designed this one to fit through an interior door. Cut the seat from 3/4″ plywood . Pad and upholster a seat cushion to be bolted to the frame uprights.

Step 4: STEERING CONTROLLER

I originally used John’s hand held steering controller, but found it was not practical for a sit down balancing machine. You need something  very rigid to hang onto when riding. The machine does a good job of balancing if you don’t fight it by trying  to balance it your self..
The stick is rigid in the fore and aft position , and will move side to side in the lateral position. a compression spring centers the stick.
Two set screws provide stops, and two more act as limit switches for the two micro switches that control the steering.
This could be done simpler by mounting the micro switched on the outside of the stick, but I wanted to have them hidden inside the stick.
I have access to a vertical mill and I never miss a chance to use it.  The micro switches are available from Radio Shack for 3 or 4 dollars.

The adjustable front training wheel serves two purposes. first it keeps you from pitching forward during any sudden stops, and secondly it limits the forward speed by limiting the pitch angle until you get comfortable with riding it. The rear wheel is fixed limiting reverse travel to a safe speed.

Step 5: ELECTRONICS

The electronics consist of the following
Arduino Uno
Shield
Sabertooth 2X25
5 Degree’s of freedom IMU
Asst. LED’s and switches
10K pull down resistors (5)
4 conductor cable and hook up wire

I don’t like to solder directly to my Arduino. Instead I used a shield. This allows me to make solid solder connections instead of plugs that can come loose due to handling or vibrations.

A good place to begin is to solder the (5) 10k resisters to the shield. These are the pull down resisters for the balance trim, steering, and dead man circuits.

(Note) The following wires are connected to the Arduino digital pins
pin 9 is for the dead man switch circuit
pin 7 is for nose down trim  circuit
pin 6 is for  nose up trim circuit
pin 5 is for steer left circuit
pin 4 is for steer right circuit
The other end of the resisters goes to circuit ground
pin 13 connects to the S1 input of the Sabertooth Motor Controller

The following wires are connected to the Arduino Analog pins
pin 0 to Y Rate 4.5 on the IMU
pin 2 to X Rate on the IMU
pin 3 to Y Rate on the IMU
pin 4 to ZACC un the IMU
+5 volts to he Steering controller
+3.3 volts to the IMU (NOTE) do NOT apply 5 volts to the IMU
GND to the IMU

All the Analog connections can be soldered directly to the shield.
The Digital connections can be made either by plugging directly into the headers or using a connecter. I found some in the Sparkfun catalog that fit snugly into the headers (See above pic)

I found some four conductor ribbon cable at Radio Shack that worked well for me. It is stiff enough to hold it’s shape, and the color coding makes life easier. You can use ribbon cable from an old computer just as well except for the color coding .

Mount the IMU to a small block either wood or phenolic to the floor of the machine at approximately the center line of the axles.
Be sure to mount it correctly.
The factory drilled mounting hole must be pointed towards the ground
The component side of the IMU must face forward
If you mount the IMU incorrectly bad things will happen.
The plate can be shimmed fore and aft to adjust for level balance

Fashion an aluminum plate to hold the power switch, and the trim and steer switches and mount it to be reached conveniently  while seated. I installed a power indicator LED (with a Pull down resister)  to verify Arduino power (12 volts)

Step 6: WIRING

Generally wheelchairs operate on 24 VDC.. Main power is obtained from two U1 type lead acid batteries. They are used in lawn mower or lawn tractors. They are cheap ($40) and easy to get .
There are four voltages used in this project
24 VDC for the motors
12VDC for the Arduino Uno
5 VDC for the steering and trim circuits
3.3 VDC for the IMU
24 VDC is connected to the Sabertooth 2X25 . A power switch is installed in the negative leg. Be VERY  careful to maintain the correct polarity.
The Sabertooth will be permanently damaged if you hook it up with the wrong polarity. The warranty will also be voided.
The 12 VDC is obtained with a tap between the two batteries terminated with a plug for the Arduino power input. Do not use the USB circuit as a power source, strange things happen to the gyro when you use the USB for power.

The 5VDC and 3.3VDC is obtained from the Arduino Uno.
A 5 volt source  powers  the dead man, left and right turning, and both trim switches.
3.3 volts from the Arduino powers the IMU

Bolt the Sabertooth, the IMU mounted to a block, and the Arduino Uno and it’s shield to the floor of the frame
Connect the motors to the Sabertooth. The left motor connects.  to M1A and M1B. The right motor to M2A and M2B.
S1 on the Sabertooth connects to Arduino pin 13. Connect Sabertooth ground to Arduino ground. This completes the Sabertooth wiring for now. These connections will be verified during the Motor test procedure a little later. Be sure and set the Sabertooth DIP switches for Simplified Serial operation. Set switches 1,3,5, and6 to the on position Switches 2, and 4 are set to the off position. These settings support using lead acid batteries.

SITWAY Schematic

Route the five wires from Digital pins 4,5,,and 9 plus a 5 VDC source to the steering handle.
Connect the 5 volts to one side of the two momentary on steering switches and to one side of the dead man switch.
Connect the other side of the switches to the wires coming from Arduino pins 4, 5, and 9.

Route the wires from Arduino pins 6, and 7 to the little plate with the trim switches. Connect one side to 5 VDC and the other to Arduino pins 6, and 7.Route the wires from the main power switch to this plate and attach them to a 40 amp SPST switch. Install a surface mounted LED indicator and power it with 5VDC to ground through a 10k resistor.

Install the batteries using the battery box or covers you salvaged from the wheelchair and this pretty much completes the basic construction of the project.. If you saved the charging plug from the wheel chair, install it in a handy place and wire it up to the 24 VDC source. Pad and upholster a seat cushion to be bolted to the frame uprights.

 

For more detail: SITWAY




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