Model Wheelchair Robot With Pico Controller on Stripboard

Here is a design for a small model wheelchair to provide a bit of diversity into the regular robot tanks and cars which get made. This is a design for anyone but may add extra interest for children who depend on a wheelchair by involving them with a model vehicle they are familiar with.

The robot is targeted to be 'Barbie' doll size and uses ideas of the L0Cost Robot project.

The wheelchair itself has been designed in Tinkercad so can be extensively modified to suit circumstances and may be controlled via a Raspberry Pi Pico W with a web interface. An optional example of a DIY line follower is given at the end. The L0Cost robot controller can be added to either add a camera interface, PS3 controller or scripting.

Stepper motors are used to drive the main wheels and give some precision to the operation without depending on motor encoders. This does mean that the wheelchair will not race about, but that might also be seen as a more appropriate scenario. The stepper motor specified can be supplied as either 64:1 or 16:1 step down ratios, if the 16:1 is used then the maximum speed will be four times that of the 64:1 version, about 10cm/s.

After the line follower, adding additional sensors for anti-collision, drop avoidance, wall following or anti-tipping are a few ideas for where to take this model. Experimenting with driving the wheelchair on two wheels has also been done.

2 x 28BYJ-48 stepper motors (Either 64:1 or 16:1 ratio)

2 x 5mm to 12mm hexagonal brass wheel hub adapters. Buying these is optional, a 3d printed version is in one of the steps, but a well machined brass one is expected to give a more accurate fit and be more durable.

2 x ULN2003 driver ICs

Raspberry Pi Pico or Pico W, either versions 1 or 2

Small pcb mounted push button or toggle power switch

Small LM2596 buck power regulator.

2 x Lipo batteries, 802540 800mAh drone batteries would be suitable. (852540 650mAh have been used but alternative might require the battery box to be modified). Two 3.7V batteries are used in serial with the buck power regulator reducing this to 5v for the circuits and the motors.

Suitable clips to attach to the batteries, usually JST-2P or JST-PH, check which your batteries use

Optional: Voltage display module

Dupont female to female patch leads

Matrix board offcuts, see size required

5-pin sockets for stepper motor plugs

Dupont PCB pins

Dupont PCB sockets

Small PCB mounted sounder

Hook-up wire, 0.8mm single core is easiest to work with

M3 and M4 bolts, washers, locking nuts

Suitable size rubber bands to add tyres to the wheels

Optional Line Follower:

7 x TCRT5000 IR sensors

Stripboard

MCP4051 8 to 1 analogue multiplexer

The bulk of this robot is made on a 3D printer and attached are the .stl files to print a copy. The original is on TinkerCAD here which the builder is free to copy and modify. Most of the assembly is using M3 or M4 nuts and bolts with locking nuts being used where suitable.

An idea for a printing priority....If building while printing

1. Print the battery box and power switch clip, and while this is ongoing, cut and solder the switch board.
2. Once the battery box is complete, start assembling the controller board while other parts print
3. Print the chassis, seat, wheel hubs (if used) and side arms
4. If using the line follower, this can also be assembled while still printing
5. Print the caster forks, caster wheels, foot plate and finally the main wheels

The battery box needs to be assembled and attached to the main chassis before any other construction is attempted. There is a full set of pictures related to this so make sure to view them all before beginning assembly.

1. Assemble the power switch board and solder the battery leads to the input side of the switch.
2. Solder wires to the power regulator board. I used red wire for the +ve connections and black wire for the -ve connections as this is commonly recognised.
3. Fit the power regulator into the battery box as shown in the diagram.
4. The power switch board will fit on the clip on panel on the front to the battery box. Temporarily fit the power switch board to the panel and measure the leads from the power regulator to cut them to length to easily reach the board but without too much slack wire.
5. Disassemble and solder the regulator wires to the power switch board
6. If using the voltage display, change the wire leads to two dupont female terminated leads, see picture, and attach this to the power switch clip with two M3 bolts and nuts as shown
7. Attach the battery box to the chassis with two M3 bolts, with the nuts inside the chassis and tighten.
8. Attach the power switch board to the clip on panel using two M3 bolts and nuts and tighten.
9. Complete the battery box by replacing the power regulator board back into the battery box, routing the wires out the side and clipping the panel on to the top of the box. Attach the voltage display leads to the power switch board if used.

The supply voltage to the controller board needs to be set to 5V.

If the voltmeter option is fitted, connect the red wire to the output connections for the power switch board, otherwise connect a multimeter set to a suitable range, typically a range measuring volts up to 20V.

Connect batteries, or a suitable power supply, switch on, and using a small flat bladed screwdriver through the hole in the side of the battery box, adjust the voltage until it is 5V. Switch off, wait a few minutes and repeat to ensure that the voltage is settled at 5V, adjust again if necessary.

It's important to get this right in order to avoid damaging the controller board components so take some time doing it.

A controller board for the wheelchair has been designed around a Raspberry Pi Pico and is intended to drive two 28BYJ-48 stepper motors.

This step can be done while the parts for the wheelchair are being made.

A pair of ULN2003 driver ICs are used to power the stepper motors under the control of the Pico. The board also supports the addition of an ultrasonic module if required and I2C peripherals such as the VL53L0X or MPU6050.

A line following module design shown is shown at the end of this instructable and the intention is to use a Pico W to provide manual steering via a Wi-Fi link.

Software for this board is available on github here.

The controller board is assembled on stripboard and a layout is shown in the pictures. A basic sequence of assembly would be.....

1. Cut the stripboard to size ensuring that is fits securely in the controller frame.
2. Fit and solder the IC sockets in place and then the stepper motor sockets.
3. Fit and solder the PCB sockets in place for the Pico, I2C and ultrasonic connectors
4. Fit and solder the PCB pins in place
5. Using hook-up wire, complete the surface wiring shown in the diagram

Check the wiring with the diagram and ensure there are no shorts on the board.

When complete, connect up the battery box to the controller board and the stepper motors. Load the test program (on github) on to the Pico and run to verify that all is well.

This can be a complex assembly so additional help may be required to hold items still while other items are screwed together.

The chassis should now have the assembled battery box attached. Next...

1. Attach the seat to the top of the chassis with two M3 bolts, washers and nuts, with the nuts inside the chassis. Depending on the doll used as a passenger, it may be necessary to loosen these in the future to move the seat forward or back.
2. Attach the controller frame underneath the chassis using two M3 nuts and bolts, with the nuts inside and tighten.
3. If complete, test the fit of the controller board in the controller frame

1. Fit the hex hub extensions, either printed or bought, to the shafts of the two stepper motors and tighten, making sure that they are adjusted to be straight.
2. The wheelchair will be assembled with the wires from the stepper motors facing forward. Label each stepper motor either Left or Right. It will be easier later if the plugs for these are also labelled Left and Right.
3. Fit an M4 bolt, washer and nut through the top tag of each stepper motor as shown in the picture, but leave this loose as it has to slide into the slots in the chassis.
4. Take the side arms and attach them to the stepper motors lower tags by the large end hole using a M4 bolt, nut and washer, see the pictures, but do not tighten. Ensure that the support for the caster wheel forks at the front end of the side arm is facing out.
5. Add an M4 bolt nut and washer through the other large hole in the side arms, but do not tighten.
6. For each side, holding the stepper motor with the wires facing forward, slide the stepper motor and M4 bolts along the slots in the side of the chassis, taking care to place the washers on the outside of the chassis for the the upper bolt, and inside the chassis for the lower bolts. This is to ensure that the washer spreads the load of the tightened nuts. Tighten all fixings. When inserting the stepper motors, feed the connecting plugs and wire through the lower left rear hole of the chassis for later connection to the controller.

The main chassis construction is now complete and ready for the final main parts.

1. Using an M3 bolt, washers and locking nut, attach the caster forks to the arms. Two ordinary nuts may do if they are locked together to prevent them coming undone.
2. Fit the caster wheels to the middle of the forks, fixing in place by screwing an M3 bolt though wheel and forks. Check that this rotates freely. If it doesn't, dismantle and open out the hole in the wheel with a 3.2mm drill and refit. Don't drill out the holes in the forks.
3. The foot plate can now be attached across the end of the arms using two M3 bolts. It may be that this will be sufficient to hold the plate securely but be prepared to use nuts to fully tighten it.
4. If the optional hand rails on the wheels are required, fit these to the wheels now using M2.5mm nuts and bolts, ensuring that the bolts aren't protruding above the nuts when tight.
5. Fit the wheel to the hexagonal wheel hubs and fix with an M4 bolt screwed through the centre.

Your wheelchair should be looking complete at this stage. If a doll will be used with the chair, test fit now and adjust the seat position if necessary. Two holes are provided in the seat for securing a doll in the chair if necessary.

Software for the wheelchair is held on github here where it will be updated from time to time. The test programs already mentioned are held there and a basic line following program also. A webpage based remote control facility will be available in the future.

As mentioned at the beginning, this project is to add some diversity to the standard robot catalogue and it is hoped that it will inspire builders to appreciate wheelchair use more. It could of course just be a toy for an appreciative owner.

Ideas for demonstrations, challenges and improvements as well as the basic remote control or line following might be...

1. Negotiate a model high street filled with curbs, bins, cars and street furniture.
2. Build a model house to demonstrate living in a wheelchair
3. Fit a L0Cost Robot controller to get a first person view from the wheelchair
4. Build several for model wheelchair sports
5. Experiment with improving the environment for wheelchair users by creating models

Whatever you do, I hope you build it for your own interest and enjoyment and perhaps learn a bit in the process.

Short video without rubber band tyres on

A custom line follower circuit can be fitted to the foot plate. This is based on the designs in another instructable here. The 5 sensor version is intended to provide basic line following functions of following a line and perhaps negotiating simple junctions. The 7 sensor version is intended to provide the basic functions and also detect information strips to either side of the main line to indicate corners, junctions, dead-ends and in simulations, doors or furniture placement. An automatic recharging station is another optional demonstration, details are left to the constructor to think up.

The line follower circuit consists of either 5 or 7 TCRT5000 IR sensors which are multiplexed to one ADC pin on the Pico controller and are mounted on a piece of stripboard. The construction is as per the pictures...

1. Start by cutting the stripboard to the required size, either 5 or 7
2. Cut the stripboard tracks in the required positions indicated and drill the two mounting holes.
3. Fit and solder the PCB pins
4. Cut the plastic legs off the TCRT5000 surrounds so that they fit flush with the board.
5. Fit and solder the IC socket in place, taking care with the orientation.
6. Fit and solder the IR sensors, again, taking care with the orientation, and trim the leads.
7. Using hook-up wire, make the required surface wire joins as per the 5 or seven sensor picture
8. After checking the wiring and the stripboard for solder bridges, fit the IC, checking the orientation is correct,
9. Connect the board as per the picture and test with the program on github here.
10. Attached the line follower sensor board to the foot plate of the wheelchair, with the sensors pointing down. In the printing list is a thin insulation strip to mount above the sensor to prevent accidental short circuits and to improve the appearance.