Intro: Pressure Sensitive Electric Skateboard
This instructable was created in fulfillment of the project requirement of the Makecourse at the University of South Florida (www.makecourse.com). The following instructable will explain the build process of an electric skateboard that uses a pressure sensitive pad as a speed controller. The pad works along with an Arduino Uno board as well as an electric motor and esc (electronic speed controller).
Attached is a video that gives an overview of the entire project.
Step 1: Parts List
In order to build this board, you will need the following items.
1. A complete skateboard, with deck, trucks, wheels and bearings.
2. An Arduino micro controller board. I used an Uno board, which can be found here.
3. A breadboard circuit. Half Size is more than enough for this application.
4. Velostat, a semi conductive sheeting which will be used for the pressure pad, that can be purchased here.
5. A brushless electric motor. You can use different kv motors depending on your budget and speed preferences. In my build I used a 280 kv motor which can be found here.
6. An Electronic Speed Controller (esc) for radio controlled vehicles. Make sure that you purchase an esc with a higher amperage rating than the motor requires. I went with this controller.
7. Batteries, I used four 3s Li-po batteries to fit with my budget, you can use your preferred battery types as long as they are compatible with your esc and will put out enough amperage to power your motor.These are the batteries that are used in this build.
8. Male Bullet connectors for the battery connections. You can find a pack that has both male and female connectors here.
9. Gears/Pulleys for the drivetrain. My build used a 14 tooth small gear and a 36 tooth large gear. The Solidworks Part files are attached below.
10. A timing belt.
11. A box to house the electronics. This can be a design of your own, or you can modify this case fairly easily.
Step 2: Assembly of the Velostat Pressure Sensor
Velostat is an electrically conductive material that is sold as a packaging material. It has a unique property that makes it usable as a pressure sensor, which is varying electrical resistance dependent on the amount of pressure put onto it. In order to take advantage of this property, you must run an electric current through it.
To begin assembly of the sensor, you will need to cut a piece of your velostat to your preferred size and shape. Keep in mind that this will be placed on top of the skateboard where your front foot sits, so base your size on the board you are using.
Cut two pieces of conductive foil to a size slightly smaller than the velostat. Household aluminum foil with work for this.
Next you will need to cut and strip the wiring for the sensor. Using 18-20 gage wire, strip off about two to three inches of the insulation at the end of two wires.
Connect each wire to one of your foil sheets and then place each sheet on opposite sides of your Velostat pad.
You now have your completed pressure sensor assembled.
Step 3: Wiring the Arduino Circuit
Once your pressure sensor is assembled , you will need to connect it to your Arduino Uno board. Refer to the photo above as a wiring schematic.
Solder the wires from the sensor to jumper wires for the Arduino. These will be used as your positive and negative leads.
Connect the 5V output on the analog side of the Arduino to the positive strip on a breadboard, and connect the positive lead (Red wire on left of picture) to the positive channel on the breadboard.
Connect your negative lead (blue wire on left of picture) to the breadboard and then run a 120 Ohm resistor from the negative lead on the breadboard to another part of the breadboard. This will serve as a voltage divider so you can take the output voltage from the sensor and turn it into usable data in the Arduino.
Connect the resistor to the ground of the breadboard and ground the breadboard to the Arduino.
Attach a wire to the breadboard on the strip that contains your negative lead and the resistor for the voltage divider. Make sure to attach it on the opposite side of the resistor than the negative lead. Run this wire into an analog input on your Arduino board. This will be where the Arduino receives the signal that it will turn into a throttle response.
Lastly, connect jumpers to the positive and negative strips (orange and green wires in diagram) of the breadboard along with one more jumper that connects to the Arduino. Make sure to connect this last jumper to a digital pin marked as a PWM pin. These will be the power and signal inputs to your esc.
Step 4: Programming the Arduino
Using Arduino IDE, create a sketch that will take the signal for your sensor and map it into a throttle response. You will need to include the Servo library that comes with IDE. The pictures above show my sketch and I have attached the program file below.
Read the commented lines for a more clear description of the sketch.
Step 5: Assembly of the Motor Power and Control Circuit.
Depending on the batteries you purchased for your build, this step may vary slightly.
My build required 4 batteries run in parallel in order to achieve the necessary amperage.
To connect the batteries to the ESC, you will need to solder battery connections to the ESC. Using 10 gage wire, solder a wire for each battery to the positive and negative leads of the ESC. Make sure to leave enough wire to reach your batteries so consider you battery placement before you begin this step.
Next, solder each positive and negative wire to a male bullet connector. Take into account which battery you will connect these plugs to in order to keep your wiring simple and clean.
Connect the signal output side of the ESC to the brushless motor.
Connect the small signal wires from the ESC to the jumpers on the breadboard from the end of the previous step.
Step 6: Mounting the Motor Assembly.
The motor will have a mounting point on it from the factory, but you will need to fabricate a bracket to attach it to the board. I used a thin piece of sheet metal, cut and bent to size.
Line up your motor where you would like it mounted on the bracket and drill holes. Attach the motor to the bracket.
You will want to attach your timing gears to the motor and to your driver wheel so that you can mount the motor with belt tension considered.
Attach the belt to the motor and line up where the bracket needs to be mounted. Drill holes for the motor mount into the board and bolt the motor bracket to the board.
Step 7: Final Board Assembly
Take the case for your electronics and drill a hole in the front of it, about an inch in diameter, so that it is large enough for the battery plugs to fit through.
You will need to determine the placement of your electronics case and drill mounting holes into the bottom of it. Drill holes to match the mounting holes on the case into the skateboard and bolt the case to the deck. Make sure to mount it with bottom of the case to the board for easy access to the electronics.
Put the batteries and ESC into place in the box and run the wires out of the hole in the front. Plug the 9V adapter into the Arduino and connect the batteries to the ESC. Connect the ESC to the jumpers on the breadboard and plug the motor in.
The ESC in the parts list is pre-programmed and will work right away, however not all controllers will be and you may need to see the instructions for your controller to program it.