Sunday, April 20, 2014

Starting Construction of the Pivot Board

We decided to make the pivot board from two E90 Razar Electric Scooters and a plank of wood. We disassembled and cut down the scooter to get to the motorized drive train attached to the back wheel. Below is what the cut-up scooter looked like after the plastic cover and handler bars were removed as well as a closer look at the rear wheel drive train.

 By taking the scooter apart, we can use the battery, throttle button, and rear wheel drive train as part of our pivot board. Unfortunately, the motor controller that comes with the scooter only allows for single direction drive. Since we need both motors to have bi-directional drive so we will need to use different h-bridged for the pivot board.

The rear wheel assembly has very convenient has very convenient screws holes which we will use to mount each wheel assembly to the wooden plank. The wheels will be arranged as shown below.

Baby Board Trying to Navigate around Obstacles

Since the joystick allows the user to effectively change the speed and direction of the baby board, we can remotely drive the baby board like an RC. Here is an example of Justin using the glove board to wirelessly drive the baby board around obstacles with relative success. 

Baby Board DEMO

Here is a video showing the basic driving functionality of baby board.

After the initialization, the baby board balances itself and as Justin changes the position of the servo motor to drive the board forwards and backwards. Changing the position of the servo effectually moves the center of gravity of the baby board which causes the board to tilt and the motors to change speed and direction accordingly to try and keep the board leveled given the shifted center of gravity of the baby board.  

Software Description

The baby board needs to be left still for a few seconds when it is powered up to allow the IMU to calibrate. The main board is running two main control loops once the initial IMU calibration is complete. The first loop is a a PID controller on the tilt angle and error of the board. The IMU continuously measures, filters, and updates the MBed with the current tilt angle and tilt error rate which feed into a the PID controller. The controller outputs the motor speed necessary to return to leveled position. The turning input from the wireless glove as well as the x axis angle from the IMU is also feed into a PD controller which outputs the differential turning speed that is feed to the motors. The turning controller forces the baby board to drive along the IMU's x-axis unless the turning input from the glove board is non-zero. The wireless board sends turning and servo position data to the MBed on the baby board 10 times a second. 

Wireless Glove Board

The glove board consists of a two axes joystick with a select button, an MBed, and a MRF24J40 wireless board. One axis of the joystick is used to input a desired turning angle and the other axis is used to control the position of the servo motor used for testing. The select button is used to signal for the motors on the baby board to brake and hold for 50 seconds. The wireless chip sends the desired inputs to the other wireless chip on the baby board. 

Glove Electronics:

Baby Board

Here is the completed working prototype of the pivot board, "baby board", will all of the on board electronics. The board consists of:

- Dual layer acyrilic frame separated by spacers and two acryilic wheels.
- Two 6V brushed DC motors
- MPU6050 IMU
- MBed
- Dual H-bridge IC
- 3.3V Regulator
- 6V Lithium Ion Battery
- MRF24J40 Microchip (Wireless Board)

Baby Board Electronic Set-Up:

Baby Board Set-Up:

A smaller motor is used as to balance the weight of the battery. 

To simulate a rider shifting their weight to increase or decrease the speed of he board, a servo motor with an extended wight attached to the motor blades is "mounted" to the top plane of the baby board.

Servo "Rider":

ESE350 Final Project: Pivot Board Concept

For our final project, Justin and I are creating a self-balancing, two-wheeled, electronic skateboard. The board will have two motorized wheels across along the short axis of the board. The rider will be able to shift their weight forward or backwards to increase or decrease the speed of the board and the motors will respond accordingly while keeping the skateboard leveled. Turning is accomplished with a wireless, joystick-embedded glove through which the user can communicate a desired turn angle to the microcontroller located on the pivot board.

First we will make a small scale prototype as a platform for the testing the embedded system and controls.