Ok, I think I am going to order the PCB’s through Batch PCB The cost will be $49. It is only 1 oz copper and I should probably use 2 oz. But inorder to save some $$ I am going to risk it.
I still need to develop the firmware; but it will go alot easier with the PCB in hand.
Stay Tuned!
I haven’t updated in a while because the board has been sitting in a corner.
You see the Volacano 60A ESC controller worked great for the lightweight test pilots; but not so much for my 180lb butt.
I could not take off from a dead stop. I was determined to get a reliable start was trying different things and it burned up in a glorious burst of light and smoke! You see with this type of motor; starting the motor turning is the hardest thing todo. Without sensors the controller has to guess what the motor is doing until it reaches a speed at which the BEMF sensing is reliable.
My conclusion is that the Volacano ESC controllers; although probably exceptional for R/C applications; just don’t have the stuff to push start a 180lb dude on a mountain board from a deadstop. After some searching without luck, I decided that I would design my own Heavy-Duty Sensorless Brushless Motor Speed Controller based on Arduino hardware and hopefully software. The design will be Open-Hardware and the software, I hope will be Arduino based, regarless will be released under GPL.
I have had the PCB designed for almost 3 months now and have even ordered most of the components but holidays and other stuff have kept me from doing the build. But with the New Year here I am determined to get it going.
I will post the schematics and PCB design next week, and hopefully provide the links to all the great application notes I have found for the software design.
Happy New Year and stay tuned!
As I mentioned previously, I chose a Volcano 40A ESC from www.hobbypartz.com to drive the motor. This is a R/C speed controller so it usually resides inside a remote controlled vehicle and is attached to the output of a Receiver. That isn’t how I am using it. I picked up a used 3-wire thumb throttle from a local Electric Scooter shop; Alien Scooters for $10. It was salvaged off of a kid’s toy 3 wheeler.
I did some testing and driving the Red wire with +5V, the black with Ground, I read a variable voltage from < 1 V low throttle to >4V max throttle.
Here is the thumb throttle attached to the mountain board leash. I removed the brake lever because I intend to use the motor to brake.
So the question was how do I convert this voltage to a servo type signal that the ESC understands? My electrical engineering brain went quickly to designing some hardware solution something like this. But that used a potentiometer and connecting up a direct voltage wouldn’t work the same.
After much googling; I finally had an epiphany! I think I actually saw a LED above my head! Arduino!
Arduino is a easy to use platform of software and hardware to using Atmel AVR microcontrollers. A microcontroller has an Analog to Digital converter to measure the voltage from the throttle. It also has a PWM controller to generate the servo signal. Now the traditionally I would have to design aPCB or breadboard up the microController extra components and USB interface, etc, etc. Then figure out the exact registers to make the PWM controller and the ADC work, write a routine in C language and compile it, etc. etc. But with the Arduino the boards are ready built with IO accessible. Luckily I had a Duemilanove board already from Sparkfun.com. For the programming it was even easier, because there is already a servo library which generates the PWM signal. In fact there was a example code which did exactly what I wanted. It read the ADC, re-mapped the # to a range acceptable to the servo library and generated the signal. Amazingly in about 5 minutes I had the throttle driving the motor. I don’t know why I wasted 4 hours looking for a hard way to do it?
I made a small breadboard to plug on top of the Arduino board. Then I soldered all the inter connect wires going to servo(brown, yellow, red); the switch(Red,black), the throttle(red,black,white). The power to the arduino board comes from the servo connection to the ESC because it has a Battery Elimination Circuit(BEC).
Here is what the Example Arduino code looks like:
// Controlling a servo position using a potentiometer (variable resistor)
// by Michal Rinott <http://people.interaction-ivrea.it/m.rinott>#include <Servo.h>
Servo myservo; // create servo object to control a servo
int potpin = 0; // analog pin used to connect the potentiometer
int val; // variable to read the value from the analog pinvoid setup()
{
myservo.attach(9); // attaches the servo on pin 9 to the servo object
}void loop()
{
val = analogRead(potpin); // reads the value of the potentiometer (value between 0 and 1023)
val = map(val, 0, 1023, 0, 179); // scale it to use it with the servo (value between 0 and 180)
myservo.write(val); // sets the servo position according to the scaled value
delay(15); // waits for the servo to get there
}
Another great thing about the Arduino is that it allows you to print information out of the running program through the USB interface to the computer screen. So I added some diagnostic print lines and was away testing. I figured I could squeeze out more power by maximizing servo range. It only took me about 15 minutes to screw up and toast the 40A ESC in a grand and spectacular way.
After this, I ordered the 60A volcano ESC, and decided not to tweek the servo range anymore!
In order to complete my goal, I needed a small but powerful motor. During my research I read through and viewed ALOT of different postings and experiences people had building electric bikes and other vehicles. I figured I needed at minimum 750Watt motor because that was the legal limit for electric bikes to be used on road. But 750Watt is approximately 1hp so that isn’t very strong. But there are many electric products on the market with less power. I found these electric motors used in Remote Control applications such as large scale airplanes and helicopters. They were small and powerful but I had my doubts that they could push my 180lb body around even though they had impressive power numbers greater than 750W.
But then I saw this posting. This guy cludged one upto a bicycle and it pushed him down the road. So I figured it would work on the mountain board.
A great place to get these motors and some of the other components at a Huge discount off the MSRP is www.hobbypartz.com.
So inorder to make sure I got a motor which could fit a sprocket; I went shopping to make sure I could get a sprocket with the same I.D. as the motor shaft I was considering. For the sprocket I went to sdp-si.com. I found a Sprocket with hub with 10 teeth and 8mm shaft hole. This would fit the Monster Power 60 Brushless Motor I picked out.
Last think was to make sure, the speed of the motor would match up with the rest of the drive train. I didn’t want to be stuck going too slow. This is where the math comes in!
Wheel Diameter = 8″ if I use a 1:7.8 gear ratio on the sprocket setup using this 78T sprocket Top speed for a 18.5V supply would be about 22mph.
Ok, you probably want to know how to calculate that.
First I figured out how many revolutions of the wheel per mile = 5280/(pi * 8″) /12 = 2522.3 rev/mile
So with a 1:7.8 gearing the motor would need to turn 19,674 rev/mile.
Therefore with the Monster Power 60 having a 400Kv spec. Kv = rpm/volt would turn at maximum 400*18.5 = 7400 rpm = 444000 rev/hour. I planned on an 18.5volt Lipo battery.
Now if we take (rev/hour)/(rev/mile) we get mile/hour = 22.56mph max speed.
Another sanity check, the Monster Power 60 is rated at 1200W which is almost 2x the 750W spec mentioned earlier.
Max current is rated at 60A burst or 40A continuous so I also pick out the Exceeds Volcano 40A ESC brushless motor Electronic Speed Controller also from www.hobbypartz.com. This is ESC is designed to takes a servo control output from aR/C Receiver. So there will be some work on how to connect the throttle upto the ESC. But that is a tale for another day.
I have been wanting to build an electric powered vehicle of some kind for a couple years. Everything just seemed either too difficult or not to justify the cost. I wanted something fun, but didn’t want to be lazy. So I ruled out adding electric to my bicycle because pedaling it myself is greener and healthier. Doing a car or a motorcycle was too expensive and difficult for my skill set. Besides they need really long range (>40miles) for me to use so not justifiable.
So I decided on a mountain board. There aren’t any hills near my home, but I enjoyed riding a mountain board once while on vacation. So if I added a motor to it; it would make it practical to use around the neighborhood. I’m not lazy because it takes balance and skill to operate; It’s dangerous so that makes it fun! Oh, and it’s not like I’m going to ride 10 miles on one either so range isn’t an issue, which means the cost will be low.
I’ve seen a previous instructable where someone has made one but theirs was seriously flawed. They could not steer it because the motor was big and attached to the board and the axle. And they had a huge battery. Here is a link: http://www.instructables.com/id/Electric-Mountain-Board/
So my goals where set:
1. Add Electric Power to Mountain Board
2. Do not degrade ridability
a. Motor attached to skate truck/axle not board
b. Keep added weight low
3. Faster than cheap electric scooters (>15mph)
4. Durable so I can still ride off-road
5. Kid friendly. It should have a mode that limits max speed.
So first step is to find a board. I wanted one that had a brake kit; because then there is a handle with leash attached. Also, I thought I could replace the brake rotors with a sprocket without much trouble, just as the other person did on the instructable.
I scored a good buy on craigslist, for $60 I got a MBS board with brakes. It had a flat, but I could fix that!
Next Step: picking the motor
