BoB the Biped and me with v0.91 board design
June 25, 2013
I got my pile of BoB parts from isotope yesterday and I was stoked to say the least. He even printed out a JAX name badge for me \o/ Thanks man! My BoB is glow-in-the-dark blue ABS and it luminesces as advertised. Blue Glowy BoB... mmm.
Initial test fits showed that there was going to be some grinding work in my future to make things work. One change I made to the original build list was using an HC-SR04 for the sensor. The sensor holes in the head were too close together so I removed some material on the outer edges to make it fit. It's real nice now.
The hip servo holes also needed some work to make the servos fit. Not a big deal. My rotary tool made things quick and breezy.
The brackets on the feet were snug and the servos fit tightly with no modification. A concern is the blocks on the feet are a little thin so I'll probably go with some strategic hotglue to hold the servos in place instead of using screws.
I noticed that people have been mounting the hip servos from the outside-going-in, where the servo mounting tabs are on the outside of the chassis. I thought of mounting them from the inside to lower the center of gravity some and to hide more of the servos from view. That means the 4AAA holder will be mounted even higher in the bot's head, reducing working space even more.
After looking for an appropriate sized 7.4V lipo (which I found none) I opted to power the bot with two Li-ion 3.7V 600mAH AAA/10440 batteries mounted between the servos. Hopefully 600mAH is enough to waddle around for a decent amount of time. This should reduce the weight in the head and allow more room for goods in the robot. To bring the 7.4V down to useable voltages I initially was going to use two KA378R05 ULDO regulators, one for the servos and one for the rest of the electronics. I have some of these LM2576 LM2596 modules:
but I had problems making it fit in the chassis. After some thinking and talking to BoB I found space for the module. It should be better as it will put out 6V, run cooler, and will be more efficient. I'll have to cut away a part of the PCB to give it room but that's ok.
The brains will be Arduino powered (sorry PICAXE) and the board will be mounted inside the head on the back wall. I estimated it will have around 50mm in width and about 40mm in height to work with. The 40mm limit is to clear the 2AA pack mounted on the head plate. It will have headers for the servo and the ultrasonic sensor. Although the bot usually only uses the servos and the sensor I thought I'd try to include connections for future expansion:
- Two more servo headers to general digital I/Os
- Two headers connected to ADC pins
- An I2C header
- Programming header for Arduino chip
- Reset button
- PWM channels available from timer1 and timer2
- One external interrupt pin
This should let you put in edge detection, bump detection, Sharp IR rangefinder, any I2C device, MP3 player, PWM LEDs, and so on. I tried to add SPI abilities but ran out of room with the use of PTH components.
So here are the first day's musings:
The schematic is a little busy but it is raw and unrefined, for your pleasure ;)
Only two mounting holes were used and are on the bottom of the PCB because I think mounts near the top edge would be hard to get to inside the chassis.
This is the updated v0.91 PCB with room for the LM2596 regulator module:
The gray rectangle at the top is the cross section of the LM2596 module. It will mount on the side wall and the end will butt up against the back wall. The two gray rectangles on the bottom represent where the tops of the servos will overlap the circuit board (there is room between the board and servos).
Just for the record, I'm probably going with an ATmega168 but the library in eaglecad has it labeled ATmega8. It's all the same pinout wise.