Jon's Place

Tuesday, December 5, 2006

Mechanical & Electronics

So, following are my thoughts on things mechanical. Bear in mind I have no practical experience building biped robots, but I have been building mechanical stuff since I was a kid.

I'm going to be machining a bunch of parts for MicroRaptor, simply because I'm not happy just using what they provide. Many of the servo connection parts in the kit will work fine, but there are other structural pieces I will need to do. For example, I will be machining a custom "backbone" for this raptor, which will hold the whole thing together. Because I will be using a custom battery pack, and custom control electronics, what they provide with the kit won't cut it.

Another part I will be building from scratch is the head. Because I will be using custom range sensors, plus a digital camera, I'm going to be machining whatever structural components I need to hold those parts together. You can see a CAD rendering of what I think the head might look like here. It has three MaxSonar-EZ1 sonar sensors, a C328-7640 digital camera, and a nozzle for blowing air (for doing firefighting competitions). The big black thing at the back is an AX-12 servo, which is used for pitch control of the head. Right now the plan is that the 3 sonars and the camera will be held to the main board using molex headers that plug into sockets on the board. The board will be held to the servo using stand-offs.

I'm still trying to decide if I'm going to use a single central micro-controller to handle interfacing to all my custom sensors/acuators, or if each sensor group/actuator will get its own controller. If I go with a central controller, here's what will have to be on the board:
  • 2 pin terminal block for battery
  • 3 pin terminal block for bus
  • 2 pin SIP for external motor control
  • 2 pin terminal block for aux switched power
  • 4 pin SIP socket for bump sensors
  • 3x4 pin SIP sockets for sonars
  • 4 pin SIP socket for I2C bus
  • 4 pin SIP socket for console
  • 4 pin SIP socket for HW UART (programming)
  • 2 switching regulators (5 volts, 3.3 volts)
  • 2 pin terminal block for 5 volts out
  • 2 pin terminal block for 3.3 volts out
Seems like a lot, but if I use a micro-controller like a PIC 16F876, the plugs and terminal blocks just gets clustered around the chip, and can still end up a reasonable size. After looking at the data sheets, I discovered that an 876 running at 16 MHz can use the hardware UART to talk at 1 Mbps, which is the Bioloid bus speed. However, if I'm going to use this, I will need a switching chip, which basically allows disconnecting either Tx or Rx when the other is in use.

Another mechanical thing I plan to do is build new feet for the robot, with rubber pads on the bottom. I think this whole sliding around thing that most of these robots do is silly, I realize that many of these robots need it, because otherwise you have to be a lot more dynamic on balance control. Since I plan to be more dynamic on balance, I plan to have my robot be very sure-footed. The type of walking I'm planning for it pretty much requires that.


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