Haardvark Specification - 2001
Haardvark has gone through a number of incarnations - click the links below for previous specifications
Dimensions: 1.0 x 0.9 x 0.4 m (excluding weaponry)
Ground clearance: 15 mm
Weight: approx. 100 Kg
Chassis: Inner chassis of steel, adapted from a 3-wheel scooter. Outer chassis of aluminium for added strength and 'crumple zone' factor.
Drive: Two 24V wheelchair motors driving two 12" wheels (independent control).
Power: Three 7-Amp-hour valve regulated lead-acid batteries. 1kg Liquid CO2 at 10 bar (150psi)
Armour: 5mm cold-formed polycarbonate sheet mounted on shock-absorbing material, Kevlar plate and a stainless steel front pointy bit.
Weapons: 400mm diameter friction saw (4100 rpm) driven by a 2.6kW motor, mounted on the end of a 5' (1.5m) moveable arm. The arm doubles as a self-righting mechanism and a flipper, driven by a 100mm diameter pneumatic cylinder. OR: Two eight inch hardened steel forks, driven by two 40mm diameter pneumatic cylinders.
Control: Digital proportionally-controlled power electronics, designed and built by Mike. All electronics mounted inside a vibration dampened flame-proofed die cast box. Activated by a six-channel 40MHz radio controller.
The PIC microcontroller (a PIC16F84A running at 4MHz to be precise), takes in two signals from the receiver (it plugs into where servos would normally go) and outputs two speed signals, one for each drive motor. It also has two outputs per motor for switching the forward/reverse relays.
A bit more detail about the software: The signals from the receiver are 5V pulses that occur every 20ms. The length of each pulse is proportional to the position of the transmitter stick, with 1.0ms being fully minimum and 2.0ms being fully maximum. This implies that a 1.5ms long pulse means that the stick is in a neutral position. The PIC software counts how long these inputs are at 5V for, subtracts 1.5ms worth and thus knows where the for/rev & left/right stick is. It stores the for/rev value in both left and right motor outputs and then subtracts the left/right values from each. This is the 'mixing' bit that radio control people talk about. A deadband is excluded to prevent an exiteable, jittery robot, and the appropriate relay outputs are switched to 5V if there is motion required. Whilst all this is going on the process is interrupted every few microseconds to generate two pulse-width modulation trains that eventually drive the motors.
This is the simplified version - you can see why it took Mike a while to build!