Beast, a four legged Mobot
Autonomous Mobile Robot

Viewed from the front, this mobot has a bulldog like gait.
For gait-sequence finding, a prototype was built, controlled by a PC parallel port.
The basic gait is crocodile like : quick crocodile walk.



Technical Characteristics 

 Four legs
 Eight Servo Controlers, Two per leg.
 Speed : 500 meters per hour (slighly quicker than a ant, and trust me, it runs fast)
 Autonomous
 Autonomy : 1 hour.
 Weight : 900g
 Size : 170mm x 170mm x 130mm (height)
 Range : 250 meters.
 Power consumption : 1500mA at full speed.
 Energy : 4 1.2V Nickel Cadmium batteries 1300mAH plus a 9V battery for Digital Parts
 Processor : 8 bits microcontroler ST62E25 (thomson).
 EPROM 4Kbytes. Source Code size about 64 kilobytes.
 RAM : 64 bytes !
 Processing Power : About 6.5 microseconds per instruction.
 CPU load : 80%.
 Remote controlled, Radio FM receiver, needs only two channels.
 Eight degrees of freedom.
 Digital output - current amplified and sent by an analog multiplexor to
  Loud Speaker / Leds / Infra red Leds.
 Microphone / Infra red Led receivers connected to an analog multiplexor connected to
  an ADC input after amplification and filtering
 One Red Led under software control.
 Servos refresh rate : 50hz (this ensures optimum control)
 Known Illnesses : Parkinson.

The ST microcontroller just fitted the need, but its limits were reached.
It was necessary to use tricks to get round the little stack size.
Design 1993-1994
Mechanical design & Mechanical Making : Valentin LEFEVRE
First Efficient Walk Sequence Found : Isabelle (girlfriend of Valentin)
Decoration, Electronics & Software : Laurent Chabin. (email : ).

Don't even think about making that PCB. It's a mess. 

           +-------+           +-------+
         4 ¦+------++         ++------+¦ 2
           ¦¦      ¦¦         ¦¦      ¦¦
  ¦        ++------+¦ 3     1 ¦+------++       ¦
  ¦         +-------+         +-------+        ¦
  ¦         +-------------------------+        ¦ +---------+
  ¦         ¦          Back           ¦        ¦ ¦  Over   ¦
  ¦         ¦                         ¦        ¦ +---------+
  ¦         ¦                         ¦        ¦ +---------+
  ¦         ¦          Front          ¦        ¦ ¦  Below  ¦
  ¦         +-------------------------+        ¦ +---------+
  ¦         +-------+         +-------+        ¦
  ¦        ++------+¦ 5     7 ¦+------++       ¦
 \¦/       ¦¦      ¦¦         ¦¦      ¦¦      \¦/
         6 ¦+------++         ++------+¦ 8
           +-------+           +-------+

What can you do with it ?

Walk forward, with proportionnal control of speed.
Walk forward and turn with a radius (among three)
Can do the same, walking backwards.
Crab like Side Walk, a weird sequence.

Pilot It ?

Can't be easier.
Need only one stick of the remote controler.
The vertical axis controls the gait speed (can be negative)
The horizontal axis controls the direction.

How does it work ?

Well, we found that each movement sequence could be defined as a set of two positions repeated. A position beiing a set of eight numbers specifying the orientation of each servo motor.

One may say that with only two positions, the insectoïd will just jerk without progression.
The trick is HYSTERESIS.
Each leg is composed of two servo motors, giving two axes of freedom.

Thus two axes do not reach their final position at the same time.
And the resulting sequence of positions looks something like this :

To have a very precise control of speed, the microprocessor generates the intermediate positions.
Every 20ms (50Hz), the software will/won't increase/decrease one step the servos positions in direction of the destination position.
Then call the procedure in charge of driving the servos.

Exact algorithm = line draw in a plane.
Horizontal axis = Time, Vertical Axis = Servo Position, Rate = Stick Vertical Axis Position.

The electronic provided some sensors.
Direct position Feedback from four servo controllers (a wire soldered on the portentiometer of the servo)
Infra red emission and reception in 3 directions for obstacle avoidance.
None of these last features were exploited by the software.

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