Because of the capabilities of Boadicea's pneumatic actuators, Boadicea can walk faster and can carry greater payloads than previous small robots.
On rough terrain, Boadicea's climbing abilities are an improvement over previous small walkers. The robot can climb over 4" steps, twice as high as Genghis and Attila [Ferrell 93]. Again, although the robot's versatility is hampered by the limitations of the control system and a high center of gravity, the actuation system is capable of propelling the robot up a 64° slope.
Table 1 compares the performance and weight of Boadicea's pneumatic
actuators to a similarly sized DC gearmotor. The DC Gearmotor
listed is a MicroMo 2842 motor with a 34G gearbox (90:1 ratio)
[MicroMo 93]. The weight of the pneumatic system includes
the valves and actuator; the gearmotor weight includes both motor
and gearbox. The performance numbers for Boadicea's pneumatic
actuators are measured values; numbers for the DC motor are based
on manufacturer specifications. For the electric motor, foot force
and speed calculations assume a 6" moment arm from the motor
to the foot, the length of Boadicea's legs. The performance of
the pneumatic system is clearly superior, especially the force
density which is nearly 20 times greater than for the motor.
|Max. power output (W)|
|Force at foot (lbf)|
|Max. speed at foot (in/sec)|
|Power density (W/oz)|
|Force density (lbf/oz)|
The DC Gearmotor listed is a MicroMo 2842 motor with a 34G gearbox (90:1 ratio). The performance of the pneumatic system is clearly superior, especially the force density.
Boadicea's feet move at a top speed of 35 in/sec on the propulsion stroke, and 14 in/sec on the recovery stroke. These speeds are more than adequate for Boadicea to achieve the 6 in/sec design target, although the servo controller limits its top speed to 4 in/sec.
Insects are able to walk effectively after suffering limb amputation [Hughes 57] [Kram 94]. Boadicea is also able to continue walking, without changing speed when one or two legs are disabled. Because previous robots had shorter workspaces, losing a leg was more detrimental to their mobility.
The tests I have performed with Boadicea on uneven terrain demonstrate advantages of its design. Boadicea uses the same climbing strategy demonstrated by cockroaches [Yamauchi, et al 93]. The robot can climb over 4" obstacles, and can reach its front legs onto an 8" step. For comparison, Genghis and Hannibal could climb 2" steps [Ferrell 93]. Boadicea can climb obstacles twice as high as the earlier robots.
Again, Boadicea's performance in these experiments was determined by it's high center of gravity and the legs' vertical workspace. The pneumatic actuators, however, could propel the robot over more difficult terrain.
One of the primary goals of this project was to build a robot with substantial payload capacity. With a 4 lb (38% of body weight) payload, Boadicea can still walk at it's top speed of 4 in/sec on level terrain. Each leg is capable of producing over 15 lb of vertical force, more than enough to support the entire robot and payload.
Other robot studies have compared performance based on the energetic
cost of motion, or specific resistance. A more practical measure
of locomotion capability, however, is the mechanical work the
robot is able to perform. Although it is exciting to build a machine
that can navigate treacherous environments, the value of this
exercise is created by a sensor package or other payload the robot
takes with it. Payload capacity and top speed are important factors
for evaluating the utility of legged vehicles.
The data in this table are the product of normalized speed (speed/length) and payload capacity (payload/mass) of different walking robots. Boadicea and PV-II do not carry on-board power supplies.
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