Compliant Spined Foot Development

Key design issues:

1. Ensure that with N claws you get N times the gripping force from 1 claw
2. Try to maximize the reliability so that at least a few claws engage (not just one)

• short MOVIE of climber on concrete.

SangbaeKim? has been working on a foot with several claws attached to spring-mounted toes that are supported by an elastic pad. It works fairly well on rough concrete and on soft surfaces. On soft surfaces the claws penetrate, on rough concrete they hook onto asperities. The figures below show a schematic of the "gel pad foot" and a very simple RC-controlled climber using two of the legs to climb (poorly, so far) on a vertical wall. The climber also uses SangbaeKim? 's UnderActuatedFrontLeg? . Both Compliant Spined Foot and underactuated leg are designed based on Sangbae's "Sticky model" AdhesionModels? to maximize adhesion force.

According to the adhesion model, the maximum adhesion force can be achieved by applying optimum stiffness of compliance in normal direction to the wall. Adhesion force increases as the stiffness of finger decreases. However, we lose necessary engaging force with too soft compliance. Gel type(incompressible in volume) compliance provides fairly good non-linear stiffness( softer in small range) and a little bit coupled stiffness between fingers(like tank suspension).

One presentation slide from Sangbae's Adhesion model presentation file shows moment analysis in spine design.Sticky system model presentation file

(Click on attachments below to download high-resolution versions of these figures + a CAD solid model rendering).

Click here for a short MOVIE of the claws and climber in action. The key message from the video is that strategy and behavior are all-important! Many minor design modifications can be made to the climber. For example, the legs really need 2 DOF so that they do not produce pitching moments when detaching and repositioning.

Some quick numbers:

• Weight of the RC climber = 0.1 Kg including 2 RC servos, receiver and lithium polymer batteries
• Distance from wall out to center of mass (COM) = 0.02m (see sketch above)
• So, best-case (minimum) pitch-back moment = 0.02Nm
• Maximum distance from foot to COM = 0.15m
• Distance from COM to tail = 0.2m
• Minimum tensile (adhesion) force per foot in static conditions = 0.08N

For comparison: 1atm = , so 0.1 atm would be plenty for just sticking. But this is the best case. Also, the trick is to reliably get 0.1atm over a significant foot area.

• 5-6-04email.rtf: email to RiSE? PIs and Biomimetics May 6, 2004