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Snake Robot Design

 Snoopy - Actuated Universal Joint Mechanical Drawings:

Actuated Mechanical Design Notes:

The design goals for the snake robot included maximum torque-to-weight to allow cantilever support of the snake; minimum envelope diameter to fit through small openings; minimum achievable radius of curvature, resulting from short links with maximum angular travel between links; and rugged construction. Secondary goals included minimum backlash and compliance in the structure; and "reasonable" speed of motion. From the outset, a modular design with all links identical was chosen for simplicity of design, fabrication and assembly. This is sub optimal in the sense that the joints near the fixed end of the snake will generally have much higher loads than those near the ends. Six was chosen as a reasonable number of joints for the snake, although the real manipulation capabilities depend also on the degree of travel in each joint.

An actuated universal-joint (U-joint) design was selected for its simplicity and ruggedness. In this design, U-joint "crosses" are connected to one link with a pitch pivot joint, and to the next with a yaw pivot joint. The pitch and yaw joints are always orthogonal, and intersect along the link centerlines; this leads to simple kinematics. The pitch and yaw joints are actuated by linear actuators in the two links. Links are configured such that the axes at each end of any link are parallel; thus, one link will have pitch joints at both ends actuated by its two linear actuators; the next link will have two yaw joints. This arrangement facilitates packaging of the two linear actuators side-by-side in the link. Ball screws were chosen for the linear actuators because of their high efficiency (compared to lead
screws) and effective speed reduction. The screws are fixed in bearings mounted to the links, while the nuts drive clevises connected to the crosses of the U-joints. The screws are driven by brush-type, permanent-magnet, DC motors which can be operated with simple, pulse-width-modulated (PWM) electronics. For compactness, the gearmotor and ball screw are placed side-by-side with a small toothed-belt drive connecting them. Each actuator is mounted to the link through a steel flexure that accommodated the slight lateral movement of the screw as the joint angle changes.

A novel feature of this design is the overload mechanism or "snubber." It is designed to absorb the kinetic energy of the links and motors when the mechanical stops are reached, and to accommodate imposed loads on the snake without damage to the actuators or structure. Belleville spring washers--4 series sets of 3 parallel-stacked washers--are mounted in the "snubber housing" such that the ball screw can move axially by 1mm if the preload value is exceeded. The thrust load of the screw is taken by a custom-made, 4-point-contact bearing that is integrated into the snubber housing.

Each link is 41.7mm in diameter, 96.0mm long (pivot-to-pivot), and weighs about 240g. The ball screws are 6mm diameter with 1mm lead, are rated at 700N, and are connected to the crosses at 14.7mm from the pivot. Motors are Maxon RE-13 (13mm diameter) gearmotors with 16.58:1 planetary gear reducers and 16-count encoders (64 counts per revolution with quadruature decoding). These develop about 38mNm of continuous torque; this translates to 380N of force at the ball screw (well below the rated load), considering the 2:1 belt drive and transmission efficiencies. The snubber mechanisms are preloaded to about 600N to protect the ball screws and bearings from overload; no displacement occurs until this load value is reached, so the normal stiffness of the structure is not compromised. Motor no-load speed at the nominal 12V input is 8900RPM, which corresponds to 5s time to travel the full 22.4mm of screw travel. Joint angular travel is about +/-55 degrees.

Tests of the joints showed that the actuators can produce 4.5Nm of torque at 12VDC (0.40A). That is, each ball screw produces 307N at 14.7mm radius on the U-joint cross. Based on the expected 5.08mNm at 0.40A, theoretical output would be 1060N with 100% transmission efficiency. This indicates that overall drive efficiency is only
(307N/1060N) 29%, much lower than predicted (48%). We will investigate this to see if significant increases in efficiency and output torque are possible.

The torque about a joint needed to "cantilever-lift" (lift when extended
horizontally) a single joint, assuming its center-of-mass (COM) to be at its geometric center, is 0.113Nm. The torque to lift n joints is n-squared times this. Given 4.5Nm available joint torque, the snake should then be able to cantilever-lift 6 joints. Tests on the complete snake robot confirm this capability. This ability is important to allow the snake to achieve arbitrary configurations working against gravity.

At present we have a 7-link, 14-actuator snake assembled and working. The U-joint cross at one end is mounted to a fixed base. Joint actuators are individually controlled by 14 switches, allowing the snake to be moved into arbitrary configurations. Ultimately we need to have the snake under computer control so that the tip can be moved to the desired position and orientation while the body of the snake obeys constraints of the environment, etc. To this end, we are developing and electronics "bus" system that will carry power and signals between the actuators and sensors on the snake to a control computer. Hard wiring to all 14 actuators and encoders would require (14 x 8) 112 conductors and was deemed unfeasible. The plan is to use an I-squared-C bus on the snake to connect microcontrollers on the actuators to the control computer. The technology is available, but packaging the required components (H-bridge, decoder chip, PIC microcontroller plus passive components) to fit within the link envelope, and providing interconnects between controllers, are challenging problems. At present the concept is to use one PC board per actuator, and have these mounted on each side of the U-joint crosses; a preliminary look suggests that the components may barely fit in the space available.

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