Design of the first prototype
Hardware Structure
The tensegrity structure consists of isolated compression rods and a net of tension cables. The tensegrity robot can move by changing the length of the cables. To achieve the goal of actuating and controlling the robot, actuators, sensors, PCB boards and batteries are needed to control the length of the cables. It is natural that all the actuators, sensors, PCB boards and batteries are located on the rods which more rigid than the cables.
In the first prototype, all the electronic components and designed to be in end caps at the two ends of each rod. All the end caps are the same and can be divided into 5 parts, as is shown in Figure 1.
Figure End Cap
The robot PU shoe on the top of the cap is designed to protect the rod when its end contact with the ground.
The spool cap is designed to control the length of the cable. A spool shaft is inside the spool cap and connected to the motor. The cable twines around the shaft and the length of the cable between two rods changes when the shaft rotates.
The third part mainly consists of four brackets, two side brackets, one motor drive bracket and one sensor board bracket. Two PCB boards (motor drive board and sensor board) are fixed on the motor drive bracket and sensor board bracket and the motor is located between these two boards. Along with the two side brackets, the four brackets together supports the spool cap above them.
Below the four brackets is the battery holder, which holds and protects the battery and power board inside it.
A tube containing CSC spring is connected to the battery holder by a specially designed collar. Two CSC springs inside the tube are used to sense ….
A lot of design work are done on these 5 parts, these article focuses on how the four brackets are designed and connected to other parts.
Redesign of Brackets
There is a previous design of the brackets. However, the previous design was only a rough design without considering anything about the strength and design for manufacturing. In the new design of the brackets, all the key dimensions from the old design. In the process of redesigning the brackets, first the thickness and bending radius was determined. Based on the thickness and bending radius, the base and the flange of the brackets were then designed to connect with the brackets to the spool cap, battery holder and two PCB boards. Then using the mechanical property of the Aluminum 6061 to verify whether the brackets can stand the required force.
Determine the thickness and bending radius
The functions of the four brackets are providing places to install the PCB boards and support the structure. The brackets are connected to their adjacent components by screws as in figure 2.
The relative position of the holes in the brackets are fixed so that even the brackets and other components are designed separately, there will be no conflicts when assemble all the components. The relative position of the holes in the brackets are shown in figure 3.
Assume the thickness of the sheet material is t and the bending radius is r. From the sheet metal design handbook, for relatively soft material such as aluminum sheet, the bending radius is recommended to be at least twice the thickness of the sheet. So let r = 2t.
For the motor drive bracket and sensor board brackets, there should be enough place for the motor between them. The diameter of the motor is 22 mm. Since the hole on the flange is simply for clearance and will not be tapped, there is no minimum distance required between the hole and the bending edge [1]. However, the distance do need to be larger enough so that head of the screw will not interfere with the bending edge when assembling, in this case the distance should be larger than the bending radius r plus the radius of the head of the screw, which is 3.5 mm. As is shown in Figure 3, on the bottom of the two brackets, the distance between the two holes is 46 mm,