tools
Screw fastening is a challenging task in robotic assembly. Fastening via rotating a robot joint is generally not efficient or feasible, so we use external screwing tools.
We used two types of screw tools (either metal and 3D-printed), which share some characteristics:
- They contain compliance (bit cushion) to allow easy force application, without requiring impedance control.
- They are driven by very affordable Dynamixel XL-320 motors, allowing stalling detection. The downside of this motor is the low maximum speed (114 rpm).
The most significant design difference between them is the use of suction.
The M3 and M4 screw tools used a suction cup to pick the screw and detect success. The parts used in these tools are sold by Sawa, a manufacturer in Iwate, Japan. One tool should cost between 1000-1500 USD, which is expensive for research, but they are extremely durable and reliable. The picture below shows the M4 metal screw tool:
We also propose a set screw tool make from 3D-printed and standard parts, which uses no suction and is significantly cheaper. It can be used with wrenches like this that lock into the screw head, so that screws can be picked. It is possible to complete the assembly task with tools of this kind. The total cost of this tool should be below 100 USD (the costliest part are the linear guide rails).
This section details the 3D printed tool's design and assembly, so that others can reproduce a screwing system at low cost.
We use DYNAMIXEL XL-320, a compact and precise servo motor, which costs only 24 dollars. It offers limitless rotation and a maximum torque of 0.39 Nm. Using a servo motor as a driver allows us to detect motor stalling (screw success) and . A Power Hub Board is used for power supply, and a U2D2 for controlling and operating the motor from the PC. Cable 3P-XL is the cable set exclusive for the XL-320 (packaged with the motor and U2D2).
As this motor has no threaded hole to attach it, we made a shell for it instead.
We make use of the anchor holes on the motor body for fixing, as no bolt fastening holes are available. Four clamp plates with locating pins are designed to surround and button the motor, and they are further fastened to each other by bolts, which fixes the motor via form closure. These four plates form the shell of the motor. The shell is equipped with two slide rails and springs, which enables the tooltip to have compliance in one axis.
Uncertainty is always a difficulty in screw fastening tasks, and one simple way to deal with it is to make the tooltip compliant. In this design, we introduce compliance in the motor instead of in the tooltip. this way, the screw tip can be thin and fit into narrow spaces. Thus, we mount two slider rails on the motor shell and the rails are fastened on the tool shell. This mechanism guides the motion of the motor body. On the backside of the motor, another clamp plate is attached, and the locating pins here are used to hook the spings. The holder is fixed on the back end of the tool shell. The holder does three things: giving pre-stress on the springs, constraining the extreme position of the motor, leaving paths for the cables.
The tip of the screwdriver should be strong and accurate. 3D printing or making it from scratch is impractical, so we designed a connector that allows using a commercial tool, e.g. a hexagonal (Allen) wrench. The connector holds the wrench using force closure. It has two through-holes to attach to the motor horn with screws, as well as two pins to align with the motor horn. We make use of the original motor flange to fix the connector. Two screw bolts are added to provide stable anchor points. Note: the screws here should be M2.6 counter-sunk flat-head screws to match the hold's size on the flange.
- DYNAMIXEL XL-320 x 1
- Power Hub Board x 1
- U2D2 x 1
- Slider Rails x 2
- Springs x 1 set
- M2 bolts and nuts x 1 set
- M2.6 bolts and nuts x 1 pair
- Note: Assemble and arrange the cable set of the motor at the beginning.
