PRACTICAL CONSTRUCTION

So, having explained the arm we can move on to see how to build it: all the necessary parts are available in assembly kits, in laser-cut plexiglass, complete with accessories for assembly such as screws, bolts, etc. However, nothing prevents you from making them yourself, however, by using plastic material and maybe with 3D printing.

In any case, once in possession of all the component parts, the mechanism is started by assembling the support base with the fifth wheel and adding to it the hub of the servo control which activates the rotation of the arm on the base. The plastic hub supplied with the servo must be fixed with 4 2.5×12 self-tapping screws complete with 3×6 flat washer (the screws must not be tightened so as to allow the hub to align correctly with the rotation servo pin that will be mounted later).

Once the support base has been put together, you can assemble the arm, starting with the rotating base: take the 5 mm plexiglass right shoulder, to which you must attach the 13 kg/cm servo with 4 M4x14 TCEI screws and 4 self-locking M4 nuts and then insert the cable into the indicated slot. Also, mount an analogous servo on the opposite shoulder (SX) (but interposing 4 4×4 mm ABS spacers between the two elements) securing it with 4 M4x18 TCEI screws and 4 M4 self-locking nuts. Also, in this case, you have to insert the servo cable into the special slot on the robot’s shoulder. Then assemble the lower part of the arm using the screws and the spacers with the hexagonal column, then fixing one side to the hub of the 13 kg/cm servo control mounted on the shoulder, obtaining the assembly seen in Fig. 2.

Now take the two “arm 2” elements in 3 mm plexiglass and connect them with M3x8 TB cross screws complete with 3×6 flat washers and 2 hexagonal F / F M3x15 spacers, obtaining the block seen in Fig. 3. You have thus made the forearm, which will then join the first portion of the arm and the return rod. Then take this 3 mm plexiglass rod and fix the lower end to the level of the corresponding servo, whose opposite side will be connected through the appropriate hub to the corresponding servo control (interposing 3×6 mm flat washers).

Fig. 3

At this point, apply the intermediate support of the rotating base and in it introduce the axis of rotation of the initial part of the arm, which will also support the hubs of the two opposed 13 kg/cm servos and the operating lever of the return rod which will command the forearm. The set will result as shown in Fig. 4.

Fig. 4

Before fixing everything to the base, at least it must have completed the essential structure of the arm, hinging the forearm to the first arm portion and applying the return rod and the rocker arm, obtaining what is seen in Fig. 5.

Fig. 5

Then apply the base rotating with the first part of the arm and the return rod, to the fifth wheel, using the appropriate M4 screws to be inserted from the bottom into the holes of the upper part of the base bearing and then introducing the M4 nuts that you have previously interlocked in the special cross seats made in the lower part of the shoulder or rotating base (Fig. 6). At this point you have to assemble the fitting (wrist) between forearm and gripper, hinging it through the plastic bushings (using spacers) to the final part of the forearm and the return rod that starts from the elbow bar and which will allow you to lift and lower the fitting itself. On the inside of the wrist, you will have to place the small servo that will rotate the gripper, fixing it as expected.

Fig. 6

The gripper must then be applied to the wrist with its actuator and the relative servo control, which must be coupled by means of the lever supplied to the servo itself and the return rod which will act on one of the two jaws; this jaw will be meshed in the other before closing the gripper with the appropriate 3MA screws, in order to obtain a stable assembly.

The gripper, the terminal part of the robotic arm, can be oriented horizontally or vertically; the numerical references in Fig. 7 explain how to mount the wrist in the first case (right) and in the second (left).

Fig. 7

USE

Once the robotic arm has been completed, it is necessary to fasten the base to the support surface by means of screws or suction cups. Alternatively, the tendency to overturn in extreme conditions can be countered by applying a mass-weight counterweight at the control board.

The 5 mm plexiglass support element, between the axes of the two servos that activate the arm, allows you to constrain the arm shoulder to the servo support base.

Note that as at each ignition each servo quickly returns its pin to the central position, before turning off the system it is always necessary to position the arm elements so that the servos are at rest; otherwise, the abrupt repositioning at switch-on can over-stress the mechanics and damage it in the long run. As for electronics, remember that the Octopus controller, being an I²C-Bus device, like all peripheral devices of this kind, must be addressed to ensure the univocality of the commands directed to each shield; for this purpose it has three jumpers to be set differently from one shield to the other, which allow 8 logical combinations.

Once the address has been chosen, it must be returned to the library called up by the firmware.

In Listing 1 we propose a simple test sketch that in the Octopus output section shows the assignment of the shield outputs (not to be confused with the Arduino pins) to the servo of the arm portions; you can change exists but you have to specify them in the sketch; in it, base is the shoulder, wrist etc. The loop performs the opening and closing of the gripper by moving the corresponding servo clockwise and counterclockwise alternately, with a range between 50 and 130°, with a period equal to 15 and a value frequency of 45.