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Design by GRAEME MATTHEWSON A 2-axis robot with gripper Are you a control freak? Do you wish to exert power over things animate and inanimate? Well, here’s a way to indulge yourself. Build this simple two-axis robot which has a gripper to pick up and place objects. This robot can be controlled from your PC using a QBASIC program via the serial port. Don’t worry – you don’t have to know anything about programming in BASIC to make it work. Just go to the DOS prompt, type QBASIC and run the program which is called Ausbot.bas. Apart from the simple method of control, a major attraction of this robot 40  Silicon Chip is the motive power. It is provided by cheap and readily available servos, as used in radio controlled model cars, boats and aircraft. These can be purchased from model stores everywhere or you might have some servos from model cars lying around – these will do just as well. The servos provide the two axes of operation for the robot arm; ie, up & down or sideways motion and also open and shut the gripper. So just three servos are required. As the title of this article suggests, this is a 2-axis robot with a gripper. It can rotate on its base through 90 de­grees with extremely small movements: 254 steps of 0.354 degrees. It can raise and lower its arm from desktop level in 254 x 0.807mm steps to a height of 205mm. Similarly, it can open its gripper to 105mm wide or fully close it in 254 steps of 0.413mm. With this sort of resolution this robot can pick up an egg without breaking it! Its fingertips can be made to pivot slightly so as to grasp irregular shaped objects or they can be tightened to grasp small items at their tips. The Ausbot software takes care of speed control and only allows you to set speeds within safe limits for the robot. The required speed limits for each servo are different as each one controls items of different length and weight. If the speed ranges allowed by the software are too fast or too slow, the upper and lower limits may need to be changed due to varying clock speeds of different computers. The software is easily understood and the user should have no difficulty in identifying the delay lines for each servo. As described in the “Radio Control” column in last month’s issue, a servo is basically a closed loop system, you just tell it where to go and it goes there; no argument. It will operate from between 4V and 6V DC and requires pulses of between 1ms and 2ms, at a rate of about 50Hz. Also as described last month, 1.5ms Fig.1: the robot has three servos controlled by a Mini SSC (serial servo controller). The SSC is controlled with a QBASIC program via the serial port on a PC. wide pulses will rotate the servo shaft to it its “neutral” or null position; ie, more or less its central position. Furthermore, pulses 1ms wide will rotate it to the fully anticlockwise position Base Assembly while 2ms pulses will rotate it fully clockwise. And since a servo is a closed loop system, it has a very large number of positions in between those extremes, SHOULDER ASSEMBLY DOUBLE SIDED ADHESIVE TAPE CABLE-TIE BASE PLATE RUBBER FEET Fig.2: the base assembly uses an aluminium extrusion measuring 3mm thick, 120mm long, 80mm wide and 20mm high. A servo is cen­trally located against the vertical section and simply secured with double sided adhesive tape and a Nylon cable tie. December 1997  41 Shoulder Assembly DOUBLE SIDED TAPE CABLE-TIE NYLOC NUT ARM M3 X 10 SCREW WASHER SHOULDER M3 X 10 SCREW SERVO DISC NYLOC NUT 4mm x 1mm "O"RING NYLOC NUT Fig.3: the shoulder assembly diagram is made from a piece of T-section aluminium and is attached to the arm, also made from T-section aluminium. The arm has two servos which are attached to one end using double sided tape and a Nylon cable tie. limited only by the resolution of its internal feedback pot. Servo drive How are the servos driven? Normally, you would need three variable width pulse generators, one for each servo. And then the driving computer would need to vary the pulse generators in response to the QBASIC program. That approach could have been taken but in this case a Mini-SSC has been used. Er, what’s a Mini-SSC? It stands for Mini Serial Servo Controller. In turn, the Mini-SSC is based on a PIC-series microcontroller. The Mini-SSC comes fully programmed. It accepts commands in ASCII on its serial port and then provides pulse signal outputs for up to eight servos. Fig.1 summarises the robot concept. You have a PC (well, you must have one if you want to control this robot) which feeds a serial port on the Mini-SSC and it is being used to control three servos. It generates all the puls42  Silicon Chip es to operate the servos so no other circuitry is required. That’s another bonus of this project – you don’t have to build any electronics circuit boards; the Mini-SSC can be purchased assembled and read to go. Or if you want, you can buy it in kit form. As it stands, the software supplied with the Mini-SSC does not address the problem of servo speed. Typical servos are cap­able of rotating through 90 degrees in about 350 milliseconds but that is much too fast for operating this robot, whether we are concerned with motion of the arm or the gripper. This drawback is taken care of by the QBASIC program writ­ten for this project. At startup the Ausbot software prompts the user to enter the desired speed for each servo, then which servo to move and which position to move to. The current position of each servo is also printed at the bottom of the screen, along with the speed. The Mini SSC starts in position #127, the servo neutral, on power-up, so to avoid servo damage the Ausbot software also provides a park function which parks all servos at position #127. The robot should be parked at the end of each session. If the robot is moved from this position when not in operation it should be gently moved into the park position before power-up, otherwise there is the possibility of damage to the servos as they initial­ly try to take up the neutral position. Parts availability All of the components and materials used for this robot were chosen for their availability. The arm and gripper is based on a T-section aluminium extrusion which is readily available through aluminium suppliers such as Capral or good hardware stores. Most of the other hardware involves pushrods and servo links which again are readily available from most model stores under the “Kwicklink” brand name. The parts for the prototype were purchased from Vaggs Radio The two servos attached to the arm operate the gripper and provide the vertical motion. The third servo at left rotates the arm on its base. This close-up view shows the underside of the arm at the servo end. Note that one servo is mounted upside down with respect to the other. The gripper fingers are operated by a Y-pushrod assembly linked to two bell cranks. The other end of the pushrod assembly goes to one of the servos on the end of the arm. Ausbot can open its gripper to 105mm wide in 254 steps of 0.413mm. With this sort of resolution this robot can pick up an egg (or a light bulb) without breaking it! Models at Miranda NSW. Phone (02) 9525 5797. You will be able to put the whole project together with just a soldering iron, a drill, a hacksaw and a file or emery paper, a screwdriver and a spanner. Building it There are three major assemblies in the robot. These are the base assembly, the shoulder assembly involving two servos and the arm, and the gripper assembly, the latter involving another servo, a couple of pushrods and two bell cranks. Let’s start with the Base Assembly – see Fig.2. This involves a length of aluminium extrusion measuring 3mm thick, 120mm long, 80mm wide and 20mm high. A servo is centrally located against the vertical section and simply secured with double sided adhesive tape and a Nylon cable tie. The software runs in QBASIC and is easy to drive. All you have to do is enter data at the screen prompts. The servo is fitted with a “servo disc” and this attaches to the shoulder piece. This is shown in detail in the Shoulder Assembly diagram – see Fig.3. Made from a piece of T-section December 1997  43 FINGER TIP 1/8 WASHER M3 X 20 SCREW RUBBER PAD BRASS TUBE 1/8 WASHER 5/32 WASHER M3 X 10 SCREW FINGER BELL CRANK M3 X 6 SCREW Gripper Assembly NYLOC NUT ARM Fig.4: the gripper assembly has two fingers, each operated by a standard bell crank linked to a Y-pushrod assembly and one of the servos attached the arm. aluminium, the shoulder piece dimensions are 40mm high and 40mm wide. The other dimen­sions can be estimated from Fig.3. Attached to the shoulder is the arm, made from T-section aluminium, 20mm x 20mm and 270mm long. The arm has two servos which are attached to one end, again with double sided tape and a Nylon cable tie. Both of these servos are fitted with standard servo arms, one at top to operate the gripper and the other below, to provide vertical motion. This lower servo arm is linked to the shoulder piece via a short pushrod with Kwicklink attached. Finally, there is the Gripper Assembly which is shown in Fig.4. The Gripper Assembly has two fingers, made from 10 x 3mm aluminium flat bar, 95mm long. Each finger is operated by a standard bell crank linked to a Y-pushrod assembly with three Kwicklinks. Each finger has a swivelling fingertip fitted with a rubber pad. These allow the fingers to grip SC smooth or irregular objects. Kit Availability The Mini Serial Servo Controller (Mini-SSC) is based on a PIC-series microcontroller and comes fully programmed. It can be purchased fully assembled and ready to go. 44  Silicon Chip As already indicated, this project requires little more than a soldering iron and a few other tools. It will be available as a full kit of parts and working drawings. Most people should only take a few hours to put it together. All the parts for this robot and the software are available from Oatley Electronics who own the design copyright. Their address is PO Box 89, Oatley, NSW 2223. Phone (02) 9584 3563; fax (02) 9584 3561. The prices are as follows: Software disc plus copies of detailed plans ............................................$14.00 Mini SSC .................................................................................................$55.00 Kit of machined aluminium parts .............................................................$21.00 Servo kits........................................................................................$15.00 each Please add $5 for postage and packing.