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382 BUILD A REVOLUTE COORDINATE ARM
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FIGURE 25.4 Forearm elbow joint detail. a. Complete joint; b. Exploded view.
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POSITION CONTROL 383
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FIGURE 25.5 A close-up view of the elbow joint.
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behind it. This may interfere with the operation of the arm or robot, however, so you may want to opt for counterweights attached to the end of the arm. I successfully used two four-ounce fishing tackle weights attached to the arm with a 2-inch-by-3/4-inch corner angle bracket (see Fig. 25.8).
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Position Control
The stepper motors used for the shoulder and elbow joints of the prototype provide a natural control over the position of the arm. Under electronic control, the motors can be commanded to rotate a specific number of steps, which in turn moves the upper arm and forearm a specified amount. You should supplement the open-loop servo system with limit switches. These switches provide an indication when the arm joints have moved to their extreme positions. The most common limit switches are small leaf switches. You can also construct optical switches using photo-interrupters. A small patch of plastic or metal interrupts the flow of light between an LED and phototransistor, thus signaling the limit of movement. You can build these interrupters by mounting an infrared LED and phototransistor on a small perforated board or purchase ready-made modules (they are common surplus finds). When using continuous DC motors, you need to provide some type of feedback to
384 BUILD A REVOLUTE COORDINATE ARM
FIGURE 25.6 The motor mounted on the shoulder.
FIGURE 25.7 The completed arm, with gripper (hand) attached.
POSITION CONTROL 385
FIGURE 25.8 Counterbalance weights attached to the end of the forearm help redistribute the weight. You can also use springs, which will help reduce the overall weight of the arm.
report the position of the arm. Otherwise, the control electronics (almost always a computer) will never know where the arm is or how far it has moved. There are several ways you can provide this feedback. The most popular methods are a potentiometer and an incremental shaft encoder.
POTENTIOMETER
Attach the shaft of a potentiometer to the shoulder or elbow joint or motor (see Fig. 25.9), and the varying resistance of the pot serves as an indication of the position of the arm. Just about any pot will do, as long as it has a travel rotation the same as or greater than the travel rotation of the joints in the arm. Otherwise, the arm will go past the internal stops of the potentiometer. Travel rotation is usually not a problem in arm systems, where joints seldom move more than 40 or 50 . If your arm design moves more than about 270 , use a multiturn pot. A three-turn pot should suffice. Another method is to use a slider-pot. You operate a slider-pot by moving the wiper up and down, rather than by turning a shaft. Slider-pots are ideal when you want to measure linear distance, like the amount of travel (distance) of a chain or belt. Fig. 25.10 shows a slider-pot mounted to a cleat in the timing belt used to operate the elbow joint. The value of the pot is a function of the control electronics you have hooked up to it, but 10K to 100K potentiometers usually work well with most any circuit. The potentiome-
386 BUILD A REVOLUTE COORDINATE ARM
Potentiometer
Motor
FIGURE 25.9 Using a potentiometer as a position feedback device. Mount the potentiometer on a drive motor or on a joint of the arm.
Slide potentiometer
Movement of Drive FIGURE 25.10 Using a slide potentiometer to register position feedback. The wiper of the pot can be linked to any mechanical device, like a chain or belt, that moves laterally.
POSITION CONTROL 387
Wiper connects to circuit or ADC FIGURE 25.11 The basic electrical hookup for providing a varying voltage from a potentiometer.
ter may provide a relative measurement of the position of the arm, but the information is in analog form, as a resistance or voltage, neither of which can be directly interpreted by a computer. By connecting the pot as shown in Fig. 25.11, you gain an output that is a voltage between 0 and the positive supply voltage (usually 5 or 12 volts). The wiper of the pot can be connected to the input of an analog-to-digital converter (ADC), which translates voltage levels into bytes. Now, before you go off screaming about the complexity of ADCs, you should really try one first. The latest chips are relatively inexpensive (under $5) and require a very minimum number of external components to operate. The best part about ADC chips is that most have provisions for connecting eight or more analog signals. You select which signal input you want to convert into digital data. That means you can use one $5 ADC for all of the joints in a two-arm robot system. To be useful, the ADC should be connected to a microprocessor or computer. You can also use your personal computer as the controlling electronics for your robot. Read 29, Interfacing with Computers and Microcontrollers, for more information about ADCs and computer control. Also note that some microcontrollers have their own ADCs built in. For example, the BasicX-24 from NetMedia sports eight ADC inputs; the OOPic microcontroller offers a pair of ADC inputs. Neither of these microcontrollers requires any external components to be connected to the ADC inputs. See Chaps. 32 and 33, respectively, for more information on the BasicX and OOPic microcontroller chips.
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