1、Rapid Prototyping of Small Robots Draft Grigoriy B. Reshko Carnegie Mellon University Pittsburgh, PA, USA reshkori.cmu.edu Matthew T. Mason Carnegie Mellon University Pittsburgh, PA, USA matt.masonri.cmu.edu Illah R. Nourbakhsh Carnegie Mellon University Pittsburgh, PA, USA illahri.cmu.edu 1. Introd
2、uction This paper describes our work on high quality rapid prototyping. The focus is on techniques that produce prototypes of desired quality and yet do not take long to build. We present necessary information about methods of control, power, sensors, batteries, electronics, and more. We outline mat
3、erials, methods, and tools. We also explain how to use servomotors, Lego, and electronics to achieve satisfying results. Figure 1 shows an example of the kind of project. With the right tools and materials, and with parts on hand, the omni wheel telerobot took less than half an hour from conception
4、to completion. 1.1 Why prototype? We have noticed that some designers tend to prototype very quickly 1. Sometimes you can learn something important in a few minutes from a prototype that might have taken weeks or months otherwise. Mechanical design seems to require iteration, so, the faster you can
5、iterate, especially at the beginning of a project, the better. 1.2 Levels of prototyping Prototyping ranges from an idea to a complete product. The quality and the level of detail of a prototype should depend on its purpose. For example, Figure 1 shows the first prototype of the Palm Pilot Robot 5,
6、which at this point is nothing more than three servos taped to a receiver. This simple model is sufficient to fulfill its purpose of demonstrating holonomic motion of the base. Later we built a more complex prototype with sensors to show that the robot can follow walls. 2. Control We examined and te
7、sted three types of control of prototypes: human powered prototypes, remote control, and computer control. 2.1 Human powered prototypes One of the most common goals of prototyping is to demonstrate a concept or an idea. Usually complexity of a prototype varies with the complexity of an idea. Relativ
8、ely simple or predictable prototypes do not need motors and batteries they can be powered and controlled by humans instead. An example of a human-powered prototype is a regular four-wheel LEGO vehicle in Figure 2. There is no need to attach motors and remote-control equipment to demonstrate that it
9、can move back and forth. On the contrary, it is impossible to demonstrate wheel traction simply by turning the wheels. Therefore, prototypes in which human Figure 1 Figure 2 Rapid Prototyping of Small Robots 2 intervention affects the results require a remote or computer control system. 2.2 Remote c
10、ontrol Human remote control is useful when precise or unusual methods of control are required. It is also the simplest way to control a prototype without actually physically interfering with it. For example, the fastest way to determine what happens to a robot that rapidly accelerates and then decel
11、erates is to use a remote control system. A simple and easy to use remote control system for model airplanes will suffice to demonstrate whether such robot will immediately come to a stop or skid for a certain distance. Such control system has four channels and provides good range, reliable communic
12、ation, and up to four proportional controls, which allow smooth adjustment of speed or angular position of servos. An example is Three-Wheel Mobipulator robot. Human remote control provides an easy method of control of the steered wheel and the brake at the same time, while not requiring any program
13、ming or additional electronics. 2.3 Computer control When a robot requires precise and complex operations or is involved in an application that requires sensors or computer feedback to take actions, the computer control becomes the right choice. An example of such control is a simple autonomous prot
14、otype with encoders on its wheels driving in a straight path. Companies develop a variety of boards with different capabilities that can control servo and stepper motors by means of serial or parallel communication with a computer. Some boards just translate commands from a computer to PWM signals,
15、whereas others are programmable. Pontechs SV203 board is a good example of a relatively simple and inexpensive computer control system. It uses serial communication to talk to a computer and can run by itself. However, it is very slow and simplistic compared to a desktop computer. Most boards have v
16、ery limited number of functions and the amount of memory. Although the board by itself can be sufficient for a number of applications, the two alternatives are to use a tether and connect a robot to a reasonably powerful computer, or to use a more complex board such as 68HC11 chip or BASIC stamp. An
17、other advantage of computer control over the previous two methods is the ability to use sensors. Most control boards have already built-in analog and digital inputs and can connect to a variety of sensors. 3. Power Size, shape, and voltage of batteries depend on its application. For small-scale prot
18、otypes, we use 6V or 4.5V Nickel-Metal Hydride (NiMH) battery. It is fully rechargeable and can provide steady voltage for a relatively long time. These batteries can be used for servomotors and electronics, because they provide high current. A small regular 9V or 6V NiMH with a voltage regulator ca
19、n be used to power electronics, since it does not require high current. Tethered models can be powered externally, since they already require wires for communication. External power supply has an advantage of providing stable high-current output without a need to recharge. 4. Sensors When controllin
20、g a robot by a computer, or when it is an autonomous system, sensors become an integral part of prototyping. 4.1. Digital and analog sensors There are two types of sensors: digital and analog. Digital sensors, such as micro-switches or pushbuttons, are easy to implement by interfacing them directly
21、to the electronics. Micro switches or push buttons are small switches that are useful when physical confirmation of a contact is required, such as in obstacle detection. Analog sensors provide a range of output values, but usually require an analog-to-digital converter to convert their values into a
22、 binary number, which a computer can understand. Analog sensors can also be converted to binary sensors by using a voltage divider, which usually requires one additional resistor connected to the ground. Analog sensors 2 include the following: Rapid Prototyping of Small Robots 3 Photoresistors are v
23、ariable resistors that change their resistance due to the intensity of visible light. Phototransistors are similar, but provide greater sensitivity. Photodiodes have even greater sensitivity and respond rapidly to changes in illumination. Infrared sensors are used as proximity and reflectivity senso
24、rs and are almost unaffected by visible light. Pyroelectric sensors respond to very small temperature changes, because its crystal induces a charge when it is heated. Bend sensors have conductive ink between two electrodes to give a variable resistance, depending upon the degree of bending. Total re
25、sistance changes by a factor of about 3 to 5 as the bend sensor goes from straight to maximum bend. Force-sensing resistors are also based on conductive ink technology. They are very sensitive, and their resistance is changed by several orders of magnitude as force is applied. Rubbery ruler is a uni
26、que tube-like sensor that measures change in its length. Inside the tube has two wires, and when it stretches, the distance between the wires increases and the capacitance decreases. Microphones provide a way to add sound detection ability to a robot, although it usually requires additional electron
27、ics. Piezoelectric film sensors detect vibrations, changes in applied force, and changes in temperature and far-infrared radiation. Sonar sensors measure distance to an object. Tilt sensors have a drop of electrolytic fluid between its electrodes. The amount of conduction between the center electrod
28、e and each of the outer electrodes is determined by the degree to which the outer electrode is immersed in the electrolytic fluid. Shaft Encoders are important for precise control of motors and can be implemented using IR or light sensors. Proprioceptive sensors measure the internal state of the rob
29、ot, such as battery-level sensing, stall current sensing, etc. 4.2. Infrared Rangefinders Sharp GP2D12 infrared triangulation sensors (distributed by Acroname, Inc.) are very accurate rangefinders. These devices use linear position sensitive devices (PSD) and their measurements are accurate to appro
30、ximately 1cm Appendix A. Sensors return their readings as variable voltage, ranging from 0 to 5 volts. Analog-to-digital converter is used to convert this analog value to digital reading ranging from 0 to 255. The sensors manual includes a graph of sensors value versus distance. Distance is approxim
31、ated by using a power regression: d(s) = 2141.7*s-1.07886 where d is distance in cm and s is 8-bit sensors value from 0 to 255. IR sensors require significant amount of power. When using small boards, such as SV203, the sensors should be connected to unregulated 6V, instead of regulated 5V. Outputs
32、of these sensors are connected to boards analog input pins. 5. Components 5.1 Fasteners The most common type of fasteners in small-scale prototyping are little nuts and bolts. They come in variety of types and sizes. In our prototypes, we use 4-40” and 6-32” hexagonal head bolts and nuts, since they
33、 are appropriate for small-scale models. Usually, we use regular steel nuts, however plastic lock nuts and nylon insert nuts are useful for variable tightening of a bolt. 5.2 Materials Aluminum: We found aluminum to be one of the best prototyping materials. It is lightweight and can be easily machin
34、ed. A specific advantage of aluminum is its pliability. It can be bent at virtually any angle, although it depends on its thickness. ” plates can be bent up to 90, whereas thin plates can be bent all the way to 180. Aluminum can also be drilled, tapped, or cut more easily than other materials, such
35、as steel. Rapid Prototyping of Small Robots 4 Acrylic plastic: Acrylic plastic is also an interesting material. It can be cut and filed to provide almost perfect edge. It is also easy to drill and tap. One of its advantages is that two pieces can be connected together in any fashion by using acetone
36、 or other solvent-welder. A solvent bond is similar to a weld, so it is extremely durable. Furthermore, unlike other glues, solvent securely connects two materials in very short time. Polycarbonate plastic: Like acrylic, this plastic is easily machined. It can be cut, milled, and drilled. However, i
37、t is quite difficult to tap, because the material can be compressed. Its unique advantage is that it can be bent without heating, for example, using a brake. Most plastics either break during even minor bending or require to be heated. Another interesting property of polycarbonate plastic is its str
38、ength a ” thick sheet is bullet proof. 5.3 Lego Lego by itself is a great kit for prototyping. Complex parts, such as gear trains, arms, and steering mechanisms can be implemented quickly and easily using Lego parts for construction 3. 5.3.2 Lego structures Sometimes it is faster to use Lego than to
39、 manufacture similar parts using usual materials. A good example is Lego wheels. Manufacturing a wheel is not an easy task and usually requires some time, whereas Lego wheels come in many shapes and sizes, such as a smooth-surfaced pulley or a large traction-wheel. 5.3.2 Lego Wheels Small wheels and
40、 pulleys can be glued to servos with a heat-melt glue, whereas big Lego traction-wheels can be securely attached to CS60 servos, because servos connector fits nicely between wheels plastic and rubber. An easy way to attach free spinning wheels to a prototype is to connect small Lego wheels to a 2x1
41、Lego beam with a hole in the middle by a Lego connector peg. The Lego beam is then glued to a structure, for example, an aluminum angle in Figure 3. Larger wheels require two 4x1 or larger beams and a Lego axle that goes through the beams and the wheel. 5.4 Double-sided Foam Tape and Variable-rigidi
42、ty attachment Sometimes it is useful when servos are not rigidly attached to a structure, but are rather flexible. For example, it is definitely a plus for a four-wheel vehicle, since it ensures that all wheels are going to be on the ground at the same time. (It is difficult to achieve the same resu
43、lt with a rigid attachment, since it is almost impossible to make four perfectly leveled supports for servos). One of the fastest, but also secure ways to do that is to attach servos to the base using a double-sided sticky tape. Such tape is usually quite thick, which allows servos to have some flex
44、ibility. Structures that are more flexible require a few more layers of tape. By varying area covered with tape and tapes thickness, it is possible to get any degree of flexibility of a connection from very rigid (such as one layer of tape covering the whole servo) to very flexible. In Figure 4 all
45、servos, a receiver, and a battery pack are connected by double-sided tape to a rectangular piece of plastic. Furthermore, the tape is strong enough to hold aluminum legs attached securely to plastic servo connectors. Figure 3 Figure 4 Rapid Prototyping of Small Robots 5 5.5 Velcro A simple and yet s
46、ecure way to attach a receiver or a battery to a robot is to use Velcro. A small 1”x1” piece on a robot and at the bottom of a battery or a receiver will securely hold it in place. Velcros advantage is that it can be detached this is not the case with other adhesives. 6. Servomotors Servomotors can
47、be used very efficiently for prototyping as primary drive motors, because of their mounting design, built-in gear reduction, and simple remote control. Lightweight and compact receiver-battery pack is also useful at relatively small models (ex. when a model is just twice the size of a receiver). Fur
48、thermore, servos can provide both types of actuation: angular position and velocity control. Angular position control means that servos input controls its position, whereas velocity control affects servos velocity. 6.1 Modular assembly and servo case as structural element When a prototype is relativ
49、ely small, servos take a lot of space. Instead of using servos purely as rotary actuators, it is more efficient to use their cases as structures of a robot. A simple spherical actuator in Figure 5 can be made by taping one servo to another. By using only three aluminum angles and a tape, it is possi
50、ble to make a three joint leg (Figure 6). Three servos provide not only three actuators, but also three links. Because, servos have very high torque-to-weight ratio, they make up very efficient legs. Such leg can lift a body that is much heavier then the leg itself. To increase durability, C-shape a
