Robotic Systems ECE 401RB Fall 2007

Transcription

1 The following notes are from: Robotic Systems ECE 401RB Fall 2007 Lecture 4: Actuators Part 1 Chapter 3, George A. Bekey, Autonomous Robots: From Biological Inspiration to Implementation and Control, The MIT Press, I. Actuators for Robots Dictionary definition A servomechanism that supplies and transmits a measured amount of energy for the operation of another mechanism or system. Animals Muscles are the actuators Moves a particular skeletal link around a joint. Changing its position relative to another link. Muscles receive control inputs and send back feedback signals. Robot actuators Analogous structures that move with respect to other links. Need not be imitations of animal structures. Joints can be rotary. - Like with animals. Or prismatic. - Telescoping Lecture 4, Page 1 of 18

2 Muscles can only exert force in one direction. - Pulling by contraction of the muscle. But robot actuators can exert forces both shortening and lengthening. Uses of actuators How can robots make use of the forces generated by actuators? Exert forces on the environment. Lifting and carrying. Locomotion. Grippers and other end effectors. Lecture 4, Page 2 of 18

3 Most actuators used with mobile robots are electric motors. Other actuators Artificial muscles. Electromagnetic actuators. Shape memory alloys. Plus a number of others. Pneumatic and hydraulic actuators are difficult to use with mobile robots. Since pumps and fluid reservoirs would need to be carried. But these actuators are good for industrial robots. - Due to the force they can generate. Electric motors Uses Wheel movement Joint rotations in hip, knees, or ankles of walking robots. Frequently are direct current servomotors (set an angular position and include control loop functionality). May also be stepper motors That step motors at given numbers of angular increments. May be employed in open loop or closed loop modes. Uses with rotary joint systems Can be mounted directly on the joints May increase the physical size and weight of a joint. May be undesirable for some applications. Can pull cables. Like tendons that connect muscles to bones. But can only exert force in one direction. - It is necessary to use a pair of cables to obtain bidirectional motion. Can be very complex to implement. - Example: Utah-MIT robot hand. - 4 fingers, each with 4 degrees of freedom - Sixteen joints - Sixteen motors and thirty-two cables. Lecture 4, Page 3 of 18

4 Motors produce rotational motion May need to convert this to linear translational motion. Methods Leadscrews Belt-and-pulley systems Rack-and-pinion systems Gears and chains Lecture 4, Page 4 of 18

5 Artificial muscles Animal muscles shorten when activated. Attach to bones on both sides of a joint - Longitudinal shortening produces joint rotation. Muscles shorten actively and lengthen passively. KcKibben muscle Pressurized air keeps the muscles long. Then rubberized material contracts when the air pressure is reduced. Shape memory alloys Contract when heated. - Most materials expand when heated. Lecture 4, Page 5 of 18

6 Gradually return to their original length when cooled. - Must add a cooling system if they need to return quickly. - Underwater cooling can help. Also do not generate very large forces. Electroactive polymers Similar benefits and drawbacks as shape memory alloys. Electromagnetic Actuators Devices like solenoids can be used to move short distances. Typically only two positions also. Like for closing contacts. Actuation is a major area of research. Consider ants that can carry an object several times their body weight. Robot actuators leave a lot still to be desired. The following notes are from: Chapter 19, G. McComb, and M. Predko, Robot Builder's Bonanza, Third Edition, Mc- Graw Hill, II. Choosing the Right Motor There are many kinds of motors Only a select few are truly suitable for mobile robotics. AC or DC? DC dominates the field of robotics On-board electronics Opening and closing solenoids. Running motors Few robots use motors designed to operate on AC. Not even robots in factories. Even there, robots convert AC to DC, then distribute DC to various subsystems in the machine. Lecture 4, Page 6 of 18

7 DC motors should be reversible for most applications. Few applications call for motors to just move in one direction. DC motors are inherently bidirectional. - But design limitations may exist due to the design of commutator brushes. - If brushes are slanted. - If internal wiring prevents reversal. Continuous or Stepping? Continuous motor Application of power causes the shaft to rotate continually. Shaft stops only if power is removed and the load slows the motor to a stop. Or a motor may stop if it stalls due to too much loading. Stepping motor Application of power causes the shaft to rotate a few degrees, then stop. Continuous rotation requires power to be pulsed to the motor in alternating polarities. - In the figure below, the rotor and stator have an unequal number of teeth. - When poles E-E are energized, teeth 4 and 8 line up with these poles. - Then if D-D are energized, the motor will rotate counterclockwise so that 3 and 7 are lined up. - The rotation will be 9º. - Spacing between stator teeth is 360/10 = 36º. - Spacing between rotor teeth is 360/8 = 45º. - Difference is 9º. Lecture 4, Page 7 of 18

8 Servo Motors These are a special subset of continuous motors. Commonly used in model and hobby radio controlled cars and planes. Continuous DC motor plus a feedback loop. Why would a stepper motor not need a feedback loop? We already know how far a stepping motor has rotated by counting how many pulses we have given it. We control the speed of response by how we give pulses. Lecture 4, Page 8 of 18

9 Other motor types Maybe useful for robots, maybe not. Brushless DC Commonly used in fans inside computers and for motors in VCR s and CD/DVD players. Synchronous Also called brushless AC. Note that AC motors do not always operate at 50/60 Hz. - Some operate at 400 Hz. Synchro Commonly used in pairs. Master motor electronically controls a slave motor. AC induction Ordinary type of AC motor. Motor Specifications Operating voltage DC motors will specify a range of voltages. - Typically 1.5 to 6 Volts for hobby motors. Some high quality DC motors are designed for a specific voltage V or 24 V Most motors can operate satisfactorily at voltages higher or lower than specifications. - A 12 V motor can operate at 8 V. - But will not be as powerful. - And will run slower. - Although most motors will refuse to run, or will not run well, at voltages less than 50% of specified rating. - A 12 V motor can run at 16 V. - Speed of shaft rotation and power will increase. - Not recommended to operate more than 30 or 40 percent above ratings. - Windings may overheat and cause permanent damage. - Ball bearings may not be able to handle the speed of rotation. Current draw A free-running (no load) motor may have very little current draw. But loading can require significant amounts of current to 1000 percent more current than free-running. Lecture 4, Page 9 of 18

10 For most permanent magnet motors, current draw increases with load. - Load (required torque) is measured as a product of mass and distance. - The figure below is lb-ft. Loads used by manufacturers when testing motors for current draw are not standardized. - So current draw for a particular application can be more or less than specified. Stalling occurs when the motor cannot give any more current to do more work. - The shaft will stop rotating. - Some motors (not many) are rated by their stalled current draw. - It is good to know the approximate current draw under load. - Amperages can be high (5 to 10 A), so some meters may not be equipped to measure that (typical meters handle less than 1A). Speed Rotational speed is given in revolutions per minute (r/min). Most continuous DC motors operate at 4000 r/min. to 7000 r/min (66.7 to r/sec). Some specialized motors are slower to 3000 r/min. - Like tape recorders and computer disk drives. Lecture 4, Page 10 of 18

11 These speeds are much too high for robotics. - Need to reduce speed to 150 r/min. or less. - And much less than that for robotic arms and grippers. Stepping motors are rated in steps (pulses) per second. - Equivalent to 100 to 140 r/min. Torque Torque is the force the motor exerts upon its load. The higher the torque, the larger the load that can be handled. - And the faster the motor will spin under that load. - If torque is reduced, the motor will slow down as it strains under the workload. - Reduce the torque even more and the motor may stall. - The load is too demanding for the motor. - Will draw high amounts of current. - And generate lots of heat. Torque is perhaps the most confusing design aspect of motors. - Because motor manufacturers have yet to settle on a standard means of measurement. - Motors for industry are rated one way, those for military another. - A simple approach is to attach a lever to the motor shaft. - Then measure the upward pull. - Multiplication of the length of the shaft times the weight gives the torque. Lecture 4, Page 11 of 18

12 Some motors are not designed for heavy loads. - But may be suitable for arms, grippers, and other mechanical components. Stall or running torque Most motors are rated by their running torque. - The force they exert as long as the shaft can continue to rotate. - Determines how large the load can be and still guarantee that the motor works. - To measure this requires more complicated methods than the one shown above. - Need to use slip-clutches, precision scales, and other test jigs. - Cannot be done by a hobbyist. Some manufacturers use a stall torque specification. - This is the force exerted by the motor when the shaft is clamped tight. - There is an indirect relationship between stall torque and running torque. III. Gears and Gear Reduction Normal running speed of motors is too fast for most robotics applications. And the torque is too weak. Locomotion systems need motors running from 75 to 150 r/min. For a wheel with a 3 inch circumference. How fast can a robot move when the motors rotate at 150 r/min? (150 rev/min)*(1/60 min/sec) *(3 inches/rev) = 7.5 inches/sec. Lecture 4, Page 12 of 18

13 What happens when a robot moves too fast? Wheels slip. Runs into walls and people. Cannot process sensor information fast enough. Cannot stay on a line. Cannot stop fast enough (or must also have brakes). Arms, gripper, mechanisms, and most other mechanical systems need even slower motors. 20 r/min down to 5 or 8 r/min - (5 r/min)*(1/60 min/sec)*(360 degrees/rev) = 30 degrees/sec. Two methods exist to decrease motor speed. Bigger motors Gear reduction practical. - Used cars, bicycles, washing machines, and countless othermotor operated mechanisms. Gears 101 Gears perform two important duties. Make the number of revolutions applied to one gear greater than or lesser than another gear connected to it. Increase or decrease torque. - Depending on how the gears are oriented. - Gears can also serve to transfer force from one place to another. Lecture 4, Page 13 of 18

14 Consider the following figure. The small gear is driven by a motor. For each revolution of the smaller gear, the larger gear makes ½ revolution. - A movement of 15 teeth occurs for both gears. If the small gear turns at 1000 r/min, the larger gear turns at 500 r/min. Gears are like round levers A small amount of torque applied at the edge of a large gear translates into a large amount of force at the shaft. Like a small amount of force on the long side of a lever translates into a large amount of force on the short side. - Because torque is force times distance. d1*f1=d2*f2 If d2 is less, f2 will be more. This is how gears not only change speed of rotation, but also torque. Consider the above figure - The motor produces a torque on the shaft at the center of the smaller gear. - Which exerts a smaller force on the circumference of that gear and on the larger gear. - But the force on the center of the larger gear is larger. Lecture 4, Page 14 of 18

15 - The distance to the center of the larger gear is twice that of the smaller gear (since it has twice the circumference). - So the torque will be roughly twice that given by the motor. - Less some loss due to friction between the teeth. Establishing gear reduction Teeth provide the active physical connection between the two gears. The force is transferred from one gear to another. Gears are ultimately characterized by the number of teeth. Given, of course, that the teeth are the same size at the interface between two gears. Speed always decreases when going from a smaller gear to a larger gear. And vice versa. Example: Speed reduction from 1000 r/min to 5 r/min. Reduction ratio of 200:1. Impractical to have one gear do this. - One gear with 10 teeth and another with teeth. Can start with the following arrangement. - There is a larger gear with a smaller gear permanently attached to the shaft. - Called a pinion. Lecture 4, Page 15 of 18

16 - What speed reduction does this provide? 12 to 60 1/5 12 to 48 ¼ Total: 1/20 - Then to get to 1/200, reduce even further with a single gear that accomplishes 1/10. - Like a 12 tooth pinion and a 120 tooth gear. - Or use two gears, for example that do 1/5 followed by 1/2. Motors and gear reduction It s always easiest to use DC motors with gear reduction boxes already built into them. The important specification is the output speed of the gearbox, not the actual running speed of the motor. - And maybe also the output torque of the gearbox (once again not the motor itself). Output shafts can be opposite the input shaft, on the same side as the input shaft, or at 90 degree angles (called a right-angle drive). Lecture 4, Page 16 of 18

17 One can also add reduction boxes, or make them yourself. But there are many pitfalls. - Shaft diameters of motors and gearboxes may be hard to match. - Separate gear reduction boxes are hard to find, but might be cannibalized from salvage motors. - When making your own, meshing gears must exactly match the teeth. - Even small errors can cause the gears to mesh improperly. There are several types of teeth for gears. Spur gears are the most common, with teeth around the edge of the gear. Lecture 4, Page 17 of 18

18 The of the gear is the number of teeth on a gear divided by the diameter of the gear. - Common pitches are 12, 24, 32, and Which pitch would have the smallest teeth? 48 teeth per one inch diameter verse 12 teeth per one inch diameter. - Some gears have extra-fine 64-pitch teeth, but only for miniature mechanical systems. Some gears, like worm gears and rack gears hold their position even when the motor is not energized. - Good for arm mechanisms. The pressure angle of the gear is the slope of the face of each tooth. - The most common pressure angle is 20º. - But some use 14.5º. - Meshing two gears with different pressure angles is possible, but some wear will occur. Belts and chains Similar to gears are pulleys, belts, sprokets, and chains. Belts with pulleys. Chains with sprokets. They just allow the gears to not need to physically touch. IV. Mounting considerations Mounting of a motor may need to be quite precise. And requires careful attention. Connecting the shaft of the motor to a gear, wheel, lever, or other mechanical part is probably the most difficult task of all. Lecture 4, Page 18 of 18