5Building Robot Drive Trains Building Robot Drive Trains is the first volume in the new "Robot DNA" series by McGraw-Hill/TAB Electronics.It is just what robotics hobbyists need to build an effective drive train using inexpensive, off-the-shelf parts.Leaving heavy-duty “tech speak” behind, the authors focus on the actual concepts and applications necessary to build – and understand -- these critical force-conveying systems.If you want a clear, straightforward guide to the nuts-and-bolts of drive trains, this is the way to go.
6Building Robot Drive Trains Chapters include:The Basics of Robot Locomotion *Motor Types: An Overview *Using DC Motors *Using RC Servo Motors *Using Stepper Motors *Motor Mounting *Basic Motor Control *Advanced Motor Control *Electronics Interfacing *Wheels and Treads *Locomotion for Multipods *Glossary of Terms, Tables, Formulas * 75 illustrations
7Pulleys Pulley are wheels and axles with a groove around the outside A pulley needs a rope, chain or belt around the groove to make it do work
8Lifting Pulleys Lifting Pulleys in transmission systems Single pulley Moveable pulleyBlock and Tackle
9Pulley Examples Reverses the direction of force Two or more pulleys connected together permit a heavy load to be lifted with less force
10Fixed (single) pulley Fixed Pulley: Wheel attached to surface Changes the direction of the applied forceNO mechanical advantage – same amount of force is required
11Single PulleyThe resistance arm and the effort arm are both the radius of the pulley.Since they are equal, there is no mechanical advantage.
12Single Pulley Provides change of motion, but no mechanical advantage 100 ft100 lbs
13Movable pulley Movable Pulley: Pulley moves along the rope Wheel supports the loadEffort is in the same direction as movementReduces the forces needed to move an object
14MA in a Moveable pulley A moveable pulley is a second-class pulley. The effort arm is the diameter and the resistance arm is the radius.MA = dE ÷ dRMA = 2 ÷ 1 = 2Mechanical Advantage = 2
15Another Moveable pulley Provides change of direction and a mechanical advantage of 1:2 (at the expense of having to increase the amount of line to pull)50 lbs100 lbsThese kinds of diagrams are easier to analyze
16In the second picture, the weight is held by two ropes instead of one In the second picture, the weight is held by two ropes instead of one. That means the weight is split equally between the two ropes, so each one holds only half the weight.
17COMBINED PULLEY Combined (double) Pulley: Has at least two wheels The more complex the pulley, the more the effort needed to move the object decreases
18COMBINED PULLEYThe effort needed to lift the load is less than half the weight of the load.The main disadvantage is it travels a very long distance.
19Mechanical AdvantageMechanical Advantage is the same as the number of ropes supporting the objectMA= MA= MA=4
20How about these?Calculate MA for these and similar
24Pulleys as used in robots A pulley is basically a wheel with a groove in the circumference of the wheel, with a belt or cable being fed around the groove.Pulleys create a mechanical advantage by connecting a small input drive pulley to a larger follower with a belt.Driver(Effort)Follower(Resistance)
25Mechanical Advantage of a Pulley Drive Train MA = FOLLOWER ÷ DRIVERThe Pulley Drive Train above produces an MA of 4. This means 10 lbs of input torque will create 40 lbs output, but the driver must rotate 4 times to rotate output 1 time. Speed is reduced by 4.Driver1mmFollower4mm
27Increasing Speed Driver Follower 4mm 1mm Increase the speed of the output by using a larger driver gear and a smaller follower gear. Speed increases and power decreases.Driver4mmFollower1mm
28Compound Drive Pulley Trains A Compound Drive Pulley Train has pairs of pulleys working in combination, with the follower of one pulley sharing the same axle as the driver of the next sequential pulley train.
30MA = (FOLLOWER 1 X FOLLOWER 2) ÷ (DRIVER 1 x DRIVER 2) Example 2: Compound Drive Pulley TrainsMA = (FOLLOWER 1 X FOLLOWER 2) ÷ (DRIVER 1 x DRIVER 2)Driver 1 = 3mm Follower 1 = 9mmDriver 2 = 2mm Follower 2 = 14mmCalculate MA
31Problem 2 Sketch the following two pulley trains: A: Simple pulley train with belt wrapped so both pulleys rotate in same direction.B. Simple pulley train with belt wrapped so both pulleys rotate in opposite direction.
32Solution to Problem 2A: Simple pulley train with belt wrapped so both pulleys rotate in same direction.
33Solution to Problem 2B. Simple pulley train with belt wrapped so both pulleys rotate in opposite direction.
34Problem 3Calculate the mechanical advantage of the compound pulley drive train model.Show your work.Small Pulley Diameter = 25mmLarge Pulley Diameter = 100mm
40Block and Tackle: Luff-upon-Luff Tacke 1200/100 = 12
41Pulleys are used to gain mechanical advantage trading the amount of rope you have to pull to lift an object for how heavy the object can be.
42The more lines of support (ropes) a pulley has, the more mechanical advantage it has!
43Review of PulleysGive real-world example of each of the following pulleys.Force MultiplierSpeed MultiplierDirection ChangerGive one example of a simple machine or transmission that changes output motion. Identify both input and output motions.Give examples of using Pulleys in Robotics, especially humanoid robot design.
44ElaborationPulley EngineeringDesign, Create, and Verify a pulley system with a mechanical advantage of:456Be sure to include drawings of your systems and how you verified the MA of the systemsFor block and tackle systems, what effect most greatly reduces the MA?8/23/04
45EvaluationPulley PuzzlersThe girl in the illustration is trying to lift herself and the swing using the rope.What would you predict would happen if she pulled down on the rope?If she weighs 90 pounds and the swing weighs ten pounds, how much force does she need to apply?8/23/04
46Pulley PuzzlersEvaluationWe should assume that it is an extensionless, massless rope, a massless pulley that does not hinder its rotation, and a rigid support.We should imagine a box around the girl and the swing with only the rope extending out from the box to isolate the forces acting on the objects inside the box from the external forces.The rope supports the box once, goes through the pulley and supports the box again.The tension of the rope going upwards is T on each side, so the total tension is 2T.The combined force from the tension in the rope must be greater than the combined weight of the girl and the chair for the chair to be lifted.For a 90 pound girl and 10 pound chair, she must use a force of 50 pounds of more to be able to lift herself off the ground.8/23/04
47Pulley PuzzlersEvaluationThe man in the illustration below is trying to lift himself and the block on which he is standing off the ground.Is this possible?This movable pulley allows the man to pull upwards and the block on which he stands will rise with him on top.He must apply a force equal to his weight and the weight of the block because there is only 1 supporting rope outside the system.There is a mechanical advantage of 1.Yes, it can be done.8/23/04
48EvaluationPulley PuzzlersA long rope passes over a single pulley and has bananas on one end and a monkey on the other.If the monkey and the bananas have the same mass, can the monkey ever reach the bananas if he climbs the rope?Assume that the rope is massless, there is no friction, and the rope is limitless.8/23/04
49EvaluationPulley PuzzlersThe opposite external torques produced by the bananas and the monkey will cancel each other out.Any upward movement of the monkey will be equaled by an upward movement of the bananas.If the monkey started below the bananas, the distance between the two will always stay the same.Because we assume that the rope is limitless and the bananas will never wedge into the pulley, the monkey will never be able to reach the bananas.8/23/04
50How to use pulleys in your robot? Look to the pulleys used in other robots.Look to gears, chains and other similar constructions in our robots.How to use old Goodwill/Home Depot components?
52Ancient Pulleys Instructor: Read this in your best theatrical voice EngagementArchimedes, who was kinsman and a friend of King Hieron of Syracuse,wrote to him that with any given force it was possible to move any givenweight, and emboldened, as we were told, by the strength of hisdemonstration, he declared that, if there were another world, and hecould go to it, he could move this. Hieron was astonished, and begged him to put his proposition into execution, and show him some great weight moved by a slight force. Archimedes therefore fixed upon a three-master merchantman of the royal fleet, which had been dragged ashore by the great labors of many men, and after putting on board many passengers and the customary freight, he seated himself at a distance from her, and without any great effort, but quietly setting in motion with his hand a system of compound pulleys, drew her towards him smoothly and evenly, as though she were gliding through the water.- PlutarchInstructor: Read this in your best theatrical voice What else did Archimedes study?8/23/04
53EngagementArchimedes AdvantageHave two volunteers attempt to hold two metal rods apart while the instructor attempts to force them together with a “pulley system.”Discuss the Mechanical Advantage offered by pulleysTeacher 1Teacher 2InstructorAsk volunteers to keep the bars parallel and to hold the ends not the middle8/23/04