Project Overview  Introduction  Frame Build  Motion  Power  Control  Sensors  Advanced Sensors  Open design challenges  Project evaluation

LESSON 03

LESSON 03 STARTER Parts list Quick task: Discuss in pairs all the parts shown. Agree the function each of these parts for the Tumbler motion, and write your description in your parts list. 2-Wire-269 MotorShaft Collars4.6mm Spacers 3" Drive Shafts 5" Wheel Motor Controller 29

CREATING MOTION Gears VEX uses a four-sided polygon (square) shaft as part of its motion system. This means that the shaft will transfer torque directly to anything which has a matching square hole. This square shaft also has rounded corners, which allow the shaft to spin freely in a larger-size round hole. The most common type of gear used in VEX robotics is called a spur gear. Spur gears transfer motion between two shafts running parallel to each other. Spur Gears are characterized by their teeth, which are straight and parallel to the gear’s axis of rotation. These are the primary form of mechanical power transfer used in the VEX Robotics Design System. Learning objective: Understand how motion is created in robotics and how different types of motion can be achieved. Worksheet for lesson 3 =

Rotational motionRotational motion to linear travel To begin, your motors will create rotational motion. By inserting a square shaft into the corresponding hole, you can drive anything (like the gears just seen) round and round. However, attach something like a winch, and you can change rotation motion to linear, in this instance, pulling a rope in a direction. CREATING MOTION

Changing the axis of motion (X, Y, Z) Bevel gears are conically shaped gears, and transmit power/motion between shafts that have intersecting axes of motion (X to Y for example). Bevel gears can transmit power between shafts at a variety of angles, but are most commonly used to transmit power 90-degrees as seen in the above example. Here two bevel gears work off the first to potentially drive wheels either side. CREATING MOTION

Changing the axis of motion (X, Y, Z) Worm gears are different from bevel gears and spur gears. They come in pairs: with a worm gear and worm wheel that mate together to transfer power between perpendicular shafts that have axes of rotation offset from each other. Worm gears resemble screws; as they spin, they turn their mating worm wheel. This type of gear pair is very useful for creating a high mechanical advantage in a small form factor. In this type of gear pair, the worm gear can drive the worm wheel forward, but it is very difficult for the worm wheel to drive the worm gear. It would be very slow for a start… CREATING MOTION

Rotational motion to linear motion Rack and pinion gear sets are commonly used to convert rotational motion to linear motion. Cars utilize this type of gear set to convert the rotary motion of a steering wheel into a linear left/right motion required to steer the car. This is why it is called rack and pinion steering. A rack gear is a gear mounted to a straight rod, such that it moves in a linear fashion when torque is applied to it by a spur gear (known as the pinion gear). CREATING MOTION

Gear Chains The driver gear is the central gear here. It is driving the other two using a motor. The gears either side rotate in the same direction, and allow the two wheels to drive the Tumbler forward using one motor on one side. You can keep adding gears in pairs, to keep the wheels moving in the same direction CREATING MOTION

Task: Note these definitions down FRICTION is the force that opposes motion when two surfaces rub together. It is a reaction force only. It occurs when two surfaces are in contact and a force is applied such they slide along one another. TRACTION can be defined as the friction between a drive wheel and the surface it moves upon. It is the amount of force a wheel can apply to a surface before it slips. Increasing Traction: Since traction is dependent on the friction between the wheel and the surface, to increase traction one must maximize this friction. To increase traction, one must either increase the coefficient of friction (grippier wheels) or increase the normal force acting on the wheel (heavier robot, or more weight on drive wheels). CREATING MOTION

Use the Step by Step build guide - MOTION Follow the instructions on page one of the build guide. Use the tools provided by your teacher and remember to check all the time that you are being accurate and working as a team! Key to getting it right… Fit the shaft couplers into each motor before fitting the motor to the chassis. Use one long screw and one short screw to attach the motors to the frame. Fit the collar to the shaft in the right place to prevent the shaft slipping. CREATING MOTION

LESSON 03 PLENARY Look at the claw shown right. What type of motion are we creating using just a single motor attached at point AA? Draw it onto the photograph and describe it in a short but detailed sentence. AA

SUMMARY Today you have:  Developed an understanding of how motors and attachments can change the direction of motion  Explored gears, bevel gears, rack and pinion, spur gears, winches, worm and worm gears  Discussed in context the terms traction and friction. Learning objective: Understand how motion is created in robotics and how different types of motion can be achieved.