UNITS 12 AND 13 Robot Interaction A robot

UNITS 12 AND 13

Robot Interaction A robot interacts with the world around it. All robots are designed with a purpose in mind, and the purposes can vary greatly. Many purposes require a robot to handle and manipulate something. To do this, it must have a mechanism specifically designed to interact with objects in its environment. UNIT 12

Object Manipulation The three categories of object manipulators are plow, scoop, and friction grabber. Most manipulator designs fall into one or more of these categories.

The plow type of object manipulator does not actually pick up an object; rather, it applies force to the side of the object to push it forward.

The scoop type of manipulator lifts an object up from underneath. Examples of scooping manipulators can be seen below.

The third type of object manipulator is a friction grabber. The friction grabber manipulator grips an object in some way and the friction between the gripper and the object holds the object in place. A grabber consists of an actuator that moves the claws or jaws together and apart. This provides a normal force between the claw and the object. This normal force is essential to the operation of the gripper; without a normal force, there would be no friction force to stop the object from sliding from the gripper jaws. The greater the normal force, the greater the friction that will hold the object in place. Of course, if the normal force is too large, the object can be damaged. The most common form of friction grabber manipulator is a claw that pinches an object.

Object Manipulator Design When designing a robot to manipulate an object, it is important to keep the object in mind and size the robot accordingly. Choose the appropriate gripper type or combine some of the types to create an effective geometry. Try to design it so that the motor does not need to be stalled when the gripper is holding the object. It is also important to think of how the gripper will pick up and deposit the object. Consider the following questions: What orientation will the object be in when it is picked up? Does the gripper need to be able to grab the object from multiple orientations? How will the gripper deposit the object? Does it need to deposit the object in multiple orientations? What orientation change does the object need to make between pickup and deposit?

Degrees of Freedom A degree of freedom is the ability to move in a single independent direction of motion. To be able to move in multiple directions is to have multiple degrees of freedom. Moving up and down is one degree of freedom; moving right and left is another; and the ability to move up and down and right to left requires two degrees of freedom. The three basic types of degrees of freedom are as follows: UNIT 13

The degree of freedom in which a robot’s arm can rotate about an axis parallel to the arm. The human wrist has this degree of freedom. Imagine placing your arm straight out in front of you and holding a pencil in your fist so it is parallel to the floor (horizontal). Twist your wrist so that the pencil is pointed straight up at the ceiling (vertical). This twisting is one degree of freedom.

The degree of freedom that is a linear movement. In this case, a component of a robot can slide in and out (or up and down or left and right). An elevator shows this linear degree of freedom (moving up and down), as does a common desk drawer (moving in and out).

The degree of freedom in which a robot’s arm can rotate about an axis perpendicular to the arm. The human elbow illustrates this degree of freedom. This rotating joint is the focus of this unit.

Rotating Joints The joint used most frequently on VEX Robots is the rotating joint. An example of this joint can be seen below

Joint Loading T= Fx. D

Joint Speed Often, it is important for a joint to move as quickly as possible. However, this is not always practical. Designing a joint to be too fast may make it uncontrollable without advanced software.

Approach 1: Start with Load Determine the applied load on the joint. *Decide the maximum load you want to be applied on the motor. (One-half Stall? Less? ) *Determine the required gearing to achieve this loading. *Calculate how fast the joint will move with this gearing. *Determine if this is a good speed. *If the speed is good, great! Build it! If the speed is too fast, you then must: *Determine how fast you want the joint to move. *Calculate the gearing required for this speed. (This should be slower than the previous calculated speed. ) *Build it! If the speed is too slow, you then must: *Add additional power to the system so it can carry this load at a faster speed. (Add additional motors to this joint. ) Recalculate.

Aproach 2: Start with Speed *Determine the speed that you want the joint to move. (90 degrees per second? ) *Calculate the gearing required to make the arm move at this speed. *Decide the maximum load you want applied on the motor. (One-half Stall? Less? ) *Determine the maximum load that can be applied to the joint, based on this desired motor loading and the gearing determined earlier. *Is this load less than what the arm is expected to experience (including a safety factor)? *If this load is good, great! Build it. If this load is too low, are you willing to reduce the speed of the joint to accommodate this load? *If yes, then recalculate and build it! If no, then additional power to the system so it can carry more load at this speed (add additional motors to the joint) and then recalculate. Note: Reducing the length of the arm attached to the joint will reduce the amount of torque a given load will apply to the joint.

Single stage reduction