Introduction to Robotics Part 2: Structural System Robotics and Automation Copyright © Texas Education Agency, 2012. All rights reserved.

Robot Systems Structural System Propulsion System (motion) Physical system that provides support and stability Propulsion System (motion) Drive system includes motors, wheels, and gears Control System Microcontroller, operating program, electrical power, and joystick Tool and Actuator system Arms, grippers, manipulators Sensor and feedback system Perception, transducers There are different ways of describing robot systems, these are sometimes called subsystems. There are often more systems described than these (examples: sensors often have their own system, programming is often considered separately, other systems include power and logic which are included here in the control system). Each of these systems works together, there will be some functional overlap. You can also separate by pure electronic parts vs. electromechanical parts. The actuator and feedback system is how the robot completes the performance objectives. Sensor and manipulation system. The control system is for the internal robot environment, the sense and manipulation system is for the external environment. Copyright © Texas Education Agency, 2012. All rights reserved.

Structural System The structural subsystem of the robot is responsible for physical support. Holds everything in place Also provides physical protection The durable “skeleton” of the robot to which all the other subsystems are attached The Structure and Motion sub- systems are tightly integrated to form the chassis of the robot. WE look at each of the systems briefly first, then go into more detail. Delicate parts such as electronics and sensors need physical (and electrical) protection. Dangerous parts (like batteries) need to be held in place as well as protected. Copyright © Texas Education Agency, 2012. All rights reserved. Photo Credit: VEX Robotics, Inc.

Before You Begin Many types of tools and parts are necessary. Some tools and parts are required, others are simply nice to have. The larger the variety of supplies, the more creative the design can be. Systems can look better and be more structurally sound. Also increases the need for a tool/part inventory and management system. Research the internet to locate excellent links to a description of how inventory management systems work. Robotics uses a large amount of parts and supplies, and learning about inventory management is one of the TEKS for this class. (130.370(c)(10)(G)) . Design of an inventory management system may be beyond the scope of this class, but knowledge about an existing system which includes ideas for improvement or efficiency is desirable. Copyright © Texas Education Agency, 2012. All rights reserved.

Recommended Tools Allen wrench set (also called an L-wrench) Open ended wrench Screwdrivers Flat head and Phillips Needle nose pliers and diagonal cutters Wire strippers Crescent wrench Photo Credit: VEX Robotics, Inc. It would also be nice to have examples of pictures and/or descriptions to show students and allow students to practice their mechanical aptitude. Copyright © Texas Education Agency, 2012. All rights reserved.

Additional Tools Drill and drill bit set Saws for metal, wood, and plastic Examples: Hacksaw, band saw, chop saw, scroll saw A variety of screws, nuts, bolts Vise Multi-purpose rotary power tool to cut and smooth metal Wire, soldering iron, electrical connectors Copyright © Texas Education Agency, 2012. All rights reserved.

Example Parts Types of bolts 6-32 and 8-32 Keps nuts Square drive shaft Bearing flat Spacers and friction reducers Metal sized by number of holes Photos Credit: VEX Robotics, Inc. The first number (6 or 8) is the diameter of the screw, larger is bigger. The second number (32) is threads per inch, a higher number is finer threads. A keps nut is a type of lock nut. The spacers are nylon or delrin. Always use bearing blocks with the shaft to avoid metal to metal contact and friction. A square drive shaft requires special motion components, but reduces the slippage of a round shaft. Keps nuts are a type of lock nut. 5 X 15 1 X 25 Copyright © Texas Education Agency, 2012. All rights reserved.

Photos Credit: VEX Robotics, Inc. These are sample parts from a kit. Holes and slots in the metal are pre-cut to allow connection and alignment without drilling and measuring. Copyright © Texas Education Agency, 2012. All rights reserved.

The Robot Base The platform or base determines the stability, the durability, the maneuverability, and the functionality of the robot. Usually made from wood or metal Provides the support structure for the rest of the robot Everything connects and mounts here The frame or skeleton Copyright © Texas Education Agency, 2012. All rights reserved.

Robot Stability Stability is defined as when the center of gravity is over the points of support for the base (called the support polygon). To increase stability, lower the center of gravity. Long arms need additional support. With one point of support an arm will rotate. If the center of gravity is moved outside the support polygon the robot will tip over. A long arm or an arm that picks up something heavy will move the center of gravity. Keep in mind that lever action can increase the effective force from the weight of an arm. Copyright © Texas Education Agency, 2012. All rights reserved.

Base Material Considerations Common materials are wood, aluminum, sheet metal, or plastic (HDPE). Consider both weight and strength. Metals have a high strength to weight ratio. Some plastics like plexiglass will crack or break when cut and drilled. Sheet metal and aluminum conduct electricity. Wood can splinter or split but is cheap. HDPE is great because it can be machined and drilled similar to wood, but is lighter and very strong. One advantage of wood is that you can use screws to hold pieces and parts together as long the pieces are stable (meaning no vibration or pulling). Copyright © Texas Education Agency, 2012. All rights reserved.

Base Material Considerations Angled metal in an L or C shape will retain its shape under load. A solid square or round tube makes a very strong support structure. Can be metal or PVC – be creative! Wood is great when load is not too great. Wood can flex, bend, or break. Works well for a superstructure or platform. Copyright © Texas Education Agency, 2012. All rights reserved.

Base Material Considerations Even if you use wood or plastic for the base, you will need to use and cut metal. Motor mounts can be made from metal bent to a 90 degree angle (like angle iron). Wheels are an assembly with a mounting hub adapter to connect motor shaft to the wheel. 2 types of adapters: set screw and collet type Other types include keyway and D hubs. These are usually for larger shafts. There are specialized adapters for special shafts, like square and hex shafts. These eliminate a problem that round shafts have with slipping. Splined shafts can fit into a plastic mount. Also called a shaft coupler. Bending metal pieces is done more effectively using a vise. Copyright © Texas Education Agency, 2012. All rights reserved.

Wheels Two drive wheels make control easier. Can be controlled with a single joystick The robot turning point will be between the two drive wheels. Large turn radius Non-drive wheels will have to slide or slip Four drive wheels move the pivot point into the center of the robot. Robot will turn in place. May require an additional joystick. Pivot Point Dots indicate the center of rotation. The top robot (with two driving wheels) is shown with a slider on the front, but can also have wheels as shown on the bottom robot. These wheels would have to slide and should not be “grippy”. Drive wheels SHOULD be grippy. Programming four drive wheels to work together can be tricky. Copyright © Texas Education Agency, 2012. All rights reserved.

Wheel Support Wheel axles should be supported at two points by the chassis or frame. Usually on each side of a wheel as shown Needed to keep the wheel straight and the axle from bending Bearing or hub Wheel The wheels can stick out from the frame supported by only the axle as long as the wheels are light compared to the strength of the axle and motor mount, and the total weight of the robot is low. Remember, the wheels provide not just the motion but the support for the robot. Frame Axle Copyright © Texas Education Agency, 2012. All rights reserved.

Types of Propulsion Systems Legs Motors, wheels, bearings Tank treads Gears and belts Supported by the structural system Includes mounting hardware Servos are used to hold a position, and are generally considered part of a system designed to complete specific objectives. Legs require balance, tank treads are slow and less maneuverable, so the most common system includes motors and wheels. Underwater robots can use fins or propellers. Servos are usually used for arms and grippers, and are not designed for use in the propulsion system. Copyright © Texas Education Agency, 2012. All rights reserved.

Movement Wheels and pulleys use DC motors. A DC motor continuously rotates (360⁰). Speed is controlled by the amount of DC voltage. Direction is controlled by polarity of DC voltage. Arms and grippers can use servos. A servo goes to a position and holds there. Typically minus 90 degrees to plus 90 degrees Position is controlled by an electronic signal. Different forms of pulse width modulation are used for each (motors vs. servos). A DC motor’s torque is how much force it uses to spin, or the amount of resistance it feels while spinning. A servo’s torque is the force it applies against a load to hold its position. A DC motor will spin even while you try to hold it still (up to a point) while a servo will hold its position even if you try to make it turn. Copyright © Texas Education Agency, 2012. All rights reserved.

DC Motor Control Signals Voltage amount given by pulse width modulation Longer “on” time means higher voltage Higher voltage equals higher motor speed Direction of rotation controlled by polarity Emphasize that this is how we control the speed of the robot. The DC voltage from pulse width modulation is the average voltage of the signal. These signals as shown all have the same polarity, so the speed would vary only in one direction. Reverse the direction of motion by reversing the polarity. Pulses would go negative for reverse (not shown). This would be performed physically by reversing the wires. Copyright © Texas Education Agency, 2012. All rights reserved.

Servo Control Signals Pulses are always 20 ms apart 50 pulses per second Pulse width varies between 1 and 2 ms 18-19 ms of “dead time” A DC motor getting this signal would spin very slowly The pulse width determines the servo position 1.5 ms = middle (or null) 1 ms = full ccw (usually - 90⁰) 2 ms = full cw (usually + 90⁰) This is how we can control the position of an arm or gripper. Servo signals are also called RC signals. It does not mean remote control, but it does come from the remote control type signals. ms reads “milli-seconds” - 90 reads minus ninety degrees The pulse width modulation for a DC motor and a servo are considered the same thing, but a servo uses a low duty cycle type of PWM. Duty cycle has no meaning for the servo, only the time duration of the pulse is used to control position. The range of the pulse is from 1 ms to 2 ms. The angle of movement can be larger than +/- 90 degree, but is never close to a full rotation. Copyright © Texas Education Agency, 2012. All rights reserved.

Kit Versus Build From Scratch You can choose to build your robot completely from scratch using common and cheap parts. The advantage of a kit is that you get all of the parts you need. The parts are designed to work together. There are standard designs and construction techniques. There is usually some help in the form of instructions and forums, plus corporate technical assistance. Kits usually have an intended design eliminating the need for student design work. Cost is definitely a major factor, though, kits can be very expensive. Copyright © Texas Education Agency, 2012. All rights reserved.

Kit Versus Build From Scratch If you choose to build from scratch you have lots of freedom for design and construction, but you need a large variety of parts. Although there are many different kinds of motors, brushed DC motors are popular because of cost and reliability. The motion and structural system must be designed to work together. Functions are different but they are integrated to form the chassis. When you build from scratch it is easy to overlook necessary parts. It is often necessary to have an inventory system to keep up with everything, because you only have to be missing one required part to be stopped dead during construction. This slide is also designed to provide a lead in to the next section, motion, so emphasize that we will be talking a lot about DC motors because they are the most common means of providing motive power to a student robot. Copyright © Texas Education Agency, 2012. All rights reserved.