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Exploring Robotics with Electronics Presentations 10 Horsepower and Torque How much horsepower does it take to power a robot? How do you choose the correct motor size and battery size for a robot? Standards 1. Investigate and explain the relationships among current, voltage, resistance, and power. 2. Compare and contrast work and power qualitatively and quantitatively. Objectives o o o o o o o o o o o o o o o Know the definition of horsepower Know the definition of torque Understand how the size of a motor affects horsepower and torque Understand how voltage and current affect Horsepower and torque Understand concept of Current and Volts and how to determine the power requirements of a light bulb and other common electrical items Know the formula and be able to calculate Horsepower Know the formula and be able to calculate Torque Know the formula and be able to calculate Velocity Understand the concept of Amps and be able to calculate Amps Understand what factors are involved in the selection of motors, batteries, wire sizes, and electronic components of hobby robots Understand the RMF factor and be able to calculate it Understand what an overcurrent protection device is and why it is used Understand how voltage affects the size of wires used Understand the concepts of velocity and acceleration in relation to motors Be able to determine the power requirements of a motor given the HP and current Inquiry Questions 1. What do you know about horsepower? 2. What do you know about torque? 3. What do you know about velocity? Presentation View Video: Horsepower and Torque (30 minutes) Presentation Outline Horsepower and Power Requirements Calculating the amount of work and the power required to generate work. Robot Snow Plow - RoboPlow Objectives Size of motor Power requirements Horsepower 769860486 Copyright 2012 Interactive Media Publishing Page 1 Exploring Robotics with Electronics Presentations Torque Velocity Electricity Select motors, batteries, wire sizes, and electronic components of robots Measuring Work Torque = The amount of force available to get work done Horsepower = How much work is done in a period of time Torque The rotational force generated by a motor Measured in foot pounds or ounce inches An arm 1 foot in length with a weight suspended from the tip of the arm. How much weight could the motor cause the arm to lift? This is torque. A lift of 100 lbs = 100 ft lb of torque. An arm 1 inch long, if the motor lifts 10 ounces, this is 10 oz in torque Example Warehouse Robot Requirement Specs Lift containers and place on a pallet for shipping. Remove containers from a conveyor system, Stack 12 containers on each pallet, Stack 3 layers high Continuously operate for 8 hour shift. How much work? Questions How much does each container weigh – or how much weight does the robot lift? What is the cycle time on the paint buckets arriving – or how fast does the robot need to move? How close is the conveyor – or what distance is to be traveled? Horsepower Calculation Each container weighs 50 lbs. Move distance of 10 feet every 10 seconds Work = 50lbs x 10 feet per 10 seconds 50 lbs x 10 ft = 500 ft lbs 500 x 6 = 3,000 ft lbs/min 1 HP = 33,000 ft lbs/min 1/33,000 = x/3,000 3,000 / 33,000 = .09 or almost one tenth HP. Motor HP Take into consideration: Weight of robot Efficiency of system Dynamics of acceleration & deceleration Motor selected = 1 or 1.5 HP Selecting Motors for Hobby Robots John Palmisano, a mechanical engineer – SocietyofRobots.com A simplified formula for selecting motors – Robot Motor Factor or RMF Velocity 769860486 Copyright 2012 Interactive Media Publishing Page 2 Exploring Robotics with Electronics Presentations Velocity = diameter of wheel * Pi * RPM 3 ft/sec = diameter * Pi * 100rpm. 3 ft/sec = diameter x 3.14 x 1.67 rps. Diameter = 3 ft/sec divided by (3.14 x 1.67 rps). Diameter = 0.56 feet or 6.89 inches Wheel with diameter of 7 inches travels at 3 ft/second with 100 rpm motor Torque and Force The larger the diameter of the wheel, the higher the rpm, or faster the robot will go Torque = Distance x force Distance = Wheel Radius Force = Torque / Wheel Radius Robot Motor Factor Torque x rps >= Mass x acceleration x velocity / (2 x Pi). Robot Motor Factor (RMF) = Torque x rps Robot Motor Factor Look up motors that you think will work Find torque and rps (rotations per second) RMF = Torque x RPS Estimate the weight Choose the velocity and acceleration Mass x acceleration x velocity / (2 x Pi) 5 lbs x 2 ft/sec times 3 ft/sec / (2 x 3.14). RMF factor >= 4.77 RMF Motor Selection Motor A’s specs are 2 lb ft and 1 rps, multiply (2 x 1) the RMF = 2 lb ft rps Motor B’s specs are 2.5 lb ft and 2 rps, multiply (2.5 x 2) the RMF = 5 lb ft rps Motor C’s specs are 2 lb ft and 4 rps, multiply (2 x 4) the RMF = 8 lb ft rps Desired RMF >= 4.77 Best choice? If none work, reduce weight, acceleration, or velocity Electrical Power Requirements One watt of electrical power = one amp of current times one volt. current (amps) = watts/volts. 100 watt light bulb operating at 120 volts 100/120 = .83 The current or power required is .83 amps. Amps are important to know for every device that uses electricity. Electrical Power Shift Increase in friction Direct connection between the amount of electrical power required and the amount of mechanical work being done Power monitoring used to determine maintenance required Overcurrent protection will shut down most motors 769860486 Copyright 2012 Interactive Media Publishing Page 3 Exploring Robotics with Electronics Presentations Power required of Motors How much electricity to power a 1 HP motor? 1 HP is approx 746 watts of electrical power. Watts / Volts = amps (746/120) = 6.21 amps 1HP = 6 amps power Add all HPs and multiply by 6 Battery Power A 100 Amp/hour battery is capable of delivering 100 amps for one hour. Operate on 6, 12, 24, 48, or 72 volts. Batteries used in a hand-held drill gun are typically 12 or 18 volts Battery Power for 1 HP 746 watts / 6 volts = 120 amps. 746 watts / 12 volts = 60 amps. 746 watts / 24 volts = 30 amps. 746 watts / 48 volts = 15 amps. 746 watts / 72 volts = 10 amps. Example Calculations 2 HP motor plugged into a standard wall outlet (120 volts)? 746 watts /120 volts Answer 2 HP x 6.2 = approx 12.4 amps 12volt battery operated 2 HP motor? 746 watts/12 volts Answer 2 HP x 60 = approx 120 amps Voltage and Current Voltage = force required Current = amount of electricity flowing As voltage goes up, current can go down and yield same force or horsepower As voltage goes down, current needs to go up to yield same force or horsepower Why Amps? More AMPS means larger wire The types of electronic components Adaptation of motors for use in your robot DC brushed motors are designed to operate within a particular voltage range Battery Power Shifts Discharge or loose power over a period of time when under load May be above 12 volts when fully charged, and below 12 volts when discharged. Power controller reports when battery charge is getting low Summary Torque measures the amount of force available Horsepower measures how much work is done in a period of time How to choose a motor using the RMF factor Power requirements: 1 HP = 10 amps at 120 volt 769860486 Copyright 2012 Interactive Media Publishing Page 4