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Exploring Robotics with Electronics Presentations
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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
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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
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Exploring Robotics with Electronics Presentations
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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
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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
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Exploring Robotics with Electronics Presentations
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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
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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
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