Robotics Module 4 Power System Sizing and Control EGG-275 Mike Hoganson 303-594-0098

Objectives for Today Project Report Out – By Team Power Systems in General Overview of Motors Motor Control Prepare for PDR and Quiz Next Week

NEWS! Check the D2L site for links to important items NASA Internships Scholarships RockOn! Using the MakerBOT

EGG-275 Module 4 - Section 1 Team Report

Report outs Registration of Teams? Team Names? New Teams? Problems and Challenges?

EGG-275 Milestones Skills Building – January 31 Preliminary Design Review – February 13 Critical Design Review – March 6 Preliminary Testing – March 20 The Challenge – April 4 Post Test Reviews - April 10 Final Demos – May 1 Final Presentations May 8

EGG-275 Module 4 - Section 2 Robot Power Systems

What is a Power System?

What Does a Power System Do? Stores Energy Converts Energy Powers the Robot to do Work Electrical Energy Mechanical Chemical

Conversion Pyramid Electrical Energy ? ? ? Mechanical Chemical

Conversion Pyramid Electrical Energy Motors Batteries Engines Mechanical Chemical

Robot (Electrical) Power Systems Electrical System Drive Systems Control Power Storage Power Generation Motors Voltage Amp Torque Electrical Voltage Amp Power Batteries Voltage Amp-Hours

EGG-275 Module 4 - Section 3 Electric Motors

Conversion Pyramid Electrical Energy Motors Batteries Engines Mechanical Chemical

Overview Motors in context of robotics, different types of robots have different types of motors Overview of motor types / characteristics All motors convert electric energy to mechanical motion Motor characteristics: AC or DC power source, torque, speed performance Industrial robotics: AC servo motor Mobile robotics & Hobby robots: dc motor, dc servo motor, and stepper motors Principle of operation of a DC motor Inside a DC motor Principle of operation of stepper motors Performance advantages of stepper motor over DC motor and DC servo motor

Motor Basics Either an AC or DC electrical energy source serves as the input to the motor. The result is mechanical motion of the output shaft, most often a rotation about the shaft, provided the load carried by the shaft does not exceed the maximum load the motor is designed to carry.

Choosing a Motor There are numerous ways to design a motor, thus there are many different types of motors. The type of motor chosen for an application depends on the characteristics needed in that application. These include: How fast you want the object to move, The weight, size of the object to be moved, The cost and size of the motor, The accuracy of position or speed control needed.

Motor Parameters The level of performance a motor can provide is described by its parameters. These include: Rated Speed Speed measured in shaft revolutions per minute (RPM) Torque Rotational force produced around a given point, due to a force applied at a radius from that point, measured in lb-ft (or, oz-in). HorsePower = Speed x Torque / 5252.11... A measure of work expended: 1 HP = 33,000 foot-pounds per minute. Torque-Speed performance of a motor

Types of Motors The different types of motors possess different operating characteristics. A brief overview of some operation characteristics of: AC motors DC motors DC servo motors Stepper motors

AC Motor Characteristics When power is applied, AC motors turn in one direction at a fixed speed. Both reversable and non-reversable models available Usually high voltage (110V AC and up) Inexpensive and commonly available Optimized to run at a fixed, usually high speed. If the applied load is greater than the capacity of the motor, the motor will stall and possibly burn out.

DC Motor Characteristics When power is applied, DC motors turn in one direction at a fixed speed. They are optimized to run at a fixed, usually high speed. Most common found in toys, hobby planes, inexpensive robots, blender, toothbrush, screwdriver, etc. Speed can be varied if a (pulse width modulation) PWM controller is added. Almost all can be reversed. Inexpensive and commonly available. Not suitable for positioning unless some kind of position feedback is added. If the applied load is greater than the capacity of the motor, the motor will stall and possibly burn out.

DC Servo Motors Applications that require Servo motors involve control of acceleration, velocity, and/or position to very close tolerances. These motors allow for fast starts, stops and reversals, and very accurate control. DC servo motors consist of a DC motor combined with feedback for either position or speed. A servo system is closed loop with a motor, feedback signal, desired input signal, and a controller which constantly adjusts the position or speed in reaction to the feedback. Servo motor controllers are complex.

Stepper Motors A stepper motor will not automatically turn when power is applied. It requires a separate controller circuit to cause the motor to move. Controllers for stepper motors are easier to implement than closed loop servo systems. Precise positioning is possible by keeping count of steps, no feedback is required. It is open loop. They are inexpensive and commonly available, especially in salvaged computer equipment. Note: If the applied load is greater than the capacity of the motor, the motor may not step, thereby making precise positioning no longer possible.

DC Electric Motors DC Electric Motors use Direct Current (DC) sources of electricity: Batteries DC Power supply Principle of How Motors Work: Electrical current flowing in a loop of wire will produce a magnetic field across the loop. When this loop is surrounded by the field of another magnet, the loop will turn, producing a force (called torque) that results in mechanical motion.

Motor Basics Motors are powered by electricity, but rely on principles of magnetism to produce mechanical motion. Inside a motor we find: Permanent magnets, Electro-magnets, Or a combination of the two.

Magnets A magnet is an object that possesses a magnetic field, characterized by a North and South pole pair. A permanent magnet (such as this bar magnet) stays magnetized for a long time. An electromagnet is a magnet that is created when electricity flows through a coil of wire. It requires a power source (such as a battery) to set up a magnetic field.

Current in a coil creates a Magnet Current Flowing through a coil or wire LEFT: Current Enters A North Pole on Top RIGHT: Current Enters B (Reversed) North Pole on Bottom

A Simple Electromagnet A Nail with a Coil of Wire Q – How do we set up a magnet? A – The battery feeds current through the coil of wire. Current in the coil of wire produces a magnetic field (as long as the battery is connected).

A Simple Electromagnet A Nail with a Coil of Wire Q - How do we reverse the poles of this electromagnet? A – By reversing the polarity of the battery! S N + -

The Electromagnet in a Stationary Magnetic Field If we surround the electromagnet with a stationary magnetic field, the poles of the electromagnet will attempt to line up with the poles of the stationary magnet. The rotating motion is transmitted to the shaft, providing useful mechanical work. This is how DC motors work! OPPOSITE POLES ATTRACT!

DC Motor Operation Principles Once the poles align, the nail (and shaft) stops rotating. How do we make the rotation continue? By switching the poles of the electromagnet. When they line up again, switch the poles the other way, and so on. This way, the shaft will rotate in one direction continuously!

Brushed DC Motor Components

How the Commutator Works As the rotor turns, the commutator terminals also turn and continuously reverse polarity of the current it gets from the stationary brushes attached to the battery.

Inside a Toy Motor

Inside the Motor, cont.

Advantages of Stepper Motor The DC motors from the workshop offer limited speed control and low torque. A stepper motor for each wheel solves this problem The stepper motors enables accurate wheel positioning with high holding torque and allows for open-loop speed control (wheel position feedback is option). But…is it necessary?

Stepper Motor Operations A stepper motor consists of: A permanent magnet rotating shaft (or rotor) Electromagnets on the stator – the stationary portion that surrounds the motor The stepper motor moves as the permanent rotor magnet attempts to line up with the poles of the electromagnets on the stator. The electromagnets are digitally switched to change their pole orientation, which when done in a sequence produces continuous rotation of the rotor.

Stepper Motor Operations The smallest step of angular rotation a stepper motor can make is called its resolution. Unlike the example, which had 90 degrees per step resolution, real motors employ a series of mini-poles on the stator and rotor to increase resolution.

Stepper Motor Operations Because the rotor is fixed by magnetism in the stationary condition, the stationary torque is large. It allows one to make a precise stop at some angle and hold it there. Speed control is achieved by digitally cycling through the phases at a desired speed of rotation. A microcontroller is used to reverse the current after each step, which changes the poles of the corresponding electromagnets.

Unipolar & Bipolar Steppers The difference between unipolar and bipolar stepper is… A bipolar provides greater torque since an entire coil is energized instead of a half coil for each state of the electromagnet. But…The unipolar is simpler to control Bipolar motors require a slightly more involved controller that must reverse the current flow through the coils by alternating the polarity of the terminals. This is done with the aid of a microcontroller.

What is an H-Bridge? An H bridge is an electronic circuit that enables a voltage to be applied across a load in either direction. These circuits are often used in robotics and other applications to allow DC motors to run forwards and backwards.[1] Most DC-to-AC converters (power inverters), most AC/AC converters, the DC-to-DC push– pull converter, most motor controllers, and many other kinds of power electronics use H bridges. In particular, a bipolar stepper motor is almost invariably driven by a motor controller containing two H bridges.

What is an H-Bridge? The H-bridge arrangement is generally used to reverse the polarity/direction of the motor, but can also be used to 'brake' the motor, where the motor comes to a sudden stop, as the motor's terminals are shorted, or to let the motor 'free run' to a stop, as the motor is effectively disconnected from the circuit.

Switch Operations S1 S2 S3 S4 Result 1 Motor moves right Motor moves right Motor moves left Motor free runs Motor brakes Short Power Supply

How Do You Make Them? Physical Relays Semiconductors Term to Know: MOSFET Metal–oxide–semiconductor field-effect transistor The main advantage of a MOSFET transistor over a regular transistor is that it requires very little current to turn on (less than 1mA), while delivering a much higher current to a load (10 to 50A or more). However, the MOSFET requires a higher gate voltage (3-4V) to turn on. How would you use the Arduino to Control a MOSFET?

EGG-275 Module 4 - Section 4 Motor Sizing

EGG-275 Module 4 - Section 5 Batteries

What is a Battery?

Design Considerations for Batteries Geometry of the batteries. The shape of the batteries can be an important characteristic according to the form of the robots. Durability. Primary(disposable) or secondary (rechargeable) Capacity. The capacity of the battery pack in milliamperes-hour is important. It determines how long the robot will run until a new charge is needed. Initial cost. This is an important parameter, but a higher initial cost can be offset by a longer expected life. Environmental factors. Used batteries have to be disposed of and some of them contain toxic materials

Disposable Battery Types Zinc-carbon battery - mid cost - used in light drain applications Zinc-chloride battery - similar to zinc carbon but slightly longer life Alkaline battery - alkaline/manganese "long life" batteries widely used in both light drain and heavy drain applications Silver-oxide battery - commonly used in hearing aids Lithium Iron Disulphide battery - commonly used in digital cameras. Sometimes used in watches and computer clocks. Very long life (up to ten years in wristwatches) and capable of delivering high currents but expensive. Will operate in sub-zero temperatures. Lithium-Thionyl Chloride battery - used in industrial applications, including computers and electric meters. Other applications include providing power for wireless gas and water meters. The cells are rated at 3.6 Volts and come in 1/2AA, AA, 2/3A, A, C, D & DD sizes. They are relatively expensive, but have a proven ten year shelf life. Mercury battery - formerly used in digital watches, radio communications, and portable electronic instruments, manufactured only for specialist applications due to toxicity

Disposable Battery Types Nickel–cadmium battery (NiCd) was created by Waldemar Jungner of Sweden in 1899. It uses nickel oxide hydroxide and metallic cadmium as electrodes. Cadmium is a toxic element, and was banned for most uses by the European Union in 2004. Nickel–cadmium batteries have been almost completely superseded by nickel–metal hydride (NiMH) batteries. Nickel–metal hydride battery (NiMH) became available in 1989.[28] These are now a common consumer and industrial type. The battery has a hydrogen-absorbing alloy for the negative electrode instead of cadmium. Lithium-ion battery is the choice in most consumer electronics and have the best energy density and a very slow loss of charge when not in use. Lithium-ion polymer battery is light in weight and can be made in any shape desired.

Relative Properties of Rechargable Batteries

EGG-275 Module 4 - Section 6 Motor Sizing

Angular Velocity The velocity (feet per second) of a robot is directly related to the angular velocity, w (in radians per second – NOT RPM) of the motor and the radius of the wheel, R (ideally in meters or feet, so when calculating v it is in meters per second or feet per second). w = v / R Re-arranging this equation, we find the velocity of the robot: v = w*R Most motor manufacturers provide the no load RPM, so to convert RPM to radians per second, you need the following equation: 1 rev/min = 2*pi/60 rad/sec Often, a “Voltage constant”, Kv is provided by the manufacturer; for example, a motor may be rated at Kv= 2.4 Volts / 1000 RPM which correlates the RPM to the voltage.

Distance Traveled To get an idea of how far your robot has traveled, you need to know the circumference of the drive wheel: Circumference (C) = 2*pi*R Distance traveled = w*C*t (where ‘w’ is in rad/sec and ‘t’ is in seconds)

Selection of Wheels Choose smaller sized wheel for flat terrain, larger wheel for more “off- road” (or large obstacles) Proportional to the size of the robot (choosing a wheel proportional to the size of the robot will usually mean the motor you will need is also proportional, and the robot has a better chance of moving at a decent speed) See what wheels are out there – not all wheels have mounting hubs designed for all motors Keep in mind that a larger wheel may require a more powerful motor; we’ll go into more detail about this in a future article. Use the same wheel for all drive motors: as you can see, changing the radius affects both the speed and the distance traveled: you don’t want one wheel “fighting” against the other.

In order for a robot to roll up an incline at a constant velocity (no acceleration or deceleration) it must produce enough torque to “counteract” the effect of gravity, which would otherwise cause it to roll down the incline. On an inclined surface (at an angle theta) however, only one component of its weight (mgx parallel to the surface) causes the robot to move downwards. The other component, mgy is balanced by the normal force the surface exerts on the wheels.

In order for the robot not to slide down the incline, there must be friction between the wheel and the surface. The motor in a heavy truck may be able to produce 250 horsepower and significant torque, but we have all seen (in person or in video) large trucks simply spinning their wheels as they fall backwards on an icy street. It is friction (f) that “produces” the torque.

To select the proper motor, we must consider the “worst case scenario”, where the robot is not only on an incline, but accelerating up it.

Total power (P) per motor can be calculated using the following relation:

T is known from above and the angular velocity (w) is specified by the builder. It is best to select the maximum angular velocity to be able to find the corresponding maximum power. Knowing the maximum power and the supply voltage (V) which the builder chooses, we can find an idea of the maximum current (I) requirements:

The two equations above are used to produce the following relation:

Finally, the capacity (c) of battery pack required can be estimated using the equation:

Notes on Battery Sizing You may wonder why such a large value is needed. This is because when choosing a battery pack, the rated amp hours are not an accurate indicate of the maximum current the pack can produce for extended periods of time. Also, the total charge is rarely retained over time. This way you will ensure the battery pack you select will be capable of producing the current your motors require, for the time you require and with the inefficiencies inherent in recharging battery packs. Note: This is the battery required PER MOTOR. To obtain a total battery pack required for the robot, multiply this value by the number of drive motors.

Motor Sizing Tool The Good News: you don’t need to do the math yourself This tool will help you decide how much power you need (in amp hours), how much current your motors will draw (in amps), and how much torque your motors will need http://www.robotshop.com/blog/en/drive-motor-sizing-tool-9698 Review the tutorial for more data: http://www.robotshop.com/blog/en/drive-motor-sizing-tutorial- 3661

EGG-275 Module 4 - Section 7 Wheel Sizing

Big Wheels or Small?

Design Trade-Offs Larger Wheels “float” on sand… …and, they travel over obstacles easier.. …But…They are heavier… …Need more Torque… …and more Power… Which means bigger, more expensive power systems What to do?

Assignments for Next Week EGG-275 Module 4 - Section 8 Assignments for Next Week

For Next Time… Team Assignment - Program management Preliminary Design Review Each team member to present

For Next Time… Individual Design Assignment: Prepare for the PDR with your team

For Next Time… Reading http://en.wikipedia.org/wiki/H_bridge Review the PDR example on the D2L Website Incorporate the basic elements into your PDR