Transcript Introduction to Motors
Introduction to Motors
Understanding the CEENBot’s muscles
Presented by: Herb Detloff (Prepared by Alisa N. Gilmore, P.E.) CEPA Summer Robotics Institute Summer 2009 at Central Community College, Columbus, NE
Robot Defined
Motors in context of robotics, different types of robots have different types of motors Overview of motor types / characteristics ◦ Motors convert electric energy to mechanical force / motion ◦ ◦ ◦ Motor parameters: AC or DC power source, torque, speed 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 CEENBot stepper motor operation/control
Overview
Motors convert electric energy to mechanical motion.
Either an AC or DC electrical energy source serves as the input to the motor.
The result is mechanical motion of the output shaft, that is a rotation about or a translation along the shaft, provided the load carried by the shaft does not exceed the maximum load the motor is designed to carry.
Motor Basics
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.
Choosing a Motor
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
Motor Parameters
The different types of motors possess different operating characteristics.
◦ ◦ Heavy Industrial applications: AC motors Mobile robotics & hobby robots: dc motor, dc servo motor, and stepper motors ◦ ◦ ◦ Brief overview of the operation characteristics of: ◦ AC motors DC motors DC servo motors Stepper motors
Types of Motors
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 RPM. If the applied load is greater than the capacity of the motor, the motor will stall and possibly burn out.
AC 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 RPM.
Torque is highest at the rated speed and lowest at low speeds.
Speed can be varied if a (pulse width modulation) PWM controller is added.
Almost all can be reversed.
Inexpensive and commonly available.
Available in wide range of speeds and power. Suitable for turning, spinning, etc.
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 Motor Characteristics
Servo motor requirements may include control of acceleration, velocity, and position to very close tolerances and 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 system with a motor, feedback, and a controller which constantly adjusts the position or speed to in reaction to the feedback is called a closed-loop system Hobby Servos require a desired position signal to tell them where to turn to.
Once told where to go, a Hobby Servo uses its built-in controller and feedback system to hold its position.
When power is applied, in the absence of a signal, a hobby servo goes to its central position The signal to control a hobby servo is non-trivial to generate.
Hobby servos can also be modified to turn continuously, in which case the control signal is speed instead of position
DC Servo Motors
Requires a separate controller circuitry or it will not turn when power is applied.
Inexpensive and commonly available, especially in salvaged computer equipment Precise positioning is possible by keeping count of steps requested, even without feedback.
Torque is highest at the full stop and decreases as speed is increased.
If the applied load is greater than the capacity of the motor, the motor may not step, thereby making precise positioning no longer possible.
Stepper Motors
Electric Motors or Motors
convert electrical energy to mechanical motion Motors are powered by a source of electricity – either AC or DC.
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
DC Electric Motors
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.
Motor Basics
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.
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
S N + -
Q - How do we reverse the poles of this electromagnet? A – By reversing the polarity of the battery!
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.
OPPOSITE POLES ATTRACT!
The rotating motion is transmitted to the shaft, providing useful mechanical work. This is how DC motors work!
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.
Controlling Motor Direction
To change the direction of rotation: ◦ Simply switch the polarity of the battery leads going to the motor (that is, switch the + and – battery leads) Direction of Rotation CW CCW + +
Inside a Toy Motor (Similar to TekBot Motor)
The DC motors on the TekBot offer limited speed control and low torque. The CEENBot uses a stepper motor for each wheel.
The stepper motors on the CEENBot enables accurate wheel positioning with high holding torque and allows for open-loop speed control (wheel position feedback is not required).
Advantages of Stepper Motor
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, or can be controlled to give precise rotation to a desired angular position.
http://www.interq.or.jp/japan/se-inoue/e_step1.htm
Stepper Motor Operation
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
.
Surprisingly, the same sequence of the 4 stepping phases is used to control this scenario.
http://www.interq.or.jp/japan/se-inoue/e_step1.htm
CEENBot stepper motors have a resolution of 1.8 degrees per step. ◦ Q: How many steps are needed to make 1 complete wheel revolution?
Because the rotor is fixed by the magnetism in the stationary condition as shown, the stationary power (Stationary torque) is large. It suits the use to make stop at some angle.
◦ The CEENBot can better hold its position on a ramp.
Speed control is achieved by digitally cycling through the phases at a desired speed of rotation.
A microprocessor is used to reverse the current after each step, which changes the poles of the corresponding electromagnets.
“The Difference Between Stepper Motors, Servos, and RC Servos” by Roger Arrick http://www.arrickrobotics.com/motors.html
Making Things – “General Information on Motors” http://www.makingthings.com/teleo/products/documentati on/app_notes/motors_general.htm
“How Stepper Motors Work” by Images Scientific Instruments http://www.imagesco.com/articles/picstepper/02.html
CEENBot Stepper Motor & PM DC Motor Testing Unit Operations Manual by Ben Barenz, CEEN Student Hansen Corp. “Servo motors” http://www.hansen motor.com/servo-motors.htm
Animated operation of a Unipolar stepper motor: http://www.interq.or.jp/japan/se-inoue/e_step1.htm