Introduction to MotorsUnderstanding the CEENBots musclesPresented by: Herb Detloff(Prepared by Alisa N. Gilmore, P.E.)CEPA Summer Robotics Institute Summer 2009at Central Community College, Columbus, NE

Robot Defined

OverviewMotors in context of robotics, different types of robots have different types of motorsOverview of motor types / characteristicsMotors convert electric energy to mechanical force / motionMotor parameters: AC or DC power source, torque, speedIndustrial robotics: AC servo motorMobile robotics & Hobby robots: dc motor, dc servo motor, and stepper motorsPrinciple of operation of a DC motorInside a DC motorPrinciple of operation of stepper motorsPerformance advantages of stepper motor over DC motor and DC servo motorCEENBot stepper motor operation/control

Motor BasicsMotors 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.

Choosing a MotorThere 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 ParametersThe level of performance a motor can provide is described by its parameters. These include:Rated Speed Speed measured in shaft revolutions per minute (RPM)TorqueRotational 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 MotorsThe 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

AC Motor CharacteristicsWhen power is applied, AC motors turn in one direction at a fixed speed. Both reversable and non-reversable models availableUsually high voltage (110V AC and up)Inexpensive and commonly availableOptimized 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.

DC Motor CharacteristicsWhen 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 Servo MotorsServo 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 systemHobby 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 positionThe 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

Stepper MotorsRequires a separate controller circuitry or it will not turn when power is applied.Inexpensive and commonly available, especially in salvaged computer equipmentPrecise 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.

DC Electric MotorsElectric 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:BatteriesDC 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 BasicsMotors 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.

MagnetsA 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 ElectromagnetA 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 ElectromagnetA Nail with a Coil of Wire

Q - How do we reverse the poles of this electromagnet? A By reversing the polarity of the battery!

+-SN

The Electromagnet in a Stationary Magnetic FieldIf 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 PrinciplesOnce 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 WorksAs 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 DirectionTo change the direction of rotation:Simply switch the polarity of the battery leads going to the motor (that is, switch the + and battery leads)

+--+CWCCWDirection of Rotation

Inside a Toy Motor(Similar to TekBot Motor)

Advantages of Stepper 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).

Stepper Motor OperationA 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

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.

ReferencesThe Difference Between Stepper Motors, Servos, and RC Servos by Roger Arrick http://www.arrickrobotics.com/motors.htmlMaking Things General Information on Motors http://www.makingthings.com/teleo/products/documentation/app_notes/motors_general.htmHow Stepper Motors Work by Images Scientific Instruments http://www.imagesco.com/articles/picstepper/02.htmlCEENBot Stepper Motor & PM DC Motor Testing Unit Operations Manual by Ben Barenz, CEEN StudentHansen Corp. Servo motors http://www.hansen-motor.com/servo-motors.htmAnimated operation of a Unipolar stepper motor: http://www.interq.or.jp/japan/se-inoue/e_step1.htm