Introduction to MotorsUnderstanding 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 dc servo motor. and stepper motors Principle of operation of stepper motors CEENBot stepper motor operation/control ◦ Performance advantages of stepper motor over DC motor and DC servo motor Overview .Motors in context of robotics. speed ◦ Industrial robotics: AC servo motor ◦ Mobile robotics & Hobby robots: dc motor. torque. different types of robots have different types of motors Overview of motor types / characteristics Principle of operation of a DC motor ◦ Inside a DC motor ◦ Motors convert electric energy to mechanical force / motion ◦ Motor parameters: AC or DC power source. Motors convert electric energy to mechanical motion. Either an AC or DC electrical energy source serves as the input to the motor. provided the load carried by the shaft does not exceed the maximum load the motor is designed to carry. that is a rotation about or a translation along the shaft. Motor Basics . The result is mechanical motion of the output shaft. size of the object to be moved. The accuracy of position or speed control needed. The weight. thus there are many different types of motors. There are numerous ways to design a motor. These include: ◦ ◦ ◦ ◦ How fast you want the object to move. The cost and size of the motor. Choosing a Motor . The type of motor chosen for an application depends on the characteristics needed in that application. ..000 footpounds per minute. Motor Parameters . measured in lb-ft (or. The level of performance a motor can provide is described by its parameters. oz-in). These include: Rated Speed Torque ◦ Speed measured in shaft revolutions per minute (RPM) HorsePower = Speed x Torque / 5252. due to a force applied at a radius from that point.11. Torque-Speed performance of a motor ◦ Rotational force produced around a given point. ◦ A measure of work expended: 1 HP = 33. The different types of motors possess different operating characteristics. ◦ Heavy Industrial applications: AC motors ◦ Mobile robotics & hobby robots: dc motor. and stepper motors Brief overview of the operation characteristics of: ◦ ◦ ◦ ◦ AC motors DC motors DC servo motors Stepper motors Types of Motors . dc servo motor. usually high RPM. If the applied load is greater than the capacity of the motor. AC Motor Characteristics . When power is applied. AC motors turn in one direction at a fixed speed. the motor will stall and possibly burn out. Both reversable and non-reversable models available Usually high voltage (110V AC and up) Inexpensive and commonly available Optimized to run at a fixed. Available in wide range of speeds and power. usually high RPM. Not suitable for positioning unless some kind of position feedback is added. When power is applied. Torque is highest at the rated speed and lowest at low speeds. Inexpensive and commonly available. Almost all can be reversed. If the applied load is greater than the capacity of the motor. DC motors turn in one direction at a fixed speed. DC Motor Characteristics . Suitable for turning. spinning. Speed can be varied if a (pulse width modulation) PWM controller is added. etc. They are optimized to run at a fixed. the motor will stall and possibly burn out. A system with a motor. feedback. DC servo motors consist of a DC motor combined with feedback for either position or speed. Servo motor requirements may include control of acceleration. in which case the control signal is speed instead of position DC Servo Motors . stops and reversals. When power is applied. Once told where to go. 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. and position to very close tolerances and allow for fast starts. velocity. 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. and very accurate control. a Hobby Servo uses its built-in controller and feedback system to hold its position. even without feedback. the motor may not step.Requires a separate controller circuitry or it will not turn when power is applied. 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. thereby making precise positioning no longer possible. especially in salvaged computer equipment Precise positioning is possible by keeping count of steps requested. Inexpensive and commonly available. Stepper 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. producing a force (called torque) that results in mechanical motion. Electric Motors or Motors convert electrical energy to mechanical motion Motors are powered by a source of electricity – either AC or DC. the loop will turn. DC Electric Motors . When this loop is surrounded by the field of another magnet. Inside a motor we find: ◦ Permanent magnets. ◦ Electro-magnets. but rely on principles of magnetism to produce mechanical motion. ◦ Or a combination of the two. Motors are powered by electricity. Motor Basics . It requires a power source (such as a battery) to set up a magnetic field.Magnets ◦ A magnet is an object that possesses a magnetic field. . ◦ A permanent magnet (such as this bar magnet) stays magnetized for a long time. characterized by a North and South pole pair. ◦ An electromagnet is a magnet that is created when electricity flows through a 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 set up a magnet? A – The battery feeds current through the coil of wire. . How do we reverse the poles of this electromagnet? A – By reversing the polarity of the battery! .A Simple Electromagnet A Nail with a Coil of Wire S N + - Q . providing useful mechanical work. OPPOSITE POLES ATTRACT! The rotating motion is transmitted to the shaft.The Electromagnet in a Stationary Magnetic Field If we surround the electromagnet with a stationary magnetic field. This is how DC motors work! . the poles of the electromagnet will attempt to line up with the poles of the stationary magnet. and so on. When they line up again. switch the poles the other way. the shaft will rotate in one direction continuously! . the nail (and shaft) stops rotating. How do we make the rotation continue? By switching the poles of the electromagnet. This way.DC Motor Operation Principles Once the poles align. Brushed DC Motor Components . the commutator terminals also turn and continuously reverse polarity of the current it gets from the stationary brushes attached to the battery.How the Commutator Works As the rotor turns. . switch the + and – battery leads) Direction of Rotation CW CCW + + .Controlling Motor Direction To change the direction of rotation: ◦ Simply switch the polarity of the battery leads going to the motor (that is. Inside a Toy Motor (Similar to TekBot Motor) . 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 . The DC motors on the TekBot offer limited speed control and low torque. http://www.or.interq.htm Stepper Motor Operation . which when done in a sequence produces continuous rotation of the rotor.jp/japan/se-inoue/e_step1. The electromagnets are digitally switched to change their pole orientation. or can be controlled to give precise rotation to a desired angular position. 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. which had 90 degrees per step resolution. The smallest step of angular rotation a stepper motor can make is called its resolution. real motors employ a series of mini-poles on the stator and rotor to increase resolution. the same sequence of the 4 stepping phases is used to control this scenario. Surprisingly.htm CEENBot stepper motors have a resolution of 1.or.8 degrees per step.jp/japan/se-inoue/e_step1. ◦ Q: How many steps are needed to make 1 complete wheel revolution? .interq. Unlike the example. http://www. Speed control is achieved by digitally cycling through the phases at a desired speed of rotation. It suits the use to make stop at some angle. A microprocessor is used to reverse the current after each step. the stationary power (Stationary torque) is large. Because the rotor is fixed by the magnetism in the stationary condition as shown. which changes the poles of the corresponding electromagnets. . ◦ The CEENBot can better hold its position on a ramp. arrickrobotics.html CEENBot Stepper Motor & PM DC Motor Testing Unit Operations Manual by Ben Barenz. and RC Servos” by Roger Arrick http://www. CEEN Student Hansen Corp.com/articles/picstepper/02.htm “How Stepper Motors Work” by Images Scientific Instruments http://www.html Making Things – “General Information on Motors” http://www. “The Difference Between Stepper Motors. Servos.com/teleo/products/documentati on/app_notes/motors_general.com/motors.htm References .htm Animated operation of a Unipolar stepper motor: http://www.imagesco.makingthings.or.jp/japan/se-inoue/e_step1. “Servo motors” http://www.hansenmotor.interq.com/servo-motors.