L E C T U R E R, E L E C T R I C A L A N D M I C R O E L E C T R O N I C E N G I N E E R I N G

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1 P R O F. S L A C K L E C T U R E R, E L E C T R I C A L A N D M I C R O E L E C T R O N I C E N G I N E E R I N G G B S E E R I T. E D U B L D I N G 9, O F F I C E ( ) R O C H E S T E R I N S T I T U T E O F T E C H N O L O G Y 7 9 L O M B M E M O R I A L D R I V E R O C H E S T E R, N Y FORWARD This lecture offers PRACTICAL guidelines and considerations on your pathway to selecting, integrating, simulating, prototyping a motor driving scheme. This is NOT a motor design lecture. If you need to understand the internal design of a motor, please search electrical motors using Wallace Library website. There are a number of very useful books on this topic. 1

2 MOTOR CONTROLS BLOCK DIAGRAM MOTOR TYPES 1. DC DC Motors Brushed DC Motor Controls H Bridge 2. Servo 180 rotation Continuous 3. Position Sensing 4. Stepper 5. Three Phase DC 6. AC Light Industrial Appendix Added info Determining Motor needs Generators Controls: Know your Specifications 2

3 DC MOTORS BRUSHED Many DC motors used in Senior Design are brushed DC motors. These are simple to control, inexpensive and effective in many applications. Operational sequence: 1. Two external motor leads are connected to two internal brushes which contacts a 180 split circular ring called a commutator. 2. This split ring acts as a mechanical inverter. The incoming DC current flips or inverts current direction every This enables the N and S magnets to push the coil of wires often called the armature. 3

4 SIMPLE DC MOTOR CONTROL If your project requires single direction rotation, then a simple switch or transistor is all that is needed. Add a diode to suppress back EMF when the motor is switched off by the transistor. That is, as the motor inductive current continues to flow, the diode will discharge back to the other terminal of the motor. If speed control is needed, then use PWM function on a microcontroller... or simply a 555 oscillator if manual control is sufficient. DIRECTION CONTROL H BRIDGE This schematic offers forward and reverse motor direction. H-bridge got its name from its schematic shape. That is, it forms an H. Four switching elements at the "corners" of the H and the motor forms the cross bar. 4

5 HOW H BRIDGES WORK The switches are turned on in pairs, either high left and lower right, or lower left and high right, but never both switches on the same "side" of the bridge. If both switches on one side of a bridge are turned on it creates a short circuit between the battery plus and battery minus terminals. Anything that can carry a current will work, from four SPST switches, one DPDT switch, relays, transistors, MOSFETs. Brake mode. Upper or lower switches closed. SUPPRESSION DIODES AND ARCING Why?? Protect the four transistors. How do Suppression Diodes help? First, note these diodes are reversed from normal power supply so no current will flow when the motor is off or when the motor is on. The issue is when a motor switches off. Since internal electrical circuitry of a motor is a coil of wire (i.e. inductor), current in an inductor needs to continue to flow. When the diagonal transistor pair switches off and thus causing I C go to zero, the motor coils (i.e. inductors) are still charged with current and must flow through the internal inductors in the motor. This is called Back EMF. Without suppression diodes, the current must flow through the transistors which are switched off. Since the transistors are off, the Back EMF will go in 100 s of voltages to push the current through the motor s inductor circuitry. This Back EMG voltage is high enough to damage transistors. So the current path when the transistor pair switches off is through the motor (Back EMF), through the suppression diode to V+, through the power supply, through the Gnd, through the lower suppression diode and then back into the motor. This current surge will end without damaging the electronics. Okay when current flows into the positive lead of the power supply, the voltage level will dip or perhaps even go negative for a short period of time. You can trigger your scope to see this. Electronics such as a microcontroller can easily reset itself when this happens. Some times at random. 5

6 BUY OR BUILD H BRIDGE? Your project s Needs! Cost? Reliability? Features? If you buy an H Bridge, there are 100 s to pick from! For your DC motor, know your voltage and current specifications. As an example of a small H bridge, This small power IC drives a 11V at 1.8A DC motor. Has a brake function, arc suppression (they all do.) 6

7 SERVO MOTOR PARTS A Servo is a motored device that has an output shaft that can be positioned to specific angular positions by sending the servo coded signal. As long as the coded signal exists on the input, the servo will maintain the angular position of the shaft. As the coded signal changes, the angular position of the shaft changes. Freescale Servo Notes: HOBBY RC SERVO 7

8 HOBBY RC SERVO MOTOR PWM CONTROL SIGNAL Note that only a single PWM signal is needed from an external controller. The servo electronics are smart enough to determine the motor shaft rotational position based off a PWM signal pulse width. No external feedback is needed to determine placement. A range of 30 to 50 milliseconds between pulses is typical. Servo may have serial interface (i.e. USB, RS , CAN) and may be able to wire several in series. Software is readily available. SERVO CONTROLLERS (non-rc) Servo Motor systems use a closed loop control systems to precisely position mechanical devices. Where are Servo s used? PC Hard drives, ink jet printers, industrial needs requiring precision and/ or speed. A good example, on campus is the pick & place PCB layout machine in building minute YouTube explanation Designing a Servo Controller requires PID (proportional integral integral derivative) control and knowledge of project dynamics. For advanced project requirements, recommendation is to purchase a control unit and Servo Motor as a matched pair. 8

9 POSITION SENSING DC motors are not necessary sufficiently accurate due to varying mechanical loads. Shaft encoder and a home position offers feedback to motor controller for precise movement. Detailed theory of encoders: /appxa.pdf 12vdc rpm-6mm-shaft.aspx 9

10 POSITIONING : SHAFT ENCODERS Color Black Red Blue Green Yellow White Function motor power motor power Hall sensor Vcc ( V) Hall sensor GND Hall sensor A output Hall sensor B output Encoder Viewed from the back side of a DC Motor. This encoder is built into the motor. POSITIONING: HALL SENSOR A AND B OUTPUTS This will tell the RPM speed. Often 400 pulses/ rev A and B pulses are offset by 90. This offers rotational direction of motor. That is, A lead edge is leading B, thus indicates CW direction. When A leads B, then CCW. Some encoders (not this one) have another sensor output (i.e. C) labeled Home Position which offers one pulse per revolution. Some encoders have serial interface and calculates RPM and periodically sends various parameters. 10

11 STEPPER MOTOR DESCRIPTION A stepper motor (or step motor) is a brushless DC electric motor that divides a full rotation into a number of equal steps. The motor's position can then be commanded to move and hold at one of these steps without any feedback sensor. Source: Wikipedia Stepper Motors by their inherent design are well controlled motors. These motors have a series of internal windings that will turn the armature to the next coil winding. Like a DC Motor, a stepper motor uses DC power. A Stepper Motor will step when given a specific pulse. 11

12 STEPPER MOTOR INTERNAL COMPONENTS This stepper motor has a rotor of 50 teeth and the stator has 8 poles with 5 teeth each (total of 40 teeth), the stepper motor is able to move 200 distinct steps to make one complete revolution. This means that shaft of the motor will turn 1.8 per step. Many stepper motors have integrated gear down transmissions to increase torque and decrease the step increment. That is a 10:1 transmission will give 0.18 degree step and increase the torque by nearly that much. STEPPER MOTOR DESIGN HELP Anaheim Automation is a primary manufacturer of quality or industrial Stepper Motors. Helpful link to select the correct stepper motor and controller. :// products.php?gclid=cower7-41ricfcdlogodq2yamq Least expensive: Permanent Magnet (PM) Stepper Motors. Texas Instruments Evaluation Kit, highly recommended to learn electronics. Less critical applications are a number of less expensive motors from several sources. 12

13 STEPPER CONTROLLER - EVALUATION KITS Recommendation: Texas Instruments Evaluation Kit: DRV8412 Integrated Motor Driver for Brushed and Stepper Motors with Piccolo F $199 includes motors. c2-kit Stepper Motor controllers are fairly simple but purchase an evaluation board for circuit and software starting point. Many on-line schematics and software are available. COMPARING SERVO AND STEPPER MOTORS AND CONTROLLERS :// One of the main differences between servo motors and stepper motors is that servo motors, by definition, run using a control loop and require feedback of some kind. A control loop uses feedback from the motor to help the motor get to a desired state (position, velocity, and so on). There are many different types of control loops. Generally, the PID (Proportional, Integral, Derivative) control loop is used for servo motors. For more information, see the related link, PID Controller: Theory and Practice. When using a control loop such as PID, you may need to tune the servo motor. Tuning is the process of making a motor respond in a desirable way. Tuning a motor can be a very difficult and tedious process, but is also an advantage in that it lets the engineer and user have more control over the behavior of the motor. Since servo motors have a control loop to check what state they are in, they are generally more reliable than stepper motors. When a stepper motor misses a step for any reason, there is no control loop to compensate in the move. The control loop in a servo motor is constantly checking to see if the motor is on the right path and, if it is not, it makes the necessary adjustments. In general, servo motors run more smoothly than stepper motors except when microstepping is used. Also, as speed increases, the torque of the servo remains constant, making it better than the 13

14 SERVO VS STEPPER - SUMMARY Advantages Servo over Stepper High intermittent torque High torque to inertia ratio High speeds Work well for velocity control Available in all sizes Quiet Advantages of Stepper over Servo Functions open loop no feedback required No tuning of control loop parameters (i.e. PID) Stepper motor system is less expensive 5. THREE PHASE DC 3 Phase motors are used in devices such as hard drives, ink jet printers and other applications where precision is needed. Recommendation: Purchase a Development Kit such as Texas Instruments 3-Phase BLDC Motor Kit 14

15 6. AC MOTOR LIGHT INDUSTRIAL Sometimes referred to as Farm Duty. Can be plugged directly into an AC receptacle and/ or controlled by a wall switch hz, single phase, 115/ 230 VAC 2. Fixed RPM. Typical range: 600 to 3600 RPM. 3. Reversible by moving wire connections under an inspection plate on the motor. 4. Motor cases come in many forms to match your mounting application. Advantages: Inexpensive, powerful, reliable, abusive environments, no DC power supply or controller needed. Disadvantages: Can not perform more advanced control unless controllers are used. APPENDIX: ADDED INFO Determining Motor needs Controls: Know your Specifications Generators 15

16 DETERMINING MOTOR NEEDS Mechanical Power Needs Torque (i.e. oz-in, ft-lbs, newton-meter) Electrical Power Needs (i.e. kw, W, Hp, voltage, current) to meet your mechanical power needs RPM active range Cost Precision? Encoders, Motor type Locking, Braking? When the motor is off, will the shaft move? Is this an issue? Vibration? (continued) DETERMINING MOTOR NEEDS (CONTINUED) Linear versus rotational motion Gear reduction versus belt & pulley reduction Heavy starting loads Safety design Kill switch, software monitoring, torque & RPM sensing Compactness Quietness Environmental situations Fire hazard Periodic Servicing needs 16

17 CONTROLS: KNOW YOUR SPECIFICATIONS Lab Bench Testing Current sensing circuitry Shaft Tachometer Torque instrumentation; torque wrench Ammeter Heat/ temperature GENERATORS Convert mechanical energy to electrical energy. Rotating an armature, which carries conductors, in a magnetic field and thus inducing an emf in these conductors. A relative motion must always exist between the conductors and the magnetic field in such a manner that conductor cut through the field. Many have manual resettable fuses. AC Generators (also known as alternators when rectifiers are integrated) 17