ME 407: Mechanical Engineering Design

1. ME 407: Mechanical Engineering Design Electric Motors/Drivers & Their Selection Assistant Prof. Melik Dölen Department of Mechanical Engineering Middle East Technical University

2. Outline – Electric Motors* • Classification of Electric Motors • Stepper Motors • DC Motors • Brushless DC Motors • Induction Motors • Fundamentals of Motor Drives • DC Motors • Motor/Driver Selection Procedure • Load Analysis • Performance Curves • Applications • Summary [*] W. Voss,A Comprehensible Guide to Servo Motor Sizing, Copperhill Tech.Corp. 2007.

3. Electrical Motors • In most industrial applications, electrical motors are extensively used as actuators. • Four motor systems are common alternatives in machine tool designs: • Stepper motors: Simple applications (e.g. desktop manufacturing tools) • DC motors: Earlier CNC machine tools and specialized machine tools • Brushless DC motors: Principle axis drives for contemporary CNC machine tools • AC (Induction) motors: High-power spindle drives.

4. Stepper Motors • Stepper Motors • Permanent Magnet • Relies on rotor magnets • Variable Reluctance • Relies on rotor saliency • Hybrid Motors • Relies on both rotor saliency and magnets • Each pulse moves rotor by a discrete angle (i.e. “step angle”). • Counting pulses tells how far motor has turned without actually measuring (no feedback!).

5. Low cost Simple and rugged Very reliable Maintenance free No sensors needed Widely accepted in industry Resonance effects are dominant Rough performance at low speed Open-loop operation Consume power even at no load Advantages / Disadvantages

6. (Simplified) Full-Step Operation • Rotor of a PM stepper motor consists of apermanent magnet: • Stator has a number of windings. • Just as the rotor aligns with one of the stator poles, the second phase is energized. • The two phases alternate on and off to create motion. • There are four steps.

7. (Simplified) Half-Step Operation

8. Half-Step Operation (Cont’d) • Commutation sequence has eight steps instead of four. • The main difference is that the second phase is turned on before the first one is turned off. • Sometimes, both phases are energized at the same time. • During the half-steps, the rotor is held in between the two full-step positions. • A half-step motor has twice the resolution of a full-step motor. • Very popular due to this reason.

9. Actual Stepper Motor* • The stator of a real motor constitutes more coils (typically 8). • These individual coils are interconnected to form only two windings: • one connects coils A, C, E, and G: • A and C have S-polarity • E and G have N-polarity • one connects coils B, D, F, and H: • B and D have S-polarity • F and H have N-polarity [*] Courtesy of Microchip.

10. Full-step: Half-step: PM Stepper-Motor Animations* [*] Courtesy of Motorola, Inc.

11. Courtesy of Motorola, Inc. Conventional DC Motor • The stator of a DC motor is composed of two or more permanent magnet pole pieces. • The rotor is composed of windings which are connected to a mechanical commutator. In this case the rotor has three pole pairs. • The opposite polarities of the energized winding and the stator magnet attract and the rotor will rotate until it is aligned with the stator. • Just as the rotor reaches alignment, the brushes move across the commutator contacts and energize the next winding. • A spark shows when the brushes switch to the next winding.

12. Brushless DC Motor • A brushless DC motor (BLDC) has a rotor with permanent magnets and a stator with windings. • It is essentially a DC motor turned inside out. The brushes and commutator have been eliminated and the windings are connected to the control electronics. • The control electronics replace the function of the commutator and energize the proper winding. • he energized stator winding leads the rotor magnet, and switches just as the rotor aligns with the stator. • BLDC motors are potentially cleaner, faster, more efficient, less noisy and more reliable.

13. AC (Induction) Motor • Motor is essentially driven like an AC synchronous motor by applying sinusoidal current to motor windings. • The drive needs to generate 3 currents that are in the correct spatial relationship to each other at every rotor position. • High-resolution optical encoder is needed to control the commutation accurately. • Very smooth low speed rotation. • Negligible torque ripple.

14. Servo-Motor Drivers • Most servo-motor drivers incorporate motion controllers that allow the user to control • Torque (phase currents) • Speed • Position • Once the user selects the control mode, the motor drivers must be connected to multi-axis controller unit (industrial PC, motion control card etc.): • Wiring configuration • Setting motor parameters via • Manually (Control Panel / Memory Stick) • Software assistance

15. Torque Control Mode • In this mode, the motor driver accurately regulates the motor phase currents in respect to the rotor’s position (namely, rotor magnetic flux linkage vector). • Servo-motor acts like an ideal torque modulator to yield the electro-magnetic torque being demanded by the (position) control system. • Torque command is issued through an analog input (usually a bipolar voltage). • In precision motion control applications, this mode is frequently preffered.

16. Velocity Control Mode • Motor driver regulates the rotor’s angular velocity. • Relies on built-in incremental position encoder to measure velocity. • Generally, a digital PI controller is employed to control the velocity. • Velocity controller feeds torque commands to the current/torque regulator. • User must upload the relevant gains and parameters of the “hardwired” controller to the driver. • Velocity command is usually issued through an analog input. • Use of control data buses (such as CAN, SERCOS, Profibus, RS-485, etc.) to send digital commands out to the driver is also common in industry.

17. Position Control Mode • Motor driver regulates the rotor’s angular position. • Motor driver again employs built-in incremental position encoder to measure position. • Generally, a digital PID controller is utilized to control the position. • Position controller feeds torque commands to the current/torque regulator. • User must upload the relevant gains and parameters of the “hardwired” controller to the driver. • For convenience, command is usually issued through two digital inputs (i.e. direction and pulse). • The servo-motor behaves like a position controlled stepper motor. • Advanced drivers support data buses (such as CAN, SERCOS, Profibus, RS-485, etc.) to send/receive digital information.

18. Operating Modes of DC Motor • In motor mode, the machine drives the “load” and needs energy from the supply. • In generator mode, the “load-side” drives the machine and it generates power.

19. “Forward Motor” Control • Electronically-controlled (unidirectional) switch is turned on/off rapidly. • Pulse width modulation • Desired (average) voltage at the terminals of DC motor is obtained via controlling switching times: where Tp is PWM period (constant) and Td/Tp = d is called duty cycle.

20. Mode 1: Mode 2: Forward Motor Control (Cont’d) • When S1 is turned off, ia flowing through the motor cannot be cut offimmediately. • It must flow somewhere! • The “clamp” diode allows current flow in Mode 2: • La drives a decaying current. • If D1 isn’t inplace, a very large voltage will build up across S1 and blow it up.

21. Four-Quadrant Motor Control • “H” bridge is used to operate the motor in four quadrants. • Driver is composed of two half-bridges. • Switches in a half-bridge cannot turned at the same time. • causes short-circuit. • If one of the switches is turned, the other must be off.

22. Mode 2: Mode 1: Forward Motor • To go forward, • S3 is fully turned on; • PWM and ~PWM (inverted PWM) signals are applied to S2 and S1 respectively. • Unidirectional switch S1 can carry current only in the indicated direction.

23. Mode 2: Mode 1: Reverse Motor • To go backward, • S1 is fully turned on; • PWM and ~PWM signals are applied to S4 and S3 respectively.

24. Indirect Control System Courtesy of Heidenhain Corp.

25. Direct Control System Courtesy of Heidenhain Corp.

26. Generic Servo-Control System

27. Servo-Control (Cont’d)

28. Factors to Consider • The drive requirements must be defined before proceeding to motorselection: • How fast and at which torques does the load move? • How long do the individual load phases last? • What accelerations take place? • How great is the mass-moment of inertia?

29. Factors (Cont’d) • Often the motor is indirectly coupled to the load shaft, this means that there is a mechanical transformationof the motor output power using belts, gears, screws and the like. • The drive parameters, therefore, are to be reflectedonto the motor shaft.

30. Motor Selection • Decide the motor technology to use (DC brush, DC brushless, stepper, etc.) • Select a motor/drive combination • Does motor support the required maximum velocity? If no, select next motor/drive. • Use rotor inertia to calculate system (motor plus mechanical components) acceleration (peak) andRMS torque

31. Motor Selection (Cont’d) • Does motor’s rated torque support the system’s RMS torque? If no, select next motor/drive. • Does motor’s intermittent torque support the system’s peak torque? If no, select nextmotor/drive. • Does the motor’s performance curve (torque over speed) support the torque and speedrequirements? If no, select next motor/drive.

32. Selection Procedure* [*] Courtesy of Omron, Corp.

33. Load Analysis • Calculate inertia of all moving components • Determine inertia reflected to motor • Determine velocity, acceleration at motor shaft • Calculate acceleration torque at motor shaft • Determine non-inertial forces such as gravity, friction, pre-load forces, etc. • Calculate constant torque at motor shaft • Calculate total acceleration and RMS (continuous over duty cycle) torque at motor shaft

34. Load Calculation Load Side: Gearing Ratio: Motor Side: When combined: where

35. Other Load Types* [*] Courtesy of Omron, Corp.

36. Common Motion Profiles* [*] Courtesy of Omron, Corp.

37. Performance Curves* (Brushless DC) • There are two operating regions: • Continuous Duty Region: Motor can deliver the torque continuously without overheating. • Steady-state regime • Limited (Intermittent) Duty Region: Large torque can be developed with decreased overall efficiency. • Transient regime (during acceleration/deceleration)   [*] Courtesy of Pasific Scientific, Inc.

38. Performance Curve* (Brushed DC) [*] Courtesy of Baldor, Inc.

39. Selection Criteria • The motor’s rated speed must be equal to or exceed the application’s maximum speed. • The motor’s intermittent (max) torque must be equal or exceed the load’s maximum (intermittent)torque. • The motor’s (continuous) rated torque must be equal to or exceed the load’s RMS torque. • The ratio of load inertia to motor inertia should be equal to or less than 6:1.

40. Selection Criteria (Cont’d) At Steady-State:(0 < M,ss < R) In Duty Cycle:(0 < M,max < R) Worst Case: (0 < M,max < R) Note that duration to stay in the intermittent duty zone varies from one servo-motor to another (0.05 s to 30 s)

41. RMS Torque • The RMS torque (“Root Mean Squared”) represents the average torque over theentire duty cycle.

42. Inertia Matching • For optimum power transfer, the rule of thumb is that the motor’s (mass) moment of inertia should match to that of the load. • Ratio of 1:1 between load and motor inertia would be the ideal scenario. • Rather, impractical – some mismatch is allowed.

43. Inertia Mismatch • If the ratio of load over rotor inertia exceeds a certain range (for servo motors 6:1) consider theuse of a gearbox or increase the transmission ratio of the existing gearbox. Servo motors shouldnot be operated over a ratio of 10:1. • Bosch Rexroth, for instance, recommends the following for inertia mismatch: • 2:1 for quick positioning • 5:1 for moderate positioning • 10:1 for quick velocity changes

44. Some Applications Proper Application: TM,rms < TC(w) TM,max < TP(w) Failure: TM,rms > TC(w) TM,max > TP(w)

45. Applications (Cont’d) Low Speed Operation: w < wR TM,rms < TC(w) High Speed Operation: w >wR TM,rms < TC(w)

46. Typical Data Sheet* [*] Courtesy of Pasific Scientific, Inc.

47. Data Sheet (Cont’d)

48. Data Sheet (Cont’d)

49. Summary