ME 440: Numerically Controlled Machine Tools

1. ME 440: Numerically Controlled Machine Tools Electric Motors and Drive Systems Assistant Prof. Melik Dölen Department of Mechanical Engineering Middle East Technical University

2. Outline – Motors • Motors in CNC Technology • Classification of Motors • Stepper Motors • DC Motors • Brushless DC Motors • Induction Motors • Fundamentals of Motor Drives • DC Motor • Stepper Motor ME 440

3. Electrical Motors • In most CNC machine tool applications, electrical motors are extensively used as actuators: • Axis motion • Spindle motion • 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. ME 440

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!). ME 440

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 ME 440

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. ME 440

7. (Simplified) Half-Step Operation ME 440

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. ME 440

9. Bipolar (4-wire): Unipolar (5-wire): Unipolar (6-wire): Winding Connections • Unipolar motor: • Current flows through a coil only in one direction. • Bipolar motor: • Current flowing through a winding changes direction during the operation. ME 440

10. 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 ME 440 [*] Courtesy of Microchip.

11. Full-step: Half-step: PM Stepper-Motor Animations* [*] Courtesy of Motorola, Inc. ME 440

12. VR Full-Step Motor • Rotor and stator saliency • Unequal number of poles • Stator current effectively pulls rotor pole in line with stator pole. ME 440

13. VR Half-Step Motor • Possible to move rotor by half steps by exciting two windings equally. • Finer steps (a.k.a. “micro-steps”) are possible by exciting two windings unequally. ME 440

14. 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. ME 440

15. 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. ME 440

16. 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. ME 440

17. 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. ME 440

18. “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. ME 440

19. 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. ME 440

20. 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. ME 440

21. 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. ME 440

22. 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. ME 440

23. DC Motor Drivers • Commercial Motor Drivers • Include all bells and whistles! • Custom Solutions (high-power) • Switches: Power MOSFETs, IGBT • Needs gate drivers and signal isolation barriers. • Bridge ICs (upto a few-hundred Watts) • LMD 18200 • L298 • For driving small DC motors, • L293D • ULN 2003A ME 440

24. Electromagnetic Relays • Relays are electromagnets connected to mechanical switches. • When the electromagnets are energized, the switches are pulled into contact. • Hence, the corresponding circuit is powered up. • Relays allow the control of high-power devices. • Small power is sufficient to energize electromagnets in relays. • Suitable for on/off control of slow devices: • Pump (AC/DC) motors, solenoids • Heaters, lamps, etc. • If compared to solid-state switches, relays are more susceptible to malfunction. ME 440

25. Simple On/Off Control ME 440

26. Drives for VR Stepper Motors • Currents do not need to reverse. • Circuit uses incomplete switch poles that can pass current only one direction through motor phase. ME 440