2012 Texas Instruments Motor Control Training. -V th

Transcription

1 2012 Texas Instruments Motor Control Training -V th

2 Stepper Motors Popularized in the early 60s as an economical replacement to closed-loop DC servo systems. Found immediate acceptance in the emerging computer peripheral industry. lthough open-loop stepper control is conceptually easy, complex motor dynamics are often underestimated. vailable in many topologies and step sizes.

3 Exploded View of Stepper Motor N S

4 Stator-Rotor Teeth lignment 48 Stator Teeth 50 Rotor Teeth Unaligned Whole Step (1.9 o Rotor Movement) Half Step (0.8 o Rotor Movement) S ligned North Pole South Pole

5 Stepper Excitation Modes One Phase Two Phase One-Two Phase Waveform period is 4 steps (Full Stepping) Waveform period is 4 steps (Full Stepping) Waveform period is 8 steps (Half-stepping)

6 Current Vectors for Various Stepper Excitation Modes Source: Monolithic, Programmable, Full-ridge Motor Driver Integrates PWM Current Control and Mixed-Mode Microstepping, Paul Emerald, Roger Peppiette, and natol Seliversto,, llegro MicroSystems, Inc. Technical Paper STP 97-5

7 Microstepped Current Waveforms

8 Static Torque Curves The torque generated can be thought of as coming from a mechanical spring, where the K factor of the spring is proportional to this slope. Source: Making Stepper Motors ehave, Timothy J. Harned, Eastern ir Devices, Dover, NH, Reprinted from Machine Design, 1985

9 ll Steppers are NOT Created Equal! High cyclic errors resulting from winding impedance mismatch and poor tolerances on spacing of teeth. Higher precision device used for microstepping applications. Source: Monolithic, Programmable, Full-ridge Motor Driver Integrates PWM Current Control and Mixed-Mode Microstepping, Paul Emerald, Roger Peppiette, and natol Seliversto,, llegro MicroSystems, Inc. Technical Paper STP 97-5

10 Stepper Resonance Source: Technical Information on Stepping Motors, Oriental Motor Ways to reduce resonance. Damping 1. Use voltage drive instead of current drive 2. For L/nR drives, lower the value of n 3. Interphase capacitors. Excitation mode 1. Half-stepping 2. Micro-stepping (next slide )

11 That s One Small Step for Man ase ase i Phase Phase Microstepped Phase Currents Step Response Source: Compumotor Microstepping Linear Motor Systems, 1986, Parker Hannifin Corporation

12 Unipolar Drive Topology Used primarily for simple stepped mode operation (Commutation diodes not shown) Source: irpax Stepper Motor Handbook, 1989

13 ipolar Drive Topology Compatible with more sophisticated control techniques (Commutation diodes not shown) Source: irpax Stepper Motor Handbook, 1989

14 Closed-loop Current Mode Control MSP V sin( ω o t) + - Gain PWM1 PWM2 PWM1 PWM2 PWM2 PWM1 DC Wavetable Coil Coil Texas Instigants Dave s Control Center +24 V cos( ω o t) + - Gain PWM3 PWM4 PWM3 PWM4 PWM4 PWM3 I a I b DC

15 Stepper Model STEPPER MOTOR Va COIL L2 {Ls} R15 sine Coil Torque Torque {Rs} 1 E1 bemf_ 3 Detent Torque bemf ref V=I(L2) Coil Torque sine 2 cosine speed Vb L1 {Ls} V=-{Td}*sin((180/{step_angle})*V(angle)) COIL R1 {Rs} 1 sine 1 E4 bemf_ V=I(L1) cosine bemf ref 5 6 cosine V=-K*sin((90/{step_angle})*V(angle)) V=K*cos((90/{step_angle})*V(angle)) Refer to Spice Models 50 through 52

16 Stepper Model Results Open Coil Voltage (Moving Motor) ack EMF (V) V 0.4V 0.2V 0.0V -0.2V -0.4V -0.6V -0.8V -1.0V MMT STYLE STEPPER MOTOR SINGLE FULL STEP MOVEMENT (Single -V(bemf_) Phase Driven) Non-Driven Coil back EMF Flux (integral of back EMF) Flux (V-S) ms 175ms 225ms 275ms 325ms 375ms Time (S) -1.2V Green waveform: Spice model prediction lue waveform: ctual measured back-emf

17 DRV88xx Motor Drivers - Gate Drivers - Power FETs - Protection - indexer / Control Logic

18 Stepper Motors MSP430

19 Stepper Motors Stellaris

20 Stepper Motor Summary dvantages Very economical solution for position control systems. Inherently high torque/position gain. Easy to interface to digital controllers. Heat is generated in stator: easy to remove. Excellent holding torque. Holding torque even when not energized. Precise speed control economically achieved. Inherent zero following error. Disadvantages Plagued by resonance problems. High inductance and frequent commutation limit high speeds. Certain speeds not permitted due to dynamic instabilities. High vibration resulting from oscillatory behavior. Low efficiency (often operates with high d-axis current).