2013 Texas Instruments Motor Control Training Series. -V th. InstaSPIN Training

Transcription

1 2013 Texas Instruments Motor Control Training Series -V th InstaSPIN Training

2 How Do You Control Torque on a DC Motor? Brush DC Motor Desire Current + - Error Signal PI Controller PWM Power Stage Texas Instruments Dav e s Motor Control Center Measure Current ADC1 Commutator keeps rotor an stator fiels properly aligne! Measure current alreay flowing in the motor. Compare the measure current with the esire current, an generate an error signal. Amplify the error signal to generate a correction voltage. Moulate the correction voltage onto the motor terminals.

3 How Do You Control Torque on a Permanent Magnet AC Motor? PMAC Motor Desire Currents + - Error Signals PI Controllers PWM Power Stage Texas Instruments Dav e s Motor Control Center Measure Currents ADC Measure currents alreay flowing in the motor. Compare the measure currents with the esire currents, an generate an error signals. Amplify the error signals to generate a correction voltages. Moulate the correction voltages onto the motor terminals. Torque expression base on amplitue invariant form of Clarke Transform. i.e., The Same Way!

4 Fiel Oriente Control in Real Time Low Torque Meium Torque High Torque Constant Torque 3 2 P 2 r I qs Constant (for now) Ajustable Torque expression base on amplitue invariant form of Clarke Transform. Interrupt: Measure rotor flux angle Regulate current vector to be 90 o wrt rotor flux Exit ISR Interrupt: Measure new rotor flux angle Regulate current vector to be 90 o wrt rotor flux Exit ISR Interrupt: Measure new rotor flux angle Regulate current vector to be 90 o wrt rotor flux Exit ISR

5 1. Measure currents alreay flowing in the motor. Only 2 phases are measure! WHY? A, B, an C axes are fixe with respect to the motor housing. This reference frame is also calle the stationary frame or stator frame. net current vector B i a i b A i c (implie) B C Texas Instruments Dav e s Motor Control Center i b i c i a A Controller with A/D i a i b i c C

6 2. Compare the measure current (vector) with the esire current (vector), an generate error signals. The esire phase currents can be calculate via these equations: i a = -I m sin(q ) i b = -I m sin(q o ) i c = -I m sin(q o ) I m is proportional to motor torque q is the angle of the rotor flux B C Commane i s i b i a Error i s i c A So how can we accomplish this?

7 2. Compare the measure current (vector) with the esire current (vector), an generate error signals. First, we nee the angle of the rotor flux. q or q Magnetic axis for phase A Usually accomplishe with a resolver or encoer.

8 2. Compare the measure current (vector) with the esire current (vector), an generate error signals. The CLARKE transform allows us to convert three vectors into two orthogonal vectors that prouce the same net vector. B i i s i c i b In other wors, convert a 3-phase motor into a 2-phase motor. This is the Amplitue C Invariant form of the Forwar Clarke Clarke Transform A B C A i a 3 i B - C A

9 2. Compare the measure current (vector) with the esire current (vector), an generate error signals. Jump up on the rotating reference frame, whose x-axis is the rotor flux axis. B i q i i s i This is calle the Park Transform i A q C i i cosq i sinq i q -i sinq i cosq

10 Pictorial Example of the Park Transform (Still on Step 2!) -value q-value Synchronous Frame Stationary Frame

11 2. Compare the measure current (vector) with the esire current (vector), an generate error signals. i i q an are hanle inepenently. Since the comparison is performe in the synchronous frame, motor AC frequency is not seen. Thus, they are DC quantities! Uner normal conitions, we have all the flux we nee supplie by the permanent magnets on the rotor. So commane i is set to zero. i (commane) + i (measure) - error (t) This is how much torque we want! i q (commane) + error q (t) i q (measure) - i i q can be use to weaken the fiel of the machine. controls the amount of torque generate by the motor

12 3. (Finally!) Amplify the error signals to generate correction voltages. i (commane) + error (t) - I i (measure) P + + v i q (commane) + - i q (measure) error q (t) P I + + v q The PI regulator is a goo choice for current regulation

13 4. Moulate the correction voltages onto the motor terminals. Before we can apply the voltages to the motor winings, we must first jump off of the rotating reference frame. B v v q v Voltage vector q v A Part A. Transfer the voltage vectors back to the stationary rectangular coorinate system. C v (t) v q (t) v v v v cosq sinq - v v q q sinq cosq

14 4. Moulate the correction voltages onto the motor terminals. Part B. Next, we transform the voltage vectors from the rectangular coorinate system to three phase vectors. B v b v v c v a v Voltage Vector A C Reverse Clarke Transformation A A B C B C

15 4. Moulate the correction voltages onto the motor terminals. Phase A - top Phase A - bottom Phase B - top Phase B - bottom Phase C - top Phase C - bottom Over time, uner steay-state conitions, the correction voltages v a, v b, an v c will be sine waves phase shifte by 120 o.

16 Facts about Fiel Oriente Control 1. F.O.C. was first propose in the late 60 s by Felix Blaschke, a researcher at Siemens who emonstrate F.O.C. on an AC inuction motor. He use magnetic sensors in the airgap of the machine to etermine the rotor flux angle. 2. F.O.C. is a TORQUE control algorithm. Notice that at no time i spee enter into the calculations in the previous slies. However, you can wrap a spee loop aroun an F.O.C. loop to o spee control if you want. The output of the spee control section fees irectly into the commane q-axis current variable. 3. F.O.C. works best on motors which exhibit a sinusoial back-emf waveform. This inclues AC inuction motors, Permanent Magnet Synchronous Motors, an many BLDC motors. 4. Since the voltage an current waveforms are sinusoial, F.O.C. exhibits the lowest torque ripple of any motor control topology. Low torque ripple in turn results in low auible noise. 5. Since F.O.C. requires all of the motor phases be continuously riven, sensorless techniques use with BLDC motors (which require that one phase be unpowere to rea the back-emf signal) will not work with F.O.C. Fortunately, TI has evelope other algorithms that will rea the motor flux angle with high accuracy (a few egrees) without requiring a shaft sensor OR having an unpowere phase. 6. A typical F.O.C. implementation with a shaft sensor takes about 15uS on an 90 MHz F2806x processor. However, to o sensorless F.O.C. without a shaft sensor requires 2 to 3 times as many calculations. Most of the time is use to calculate the rotor flux angle. 7. The integrity of the current measurement process is critical to F.O.C. performance. 10-bit ADCs are aequate, but 12-bit converters are preferre.

17 Trigger Sync Fault Bus Voltage Bus Current Overcurrent Motor PWMs AC In AC to DC Converter DC Bus Gate Driver Gate Drivers Three Phase Inverter Processor Groun Analog Conitioning Power Supplies GPIO or PWM Bus Over- Voltage Phase Current Reconstruction i bus 12 Bit ADC epwm Moule eqep Moule θ(t) V bus i a i b i c Commane i Commane i q θ(t) Fiel Oriente Controller V α V β Actual Spee Space Vector Moulation Spee Calculation F2803x PI Controller - + Commane Spee Serial Interface Isolation

18 Vehicle Spee (CAN) FOC in Electric Power Steering θ. θ Torque PWMs Current Feeback Power Inverter Encoer I/F torque assist PMSM Motor θ feeback Texas Instruments Dav e s Motor Control Center resolver Transmission Controller Essentially, a torque amplifier!