SPECIAL MACHINES
LOW-COST SENSORLESS CONTROL OF BRUSHLESS DC MOTORS WITH IMPROVED SPEED RANGE
ABSTRACT:
“The Right to privacy….is the most comprehensive of rights and the Right most valued by civilized man”
-Justice Louis Brandies, US Supreme Court,1928.
Our paper presents a low cost sensor less control scheme for BLDC (Brushless DC) motors. In this the rotor position information is derived by filtering only the motor sensing circuits. The cost saving is further increased by coupling the sensor circuits with a single chip microprocessor or DSP (Digital Signal Processing), for speed control. In addition a look-up table based correction for the non ideal phase delay introduced by the filter is suggested to ensure accurate position detection even at low speed. This extends the operating speed range and improves motor efficiency. The BLDC motors with trapezoidal back EMF is usually powered by a set of currents having a quasisquare waveform. It also provides an attractive candidate for sensor less operation because the nature of its excitation inherently offers a low cost way to extract rotor position information from motor terminal voltages. But instead of motor terminal voltages filters can also be used to extract rotor position information. This rotor position information is then fed to a microprocessor or DSP for phase commutation and speed control.
INTRODUCTION:
Because of their higher efficiency and power density, permanent magnet (PM) motors have been widely used in a variety of applications in industrial automation and consumer electric appliances. PM motors can be classified into two major categories with respect to the shapes of their back EMF wave forms, PMAC synchronous (PMAC) motors with sinusoidal back EMF and brushless DC (BLDC) with trapezoidal back EMF. BLDC motor is usually powered by a set of currents having a quasi square wave form. This excitation can be conveniently accomplished with a full bridge voltage source inverter. PM motor drives require a rotor position sensor to properly perform phase commutation and / or current control. For BLDC motors, only the knowledge of six phase-commutation instants per electrical cycle is needed; therefore, low cost halleffect sensors are usually used. To further reduce cost and improve reliability, such position sensors may be eliminated. BLDC motor except for the phase commutation periods, only two of the three phase windings are conducting at a time; and a non-conducting phase carries the back EMF. Exploring this feature, many indirect position detection methods, which sense the back EMF from the non-conducting phase, have been reported in the literature.
BRUSHLESS DC MOTORS
A. Brushless dc Motor
Fig. 1 shows the excitation of a three-phase BLDC motor that consists of a PM motor
character- by a trapezoidal back EMF and a voltage source inverter. The PM motor is represented by an equivalent circuit consisting of a static resistance, inductance, and back EMF connected in series for each of the three phases with the mechanical moving portion omitted. The figure also shows the desired stator excitation currents, Ia, Ib, and Ic, that the inverter should provide and their relationship with the back EMFs, ea, eb and ec. The currents in each phase should have a rectangular wave shape and must be in phase with the back EMFs of the corresponding phase so that
the flat top of the trapezoidal back EMF waveform is well matched to the qnasi-square wave current waveform. Such currents will develop a constant power and thus a constant torque delivered to the rotor.
In the BLDC mode, only two of the three-phase stator windings that present the peak back EMF are excited by properly switching the active switches of the inverter to produce a current with a quasi- rectangular shape. There are six combinations of the stator excitation over a fundamental cycle; each combination lasts for a phase period of i /3, as depicted in Fig. 1. The corresponding two active switch in each period may perform pulse width modulation (PWM) to regulate the motor current. To reduce current ripple, it is often useful to have one switch doing PWM while keeping the other conducting, instead of having the two switching simultaneously. It is also possible to split each of the six phase periods into segments and alternate the switch doing PWM during each segment to improve the current waveform or to prevent the unwanted circulating current that may occur in the inactive phase. It is assumed in the rest of this paper that only the upper three switches perform PWM, because this method is commonly used due to ease of implementation. In order to provide such excitation currents, the rotor position information, i.e., the angular phase orientation of the back EMFs, must be known. Only the phase information at the six commutation instants per electrical cycle marked by arrows in the figure is required to control a BLDC motor.
B. Position Detection Based on Indirect Back EMF Sensing
As indicated in Fig, I. only two of the three state-windings are excited at a time; and the third phase is open during the transition periods between the positive and negative flat segments -of the back EMF. This arrangement provides a window to sense the back EMF, and this window rotates among the three phases as the stator curren commutates from one phase to another. Therefore, each of the motor terminal voltages contains the back EMF information that can be used to derive the commutation instants. The simu only the upper three switches perform PWM, because this method is commonly used due to ease of implementation. In order to provide such excitation currents, the rotor position information, i.e., the angular phase orientation of the back EMFs, must be known. Only the phase information at the six commutation instants per electrical cycle marked by arrows in the figure is required to control a BLDC motor.
B. Position Detection Based on Indirect Back EMF Sensing
As indicated in Fig, I. only two of the three state-windings are excited at a time; and the third phase is open during the transition periods between the positive and negative flat segments -of the back EMF. This arrangement provides a window to sense the back EMF, and this window rotates among the three phases as the stator curren commutates from one phase to another. Therefore, each of the motor terminal voltages contains the back EMF information that can be used to derive the commutation instants. The simulated waveforms of terminal voltages, Va, Vb and Vc, referred to the neutral point of a lated waveforms of terminal voltages, Va, Vb and Vc, referred to the neutral point of a the integrator output generates the required phase-commutation timing signals.
The commutation signals can then be fed to a microprocessor through upto-couplers or pulse transformers for isolation. The microprocessor produces gate control signals for the inverter and may perform closed- loop speed control with the motor speed information measured by the frequency of the detected signals. Alternatively, inverter gating signal generator logic may be used join closed-loop speed control is required. The actual terminal voltages, ‘Va, Vb and Vc. contain chopped pulses generated by the switching operation of the inverter, as shown in Fig. 2. The use of an integrator not only filters out these voltage spikes, but also produces a signal of fixed amplitude that is dependent on the back EMF constant but independent of motor speed. This means, that this scheme could work down to zero speed. In practice, however, one cannot use an integrator because of offsets and drifting that are inevitable in integrated circuits. Instead, low-pass filters are used to sense the terminal voltages, an arrangement which leads to a limited operating speed range for this scheme as the phase delay antle decreases with speed. This subject will be discussed in detail in the following section.
PROPOSED LOW-COST SENSORLESS CONTROL SCHEME
FOR
BRUSHLESS DC MOTORS
A.Simplified Position Detection Circuits
It is apparent from the previous section that sensing each terminal voltage can provide two commutation instants. Based on measuring the time between these two instants, it is possible to interpulate the other four commutation instants, assuming motor speed does not change significantly over consecutive electrical cycles.
The circuit for sensing the other two terminal voltages can therefore be eliminated, leading to a 66% reduction in sensing components. Fig. 4 illustrates the proposed low-cost sensorless control scheme for BLDC motors, where (a) shows a block diagram of the position detection circuit based on sensing only one motor-terminal voltage and (b) illustrates ideal operating waveforms for extracting the phase commutation timing information. Phase voltage, Va, is fed into an integrator for filtering and introducing the necessary phase delay. Detecting the zero crossing of the integrator output, produces two commutation instants per fundamental cycle. This information is then fed into the microprocessor measures the elapsed time, Tk, between these two instants and generates the other two commutation instants apart from the last sensed instant by Tk/3 and 2Tk/3, respectively. Because of the use of interpolation, this scheme works best for applications that do not require frequent, rapid acceleration or deceleration, usually enrountered in BLDC motor applications.
3. Correction of Position Detection Errors
As mentioned before, an integrator cannot be used in practice. Instead, a low-pass filter is employed to extract the phase information from the back EMF as shown in Fig. 5(a). The phase delay introduced by the filter varies with the back EMF frequency, i.e., the motor speed, and is always less than n/2.
This speed-dependent phase characteristic, if not corrected, will produce incorrect phase- commutation timing. The graph shown in Fig. 5(b) plots the phase delay versus frequency the switch that is turned off in process of phase communication between the lower switches. Take for instance the commutation from, to phase and refer to Fig. 1. At the beginning of each negative half-cycle when the motor current is commutated by switching off S6 and switching on 34 while S2 is conducting, the diode of S6 is positively biased because back EMF, ea, is greater than ec. Therefore, the two back EMF are shorted through the diode and S t, and consequently a circulating current is produced. Notice that commutations between the upper switches will not produce a calculating current because of the PWM switching operation. To satisfactorily operate a motor at low speeds, the phase errors need be corrected. Once the filter is designed, the resulting phase delay at a given frequency can be calculated. This can be done online or offline to construct a look- up table. Fig. 5(d) shows operating waveforms with phase-error correction. The correction is based on measuring the elapsed time, Tk, between the last two zero crossing instants and converting it to frequency according to fm= I /(2Tk). With this frequency information, the delay time correction, t k, can then be determined. Fig. 6 shows an alternative sensing scheme. Based on a h filter, to further eliminate two branches of the resistor network.
The terminal voltage referred to the negative dc bus rail, Va, is fed into the band-pass filter to remove the dc component and high-frequency content resulting from the PWM operation. The filtered voltage, Va, is then pass to a comparator to detect the zero-crossing instants, which are further sent to a microprocessor for phase-delay correction and generation of commutation signals in a way similar to that described in the previous section.
C. Microprocessor Based Implementation of Sensor less Control
Fig. 7 shows a microprocessor-based implementation of the suggested position detection scheme for speed control. The zero-crossing signals from the detection block 5(a) or Fig. 6, Vc, are fed through an interrupt input, which activates an interrupt service routine (ISR) to read a timer, Tml, and to calculate the time, Tk, between the last two interrupts. This measured time is converted to frequency according to fin=1/(2Tk), which is in turn used as an index to a time-delay correction table. U time-delay correction, t k, is loaded into the counter of a second timer, Tm2, which starts counting down to zero.
Upon reaching zero, it generates an interrupt to a second ISR, which generates a phase commutation signal and starts a third timer, Tm3, whose rounder was loaded with Tk/3. Tm3 counts down to zero and generates an
interrupt to a third ISR, which generates a phase commutation signal, reloads Tm3 with Tk/3, and starts counting again. Upon the second interrupt, the third ISR generates a phase commutation signal and stops Tm3. A proportional-integral (PT) controller is used for speed
regulation. A feed-forward path with a than equal to the back EMF constant, Kb emf, is also added to improve the speed control response. Speed feedback is furnished by the first ISR, which has a resolution of 2xP pulses per revolution, where P is the pole pair number of the motor. For simplicity, no current control loop is provide A fourth timer, Tm4, is used to generate a PWM duty control signal, which is gated to one of the upper switches, S1, S2 or S3, by the commutation signals from the timer, Tm3, It is assumed that only the upper three devices of the inverter are performing PWM to regulate the current of the motor, and the lower switches conduct for a fixed period of 120 electrical degrees corresponding to the negative flat portion of each phase back EMF in each cycle. A starting control block manages the timers, Tm3 and Tm4, for an initial startup of the motor when no position information is interrupt proposed scheme has been successfully verified by analytical and experimental results. REFERENCES
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