Motor control with digital feedback

Abstract: A motor feedback control system is claimed which, in addition to a quantized digital position feedback value, uses a feedback signal which represents the error between the actual motor position and the quantized digital position. The error function is combined with the quantized digital position value to produce a digital position value with greater resolution. The combined value is differentiated to pro­ duce a velocity value for velocity feedback control of the motor.

Description: The present invention relates to a method and an apparatus for feedback control of an electric motor, and more particularly to automatic feedback control of electric motors. In automatic control systems, feedback signals representing measurements of parameters under control are compared against the desired values set by the operator. A difference or an error signal is derived from the comparison and the parameter is further adjusted to reduce such error The parameter is set as desired when the error is reduced substantially to zero. For high performance control, it is essential that the parameter sensors accurately and precisely measure the parameters under control. For motor control systems, parameters such as velocity and position must be accurately and precisely measured. In prior motor feedback control systems, devices such as resolvers and tachometers have commonly been used to measure motor position and velocity. Motor velocity measurement by an analog tachometer suffers from the inherent problems of an analog system; i.e., problems of offset, scaling inaccuracies, and temperature drift. A digital velocity value can be obtained by using an analog to digital converter, but this method does not eliminate the inherent problems associated with the analog devices. Furthermore, a sufficiently precise analog to digital converter requires a large number of bits and resolution and would become too expensive for most systems. Motor angular positional information is often derived by means of a resolver and a resolver to digital converter. The angular position value is measured and then quantized into a digital feedback value. The precision of such digital feedback system is dependent on the resolution of the quantization boundary. The resolution of the digital data from the quantization is inversely proportional to the motor speed being quantized; that is, as the motor speed increases, the resolution or quantization boundary in number of bits derived will necessarily decrease. For example, when a motor increases its speed from 300 rpm to 1500 rpm, a digital resolver which produces a 16 bit quantization at 300 rpm will typically have a reduction in resolution down to a quantization boundary of 14 bits. For added precision, both the digital and analog feedback values can be combined in one system. In such a system the resolver will output both digital and analog feedback values. The digital data from the resolver represents the quantized positional value and is used in the digital feedback loop. The analog feedback output represents the error signal between the quantized positional value and the actual motor position. In such a system, the motor is adjusted in a digital feedback loop according to the digital feedback value. The system then switches over to the analog feedback loop for fine adjustment according to the analog feedback signal. The latter analog feedback loop is known to one skilled in the art as the analog lock feedback loop. Velocity feedback data for motor speed control can be similarly obtained from the positional feedback information by differentiating the feedback values from the resolver. US-A-3,611,101 describes a position control system comprising a servoamplifier connected to drive a motor in response to a position error signal; a position transducer arranged to sense the position of a machine driven by said motor and generating an analog position error signal of a magnitude that is a function of the distance between said position and a position transducer reference point; a command signal generator connected to the position transducer and adapted to reset the position reference point to a programmed final position; a digital error signal generator to generate a digital position error signal when the machine is not in the programmed final position; and a loop control switch to control the digital position error signal to drive the servoamplifier until the driven machine comes within a predetermined distance from the programmed final position, at which point it connects the analog position error signal to drive the servoamplifier. Furthermore, a positioning control servo system for impact printer programmed servo systems using a digital feedback control system and an analog feedback control system, both simultaneously generating control signals to a servo summing node, is known from US-A-4,353,020. In accordance with the invention, there is provided a method for controlling a plurality of parameters of a motor comprising digital and analog signals, that is characterized in that it comprises (a) generating a quantized motor parameter data representing the coarse value of rotor position; (b) generating an analog error signal representing the offset between the actual motor parameter data and said quantized data; (c) digitizing said analog error signal; and (d) combining said quantized data with said digitized data for providing a digital feedback data for adjusting the parameters of said motor. In accordance with the present invention, a low cost and high resolution digital data feedback of motor position is derived by digitizing an analog position error signal and cascading such digitized data to the quantized position data. The analog position error signal is a measurement from a resolver which represents the difference between the quantized feedback value and the actual motor position value. This signal is commonly used as the analog position feedback signal for the analog lock feedback loop for motor control. The combined digital data formed from the cascade eliminates the need for a motor feedback control system which switches between a coarse digital loop and a fine analog feedback loop, thus avoiding problems from offset, temperature drift, and scaling inaccuracies commonly found in analog feedback systems. The combined digital data provides the added advantage of having a higher resolution or more precise digital feedback data without the use of a higher resolution and thus more costly analog to digital converter. Similarly, a low cost and high resolution digital velocity feedback data is derived by differentiating the combined digital position feedback data. Fig. 1 is a block diagram of the digital position data feedback unit according to this invention. Fig. 2 is a system block diagram for a motor control system according to this invention. Fig. 3 is a block diagram of the motor control unit. Fig. 4 is a block diagram of the digital position feedback control loop. Fig. 5 is a block diagram of the digital velocity feedback control loop. In motor control systems there is an analog signal which represents the error between the measured and the quantized digital position values and the actual position of the motor. That analog signal is usually used in motor controllers as an analog feedback value for an analog feedback loop. In accordance with the invention, in a preferred embodiment, an improved digital feedback value can be obtained using this analog signal. For purposes of identification, the signal will be called ILSB since it is indicative of an analog signal comparable in magnitude to the least significant digital bit of the quantized digital position value. The ILSB varies proportionally to the offset of the motor from the zero crossing or the quantized position and can be defined by the formula in Equation 1. Eq. 1: ILSB = KSCALE * (POS - DPOS) Where ILSB is the error value, POS is the actual position of the motor, DPOS is the quantized value of the motor position, and KSCALE is a constant scaling factor of the resolver to digital converter. The actual position of the motor can be represented by the expression in Equation 2. Eq. 2: POS = DPOS + ILSB/KSCALE Further, actual velocity data is obtained by differentiating the actual position value POS or the sum of DPOS and ILSB/KSCALE. The principles of the invention are applied in the position data feedback unit as shown in Figure 1 which is an implementation of the Equation 2. In Fig. 1, resolver 101 is used to measure the angle position of the motor 107. The resolver 101 is of a conventional design including a rotor winding and two stator windings in quadrature. The rotor winding is energized by a suitable alternating signal sinW which may, for example, be 2500 Hz. If Q represents the angle between the rotor and the stator of the resolver, then the quadrature windings will produce the signals sinQsinW and cosQsinW. The resolver energizing signal and the signals produced by the quadrature windings are supplied to a resolver to digital converter 102 which interprets the received signals and provides a corresponding binary indication of the resolver shaft position DPOS and a corresponding analog voltage ILSB. As previously stated, the ILSB signal represents the error signal between the actual motor position and the quantized output DPOS. The magnitude of this error signal ILSB is within the least significant bit of the digitized output DPOS. The digital positional signal DPOS from the converter 102 is introduced into one of the two inputs of adder 105 labelled port A. In this embodiment, the DPOS signal is a 12 bit digital data. The analog signal, ILSB, represents the fine measurement value of the resolver shaft position. It has a magnitude no greater than the last two bits or second least significant bit of the digital signal DPOS. The ILSB signal is adjusted by KSCALE 103 to conform to the scale specific to the system of a particular embodiment to allow for parameter tolerances. Such adjustment may be made by means of a trimpot. The adjusted analog output from KSCALE 103 is then converted into digital converter 104. According to the present embodiment, the analog signal is converted to a 10 bit digital data and is added to the 12 bit DPOS data. As shown in Fig. 1, the 10 bit converted ILSB digital data is input into the ten least significant bits (0-9) of the "B" port of adder 105, and the DPOS data is input to the tenth to twentyfirst (9-20) bits of the "A" port of adder 105. The adder inputs are so arranged such that the most significant bit of the converted ILSB value is added to the least significant bit of the DPOS data. The output of the adder labelled "C" is the sum of the "A" and "B values which is the sum of DPOS and ILSB/KSCALE as specified in Equation 2. The sum is the motor position value POS. The digital position data signal POS may be incorporated in a motor control system as illustrated in the block diagram of Fig. 2. A microcontroller 210 is used to control the motor control circuit 206, which produces motor drive current to motor 107, including 3 stator windings interconnected in a wye configuration. The digital position data feedback 208 provides the function of measuring the motor position as described in connection with Fig. 1. The motor rotation or position is detected and converted into a combined digital feedback data according to the Equation 2 formula POS = DPOS + ILSB/KSCALE. A signal of POS digital data is the output from the digital position data feedback unit 208 to the microcontroller 210, which contains stored programs which provide such functions as that of the position command generator 201, the digital position feedback control loop 202, the digital velocity feedback control loop 203, the differentiator 204, and the memory 205. In the system of Fig. 2, position command is information set into the controller. The position command is in the form of digital data representing desired motor position information. The position command generator 201 is the user to microcontroller interface for data entry or selection by a user. It may be in the form of a keyboard or any equivalent data entry device. It outputs a position command to position loop 202. The position loop 202 receives the position command and the position feedback data POS from the digital position data feedback 208 and produces a velocity command. The position loop 202 is further detailed in Fig. 4. The velocity command output from the position loop 202 is in the form of digital data representing the desired motor velocity. The velocity loop 203 receives the velocity command input and the velocity feedback input from dif ferentiator 204 and outputs a current command to motor control circuit 206. The current command is a digital data representing the amount of adjustment in motor speed needed to be translated by the motor control unit 206. The velocity loop 203 is further detailed in Fig. 5. The differentiator 204 takes as input the position feedback digital data POS from position data feedback 208 and outputs a velocity feedback data to velocity loop 203 and the memory 205. A differentiator such as the backward difference type as shown in Fig. 5 is used for this embodiment. The velocity feedback data output of the differentiator 204 is a digital data representing the measured motor velocity. This data is compared to the data output from the position loop 202 to generate a current command as the output of the velocity loop 203. This data is multiplied by digital sine wave values stored in memory 205, which is programmed to receive position feedback data as memory address and to produce corresponding digital sine wave values. The memory may be a ROM or a PROM which is programmed so that in response to any particular vector indication they produce three phase angular data. The phase angle data are sine wave values displaced from one another by a specified electrical degree, generally 120° for a 3-phase motor. The phase angle data output from the memory 205 to the motor control unit 206 is time division multiplexed. This is commonly referred to as commutation. The motor control unit 206 receives input from the microcontroller 210 in the form of current command from the velocity loop 203 and phase angles form the memory 205. The motor control unit 206 processes these input information into motor drive currents output to the motor 107. The motor control unit 206 is further detailed in Fig. 3. Control systems which transform synthesized digital data into sinusoidal excitation current suitable for motor control are known in the field. An example of such a system is described in US-A 4,447,771. The motor control unit shown in Fig. 3 provides the excitation currents for windings 107A-C of motor 107 which are sinusoidal and displaced from one another by 120 electrical degrees. The digital output indications from the memory 205 are supplied in a time division multiplex manner to digital to analog converters DACs 301, 302 and 303 which convert the digital sine wave values into analog values, i.e., synthesized sine waves. DACs 301, 302 and 303 are of the analog multiplying type capable of multiplying the digital conversion result with another analog signal. The digital current command from the velocity loop 203 (Fig. 2) is converted to an analog current command by the twelve bit digital to analog converter 304. The analog current command output from DAC 304 is supplied to each of the DACs 301, 302 and 303 and controls the amplitude of the sine wave signals produced thereby. Thus, the outputs of DACs 301, 302 and 303 are three synthesized sine wave voltage signals displaced from one another by 120 electrical degrees. The sinusoidal voltage signal appearing at the output of DAC 301 is supplied to a pulse width controller 305 which produces pulse width voltage excitation to winding 107A of the motor. The pulse width controllers 305, 306 and 307 each receives triangular wave signals originating from a triangular wave generator 308. The amplitude of excitation voltage supplied to the winding is a variation of the triangular wave signal controlled by the current command from the output of the velocity loop 203 (Fig. 2). The circuitry for the current drive and control within the pulse width controllers or modulators is known in the art. An example of such circuitry can be found in US-A 4,447,771. Fig. 4 shows a detailed version of the position loop shown in 202 of Fig. 2. In Fig. 4, a subtractor 401 is used to derive an error value by differencing the POS feedback value from the digital position data feedback 208 and the desired position command value as set by the position command generator 201. The digital error value output from the subtractor 401 is adjusted by KP 402 so that the loop gain conforms to other system parameters. KP is typically a multiplier between zero and one-half. The output of KP 402 is the position error or velocity command input to the velocity feedback control loop 203. A more detailed version of the velocity loop 203 of Fig. 2 is shown in Fig. 5. The POS input from the position data feedback 208 is differentiated by the differentiator 501, converting the position feedback data into a velocity feedback data. A delay and subtract differentiator delays a signal and then subtracts the delayed signal from the non-delayed signal, the difference being a differentiated version of the non-delayed signal. This form of differentiation is one example of a differentiator. The velocity feedback output from the differentiator is subtracted from the velocity command in subtractor 502 to yield a velocity error value. Both, the velocity error value and the velocity feedback value are fed into the velocity loop compensation 503. The velocity loop compensation 503 is used to stabilize the various parameters and constants in the velocity loop. Examples of the types of loop compensation known to one skilled in the art are the Proportional-Integral-Differentiator (PID) and the Lead-lag types. The output of the velocity loop compensator 503 is the current command to the motor control circuit 106 which causes adjustments to the motor speed and position. As stated in the foregoing, the system and method for motor control with digital data feedback according to the present invention will provide for improved motor control while reducing component costs. The process of digitizing a fine analog position error signal and cascading such digitized data to the coarse quantized position data provides a higher resolution digital feedback data while components such as the analog lock feedback loop and the high resolution analog to digital converter are eliminated.

Claims: A method for controlling a plurality of parameters of a motor comprising generating digital and analog signals, characterized in that it comprises (a) generating a quantized motor parameter data representing the coarse value of rotor position; (b) generating an analog error signal representing the offset between the actual motor parameter data and said quantized data; (c) digitizing said analog error signal; and (d) combining said quantized data with said digitized data for providing a digital feedback data for adjusting the parameters of said motor. The method of claim 1 characterized in that the magnitude of said analog signal is less than the magnitude of the second least significant bit of said quantized data. The method of claim 1 characterized in that said combined data is differentiated to derive a velocity feedback data for controlling the velocity of said motor. A motor control system for deriving parameter feedback data for electric motor control, said system comprising a resolver (101) to measure rotor position of said motor; converter means (102) to convert said rotor position to a coarse quantized data representing coarse rotor position and a fine analog value; an analog-to-digital converter (104) to convert said fine analog value to a fine digital data; and an adder (105) to combine said fine digital data with said coarse digital data, whereby said combined value is used as feedback data to adjust the parameters of said motor. The system of claim 4 characterized in that it further comprises memory means (205) for storing phase angle data of said motor. The system of claim 4 characterized in that the magnitude of said fine digitized analog value is less than the magnitude of the second least significant bit of said coarse digital value. The system of claim 4 characterized in that said digital feedback is differentiated to provide feedback data for controlling the velocity of said motor.

Inventors: ELLIS GEORGE H, ELLIS, GEORGE H.
Applicants: KOLLMORGEN CORP, KOLLMORGEN CORPORATION
Application year: 1990
Main classification: G05B19
IPC codes: H02P29, G05B19