Method to estimate motor speed for stabilized motor control

Details Method to estimate motor speed for stabilized motor control Method to estimate motor speed for stabilized motor control

2002-09-04

2004-03-16

Barlow, John (Department: 2863)

Data processing: measuring, calibrating, or testing

Measurement system

Speed

C318S798000, C318S799000, C318S801000, C318S802000, C318S808000, C318S811000, C318S700000

Reexamination Certificate

active

06708134

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to motor speed control and more specifically it relates to a method to signal an encoderless motor controller to maintain low speed motor stability.
2. Description of the Prior Art
It can be appreciated that motor controllers have been in use for years. Motor controllers are used in a wide variety of environments to vary the activation, speed, torque or even the motor shaft position of a motor as required for the application. Manufacturing plants use a multitude of motor controllers for simple and complex operations, for speed control of an air handling fan, to highly precision maneuvers for each step in an automated manufacturing process. Production floor equipment that utilizes computer numerical controls (CNC) is dependent on controllable motors to accurately machine the desired product. For example, a CNC lathe may be used to make machine parts such as a transmission component for a truck transmission. The raw material is clamped to the lathe spindle and a predefined program is initiated by the station operator to manufacture the part by an automatic multi-step process. The program is essentially designed to control the speed of the spindle and the positioning of a number of cutting tools to correctly machine the part. Each movement of the process is provided by a motor and motor controller pair as directed by the specialized program. The accuracy of the finished product is directly dependent on the ability of the motor/motor controller pair to meet design tolerance, to achieve the desired shape.
Alternating Current (AC) variable speed (motor) drives (VSD) are available in three principal types: open loop, encoderless and closed loop that provide increasingly sophisticated command of induction (and permanent magnet synchronous) motors. These VSDs are designed to modulate a power source, where pulse width modulation (PWM) is common, to control small ¾ to large 60 horsepower AC induction motors. The open-loop VSD employ the simplest motor control, the so-called Volts per Hertz (V/Hz) method. These are also known as “scalar” control to differentiate it from the other closed-loop designs where the V/Hz runs in an open loop without a formal feedback device; however, current and voltage sensing is done for current limiting and slip estimation. The V/Hz VSD does not offer torque control or high torque values at low speeds. The V/Hz method is recognized as a lower cost approach to basic motor speed control providing relatively low speed and torque response. It has the advantage to easily control several motors from one drive, popular for driving pumps, fans and other continuous process applications.
FIG. 1
shows a simplified block diagram
100
of a closed loop type variable speed drive and motor. The motor speed controller
101
accepts speed commands form a speed reference
102
and power
104
to be managed by the motor controller to the motor
103
. A tachometer
105
is shown to generate the feedback information to close the control loop where the tachometer input shaft is mechanically attached to the rotor of the motor to generate electrical feedback signals to the controller. The tachometer shown represents old technology where today's designs use an encoder. Motor control designs for the synchronous DC servo motor employ a resolver for feedback information. The speed reference
102
is derived from a 0 to 10 VDC level, typically delivered by a digital to analog converter common to integrated circuits or, for simple manual control, generated by a potentiometer referenced to a VDC source and ground. The input power
104
is selected from a common single phase or three phase (120/208 VAC) bus for compatibility with the basic rating (and motor controller) of the motor. In this approach to VSD, the performance characteristics are much improved in every respect with the additional capacity for torque control. However, the external mechanical encoder requires extra wiring, careful mount alignment with the motor shaft (or geared output) and generally detracts from an intrinsic robustness of the AC motor drive. Moreover, for low fractional horsepower systems, the cost of the sensor can approach 50% of the motor price.
Several manufacturers utilize flux-vector control (FVC) in their closed loop VSD designs. FVC has several variations and is considered the “high end” in induction motor control performance today as opposed to the other methods. The field oriented FVC is the most capable embodiment where it models characteristics of the DC motor through independent control of flux-producing (magnetizing) and torque-producing current components to derive optimal control of motor torque and power. In this version, an actual feedback device, most often an encoder is used for motor position and speed information with very sophisticated motor models used in the control algorithms. FVC allows true torque-mode operation and employs separate speed and torque loops. An adaptive controller adds higher dynamic torque regulation and can account for motor temperature changes and other control disturbances and still deliver optimal torque output. This type of FVC delivers high torque at low speeds and offers very linear parameters over the whole speed range. Other “vector” drives also treat flux and torque currents as a vector sum of total motor current to improve on speed control and torque output but these drive products are derived from a V/Hz base and do not deliver field oriented FVC performance.
The encoderless variable speed drive (EVSD) is the third type of VSD offered today and provides a performance compromise between the expensive closed loop design and the basic V/Hz drives. The EVSD senses the voltage and/or current waveform impressed on the motor drive to motor connection by the running motor to estimate torque and magnetizing components as well as the vector relationship between them. EVSD performance is proportional to the number of motor parameters measured. Without a mechanical feedback device, the EVSD avoids the cost, wear and maintenance problems associated with the closed loop systems; however, the EVSD cannot match the performance of the closed loop VSD. The performance of the three types of variable speed drives available today are shown for comparison in the following table:
V/Hz VSD
Encoderless VSD
FVC VSD
Speed Regulation
1%
0.5%
0.01%
Speed Range
40:1
120-60:1
600:1
Low Speed @ 1800
45 RPM
15-30 RPM
3 RPM
RPM Base Speed,
(1.0 Hz)
60 Hz Motor
Torque Regulation
None
+/−5%
+/−2%
Starting Torque
150%
150%
150-300%
Encoderless speed controls or estimators are made based on the principle that the motor speed can be estimated from a measurement of the current and voltage waveform at the motor connection. AC induction motors are driven or speed regulated by a known frequency, the fundamental frequency, and a controlled bus voltage. Motor rotation induces other frequencies in the current waveform due to the physical construction (rotor bars or winding gaps) of the motor rotor. A speed estimate can be generated by detecting these induced speed related frequencies and comparing them to a known mathematical model relating the induced frequencies to the motor speed. This mathematical model is described in P. Alger, “The Nature of Induction Machines”, Gordon and Breach, New York, 1965. Current EVSDs vary in capability and performance, depending on their derivation from either a V/Hz or vector control base. The ability of the EVSD is very dependent on the latest modeling and adaptation methods since they infer rather than sense motor shaft information.
Current EVSD designs exhibit instability below 1% rated speed which puts a limit on the speed range due to the limitations of current technology. Instability at low speed is exasperated by harsh load dynamics (fast start/stop of high inertial loads) and determines the low speed set point of each application generally determined during system commissioning. It is known to be costly and complex to build a

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