KR20140000109A - 2nd surface approximation of estimated parameters for linear compressor sensorless control - Google Patents

Abstract

The present invention relates to a sensorless control of a linear compressor and, more particularly, to a method for significantly reducing a memory capacity of a controller by estimating parameters of a linear motor and approximating the same as a quadratic curved surface. Because of this, the sensorless control of the linear compressor can be facilitated by a single-chip microcomputer as the memory capacity for storing the estimated parameters is reduced even though control errors are increased. According to the present invention, the memory capacity can be significantly reduced by approximating a database of inductance and torque constant which is a function of current and stroke as a two-dimensional curved surface and a structure is to be simple through replacing by a simple calculation. [Reference numerals] (AA) Operating frequency; (BB) Operating stroke; (CC) Determining the load; (DD) Calculating current stroke; (EE) Motor constant; (FF) Detecting motor transfer; (GG) Detecting motor current

Description

2D Plane Approximation of Estimated Parameters for Linear Compressor Sensorless Control

In the conventional reciprocating compressor as shown in FIG. 1, the motion of the rotating rotary motor is changed into a straight line by the crank shaft, whereas the linear compressor is a piston type compressor in which a piston is directly driven by a linear motor as shown in FIG. 2. In the linear compressor, since all driving forces are applied in the linear motion direction, no lateral pushing force is generated by the piston. As a result, friction loss is reduced and noise during operation is reduced compared to conventional reciprocating compressors. The principle of operation of the linear motor in the near compressor is shown in FIG. As shown in ⓐ of FIG. 3, when the magnitude of the alternating current is increased, the magnetic field in the counterclockwise direction becomes larger, and the permanent magnet in the center is pushed to the left, and as shown in ⓑ, when the alternating current decreases to 0, the permanent magnet is biased to the leftmost side. As time passes, AC current flows in the opposite direction. As in ⓒ, the magnetic field also changes in direction and occurs clockwise. As a result, the permanent magnet is forced to the right, and in ⓓ, the permanent magnet is biased to the rightmost side. When the AC current is 60Hz, the permanent magnet vibrates left and right 60 times a second. If the frequency of AC current is kept constant at 60Hz and the amplitude is increased, the left and right oscillation widths of the permanent magnets, i.e., the stroke, become larger. As the stroke of the permanent magnet increases during the unit time, the linear speed of the piston connected to the permanent magnet increases, increasing the refrigerant flow rate of the linear compressor, and consequently obtaining greater cooling.

  As can be seen in Figure 1, the conventional reciprocating compressor uses a crankshaft to change the rotational motion of the motor into a linear motion. This reduces energy efficiency, but the piston is constrained by the crankshaft to maintain safe operation without leaving the top and bottom of the structurally designed cylinder. However, in the linear compressor as shown in Fig. 2, since it is not mechanically constrained, it is necessary to control the stroke of the piston so that the piston vibrates safely within a certain area and does not hit the cylinder head. In addition, the stroke control of the piston is required to adjust the outflow rate of the refrigerant for cooling.

  To do this, of course, you need to know the position of the piston correctly. Although position sensors such as LVDTs can be used to measure the exact position, these sensors are not only inexpensive, they are also difficult to mount, and inconvenient to have several strands coming out of the compressor vessel. Therefore, there is a need for an efficient way to estimate this indirectly.

Listed in the technical field

The linear motor in the linear compressor may be represented by an electric circuit equivalent model as shown in FIG. 4, and the circuit equation may be represented by a linear differential equation as shown in Equation (1) below. Also, the thrust F _e of the linear motor can be expressed linearly as in Equation (2).

(One)

(2)

In equations (1) and (2), α is a constant representing the relationship between the thrust and the current of the linear motor. Since the magnetic flux density varies depending on the position of the piston, α should be expressed as a function of the position of the piston. However, in this motor, the change of α according to the position of the piston is small and its influence is also small. In the case of the effective inductance Le and the effective resistance Re, however, the value varies depending on the position of the piston, but it is assumed that v (t) is the power supply voltage, i (t) is the current flowing through the winding, and

Is the counter electromotive force generated in the winding by the movement of the linear motor. Equation (1) can be summarized as Equation (3) with respect to x (t).

(3)

Estimated value of piston position integrating equation (3)

Is the same as Equation (4).

(4)

Equation (4) digitally calculated position estimate of the piston

The nth value of can be expressed as Equation (5).

(5)

  Where T is the sampling period.

  5 shows a closed loop control block diagram for sensorless stroke control of a linear compressor piston. The voltage applied to the linear compressor measures the voltage at both ends of the linear compressor, and the current value is measured by attaching a sensing resistor to the inlet of the linear compressor, sampling them with the A / D converter, and inputting them to the DSP processor. The measured voltage and current of the linear motor, together with the motor constant, are used to estimate the position of the piston connected to the linear motor by equation (5). Command values of strokes depending on the load are input and strokes obtained from the estimated piston position are compared to determine the amplitude of the voltage input to the linear motor via a suitable controller. The frequency of the voltage is determined and input at a frequency that meets the design specifications of the linear motor.

  In the case of controlling the sensorless stroke as shown in FIG. 5 on the assumption of the determined motor constant, the position estimation error of the piston becomes large, which affects the control performance of the system. Therefore, we want to reduce the position estimation error of the piston by estimating the most influential α and the effective inductance as a function of the position of the piston. Reordering Eq. (3) gives Eq. (6).

(6)

Equation (4) is the position value of the piston estimated by Equation (4) from the motor constant, motor voltage and motor current, and x (t) in Equation (6) is the direct position using a sensor such as LVDT. It is the measured value. When the piston makes a linear movement from side to side, let one cycle be t _n at steady state and divide each n into 0, t ₁ , t ₂ , ..., t _n-1 , t _n . Then n equations as shown in equation (7) can be obtained.

(7)

If Equation (7) is rearranged, it can be expressed as Equation (8).

(8)

In controlling the stroke of the linear motor inside the linear compressor, for accurate control of the stroke, it is necessary to know the exact stroke value. It is expensive to measure this, and thus indirectly estimates and uses this value for stroke control. The estimation requires the motor parameters such as resistance, inductance and torque constant. However, inductance, torque constant, and the like do not have a constant value and have a characteristic that varies greatly with current or stroke. Therefore, in order to accurately estimate the stroke, an inductance or torque constant, which is a function of current and stroke, is stored in a database and stored in a memory, and when the stroke is estimated, the error can be greatly reduced. However, this requires a lot of memory. Therefore, in the present invention, the database is approximated to a two-dimensional curved surface, and the memory capacity is drastically reduced even though a slight error is large, which greatly benefits the commercialization.

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1 is a reciprocating compressor
2 is a linear compressor
3 is a principle of operation of the linear motor
4 is an electric circuit equivalent model of a linear motor
5 is stroke control of the linear compressor;
6 is a database of motor torque constants.
7 is a database of motor inductance
8 is a two-dimensional surface functionalization of the motor torque constant
9 is a two-dimensional surface functionalization of the motor inductance
<Definitions of terms and abbreviations used in formulas>
α: torque constant of the motor
Le: Effective inductance of the motor
Re: Effective resistance of the motor
v (t): power supply voltage
i (t): current flowing through the winding
x (t): Stroke value

: Stroke estimate

Claims (1)

In the sensorless control of the linear compressor, the motor parameters required for estimating the stroke of the linear motor are expressed as a quadratic surface function rather than a database, which greatly reduces the memory capacity.

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