JPS626433B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a speed control system for a DC motor that is capable of precisely controlling the speed of a DC motor driven by a chopper and that allows the transient characteristics of the control system to be arbitrarily set.

The speed control of a DC separately excited motor will be explained below as an example.

FIG. 1 is a basic circuit diagram of a speed control system according to the present invention. Reference voltage E ₁ , speed generator T・
G output, load terminal voltage (chopper output) is resistance
The signals are added and integrated by R ₁ , R ₂ , R ₃ and the OP amplifier integrator 1. However, the load terminal voltage is capacitor C
This blocks the DC component and creates AC coupling. The upper and lower limits of the integrator output are detected by a comparator 2 having hysteresis, and the chopper 4 is turned on and off through an output circuit 3. The electric motor M is driven by a DC power supply Es through a chopper 4.

If we assume that the resistor _R2 is not connected and the capacitor C is short-circuited, the chopper 4 will alternately repeat the on state and off state determined by the integral constant, the hysteresis width of the comparator 2, and the reference voltage _E1 . The average value of one cycle (hereinafter simply referred to as average value) Va is controlled to a constant value by Va=-R ₃ /R ₁ E ₁ (1). The AC component of the load voltage (square wave generated by turning the chopper on and off) and the average value of the load voltage, that is, the DC component, are fed back simultaneously by the resistor R3 _, and the AC component is fed back to the integrator 1, comparator 2, and chopper 4. A triangular wave-square wave oscillation circuit is formed in the loop to cause the chopper 4 to perform on/off operations, and at the same time, the average voltage of the load is controlled as shown in equation (1) by feedback of the DC component. be.

Considering only the DC component, the load voltage average value Va is determined by the resistors R ₁ and R ₃ and the reference voltage E ₁ . Comparing it with an OP amplifier (operational amplifier) circuit, consider that the input impedance Z〓i is R ₁ , the feedback impedance Z〓f is R ₃ , the input voltage E〓i is E ₁ , and the output voltage E〓 ₀ is Va and E〓 ₀ = -Z〓f/Z〓iE〓i...(1)', and the same relationship as in the OP amplifier circuit is established. (The upper point of letters such as E〓, Z〓, etc. indicates that the phase is also taken into account.) The gain of the amplifier itself is not related to the input/output characteristics, and is output by external elements Z〓i, Z〓f. The ratio of voltage to input voltage E〓 ₀ /E〓i is determined. That is, integrator 1, comparator 2,
The output circuit 3 and chopper 4 have characteristics equivalent to one OP amplifier (operational amplifier), and various input/output characteristics can be obtained by using external elements. However, there is a restriction that the AC component must be fed back and the above-mentioned oscillation circuit must be formed. (Refer to Japanese Patent Publication No. 56-2503.) Here, the DC component refers to all frequency components including DC that are lower than the on/off repetition frequency of the chopper. Furthermore, the AC component refers to a frequency component that is higher than the repetition frequency, and the AC component does not affect the DC component because it is treated as an average value.

In the connection shown in Figure 1, unlike the above example, capacitor C is connected in series with resistor R ₃ , so only the AC component is fed back in steady state, and the DC component in steady state is fed back to resistor R ₂ which is connected separately. It has been returned by. In the event of a transient, the load voltage average value Va changes, so the series circuit of resistor R ₃ and capacitor C also feeds back the DC component, which acts to delay the change in Va, so it has the effect of damping transient vibrations in the entire control system. I can do it. Typical choppers can be operated at repetition frequencies of 1 KHz or higher, while electric motors M
The speed response time constant of is usually larger than several hundred milliseconds, so if the time constant of the series circuit R ₃ C is set to the optimal value determined by the time constant of the motor M (described later), the on/off repetition frequency of the chopper 4 is R ₃ −C
The series circuit of can be considered as an AC coupled circuit. Note that the repetition frequency of the chopper 4 can be increased arbitrarily by reducing the size of the integrating capacitor.

FIG. 2 shows the on/off relationship between the integrator 1 input, the integrator 1 output, and the chopper 4 in a steady state with the connections shown in FIG. 1. The current _i3 has no DC component due to the capacitor C, its average value is zero, and the amplitude (P-P
value) Es/R ₃ square wave. The current i ₃ is converted into a triangular wave by the integrator 1 and converted into a square wave by the comparator 2, which controls the on/off of the chopper 4. When the output of the integrator 1 reaches the upper or lower limit of the comparator 2, the on/off state of the comparator output and the chopper 4 is reversed, so that the polarity of the current i ₃ is reversed and the slope of the integrator output is reversed. The duty factor of the chopper 4 has reached a value at which the average voltage Va required for the speed of the electric motor M to reach a predetermined rotational speed Wm is obtained.

In this way, the alternating current component of the current _i3 acts only on the on/off operation of the chopper 4, and the motor does not respond to the alternating current component, so it can be handled separately from the direct current component. The control characteristics of the DC component will be described below.

Since the circuit is considered to be open with capacitor C, the magnitude of the DC component of the integrator input in a steady state is the sum of currents i ₁ and i ₂ . However, it is given by i ₁ =E ₁ /R ₁ ...(2) i ₂ =Kt·Wm/R ₂ ...(3). Here, Wm: Rotational speed of the motor Kt・Wm: Output voltage of the speed generator TG Since the input of the integrator 1 is the input terminal of the equivalent OP amplifier mentioned above, the sum of the inputs is zero, that is, Σi=0... …(4). In steady state, the DC component of current i ₃ is zero, so from equations (2) to (4), Wm = -R ₂ /R ₁ Kt・E ₁ ...(5), and the rotation speed is exactly equal to the reference voltage E. ₁ and is not affected by the size of the load or fluctuations in the DC power source Es.

To explain in more detail with reference to FIG. 2, if the rotational speed of the electric motor M is now smaller than the predetermined value shown by equation (5), i ₁ + i ₂ = 0 does not hold, and the integrator The input is the square wave of the AC component i ₃ plus the DC component |i ₂ |−|i ₁ |. Therefore, the slope of the integrator output is gentle when the chopper 4 is on, and steep when it is off. Since the widths of the upper and lower limits of the comparator 2 are constant, the on time of the chopper 4 becomes longer and the off time becomes shorter, and the output average voltage of the chopper 4 increases. The speed of motor M increases as the terminal voltage increases. As a result of the series of operations, if the rotation speed has not yet reached the predetermined value, the above changes are repeated in the relationship between the new DC components i ₁ , i ₂ , i ₃ and the rotation speed increases, i ₂ increases and the DC The component i ₁ +i ₂ = 0, resulting in a steady state.

Next, we will discuss transient characteristics as well. If the on/off cycle of the chopper 4 is sufficiently shorter than the various time constants of the motor M, and the average value of the voltage at the terminals of the motor M is Va, then the system shown in FIG. 1 is expressed as a transfer function as shown in FIG. 3. The AC component is omitted because it has a high frequency and the motor does not respond. In the figure, Km is an electrical system-mechanical system conversion constant, τa is an electric time constant of the electric motor, τm is a starting time constant of the electric motor M, and T _L is a load torque. Further, the moment of inertia of the electric motor M is represented by J.

The transfer function of electric motor M only is Wm(S)=1/τmτaS ² +τmS+1・Va(S)/Km−(τaS+1)τm/τmτaS ² +τmS+1
・Since it is given by T _L (S)/J...(6), the transfer function Wm(S)/Va
(S) is It is.

According to automatic control theory, if the quadratic system transfer function G(S) is expressed by the natural angular frequency Wn, the damping coefficient ζ, and the gain H, then G(S)=HWn ² /S ² +2ζWnS+Wn ² ...
(8), and from equations (7) and (8), the electric motor itself
Wn, ζ is It is expressed as The larger Wn is, the faster the response is. ζ
If is small, the transient response will be oscillatory, and if it is large, it will be over-controlled. It is said that the characteristics are best when ζ is around 0.7.

The transfer function of FIG. 3 according to the configuration of the present invention is set as τ ₃ =R ₃ C, and Va(s) = −R ₃ /R ₁ 1+τ ₃ S/τ ₃ S(E ₁ +R ₁ /R ₂ Kt
Wm(S)...obtained by substituting Va(S) of (10) into equation (6). becomes. From equation (11), if E ₁ and T _L are constant, Wm
The steady value of is expressed by equation (5) from the final value theorem of the S function, and there is no steady error.

Equation (11) is a cubic equation of S, and the control system is a cubic system, but the time constant of τ ₃ is In other words, the value of C is Then, from equation (11), the transfer function Wm (S) / E ₁ (S)
teeth becomes. That is, the control system operates as a secondary system. Comparing equation (14) with equation (8), the natural angular frequency of the system is
Wn′, damping coefficient ζ′ is This shows that the best transient characteristics can be adjusted by the system loop gain R ₃ Kt/R ₂ Km, that is, by R ₃ /R ₂ .

Note that since the damping coefficient of an electric motor is usually a large value, equation (12) becomes τ ₃ τm (17), and normally τ ₃ should be selected to be approximately equal to the starting time constant of the electric motor.

As described above, according to the speed control method of the present invention, it is possible not only to precisely set the rotational speed of the electric motor, but also to improve transient responsiveness.

[Brief explanation of the drawing]

FIG. 1 is a circuit configuration diagram showing an embodiment of the speed control method for a DC motor according to the present invention, FIG. 2 is an explanatory diagram of the same operation, and FIG. 3 is a diagram showing the same system in a transfer function representation. In the figure, 1: integrator, 2: comparator, 3: output circuit, 4: chopper, M: motor, TG: speed generator, E ₁ : reference voltage.

Claims (1)

[Claims] 1 Connect the reference voltage, the speed detection voltage of the DC motor, and the output voltage of the chopper to the input terminal of the OP amplifier type integrator through a series circuit of the first resistor, second resistor, third resistor, and capacitor, respectively. The integrated output is added to a comparator with hysteresis, and the output of the comparator controls the on/off of the chopper.
The DC motor is driven from a DC power source via the chopper, and the rotational speed of the motor is controlled according to a reference voltage, and the time constant and the second resistance are determined by the product of the values of the third resistance and the capacitor. A speed control method for a DC motor, characterized in that the ratio of the third resistance is set to a predetermined value to improve transient response characteristics.