JPH04351488A - Speed control circuit for dc motor - Google Patents

Abstract

PURPOSE:To realize a speed control circuit which has a small size and a high power efficiency and in which rotation irregularity does not occur even in a low speed rotation in the circuit for controlling a rotational speed of a DC motor. CONSTITUTION:A circuit for controlling a rotational speed of a DC motor M based on a reference voltage Vref for instructing the speed of the motor M, has a comparing timer 1 in which its output voltage Vo becomes active only for a previously predetermined time PW when at least one reference voltage Vref coincides with other comparison voltage Vfb, and a transistor TR to be switched by the drive of the voltage Vo of the timer 1. One end of the motor M to be controlled is connected to a hot end side of a power source Vcc, the other end is connected to the transistor TR connected to a cold end side of the source Vcc, and a voltage of a point (a) for connecting the motor M to the transistor TR is fed back as the comparison voltage Vfb of the timer 1.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit for controlling the rotational speed of a DC motor.

The rotational speed of a DC motor is easier to control than other motors, and its control range can be widened. Furthermore, the rotational torque is large from low rotational speeds to high rotational speeds.

Due to the above-mentioned characteristics, DC motors are widely used as a drive source for devices that require speed control.

On the other hand, in order to make effective use of limited space, it is essential to downsize the device.

[0005] Therefore, there is a need for a speed control circuit for a DC motor that can be miniaturized.

[0006]

2. Description of the Related Art (1) Speed control circuit FIG. 4 is an example of a circuit that controls the rotational speed of a motor.

That is, this circuit controls the power supplied to the motor M using the transistor Tr1 and controls the rotational speed.

[0008] IC1, which is a control IC, is generally a "5"
This is a timer IC called ``55'', which has a built-in comparator, and is configured to start the timer operation based on the output of the comparator.

[0009] The No. ■ terminal of IC1 is one input of the comparator, and the voltage Vre divided by the variable resistor VR1
f is input via a resistor R1. That is, VR1
A speed command is given using the voltage Vref divided by as a reference voltage.

On the other hand, the terminal ## of IC1 is the other input of the comparator, and inputs the terminal voltage Vfb1 of the motor M via the resistor R4. That is, Vfb1 is a feedback voltage for feedback control.

Further, a resistor R6 is connected in series with the motor M, and a voltage Vfb2 generated in proportion to the current Im flowing through the motor M is inputted to the No. 2 terminal of the IC1 via the resistor R5. That is, Vfb2 is also a feedback voltage for feedback control.

[0012] Then, Vref and Vfb1, Vf
By b2, the control voltage Vo is output from the terminal No. 2 of IC1, and the transistor Tr1 is switched via the resistor R3.

(2) Operation The circuit shown in FIG. 4 controls the power supplied to the motor M by switching it with a transistor Tr, thereby controlling the rotational speed of the motor M.

FIG. 5 is a waveform diagram illustrating the operation.
(a) and (b) are diagrams when only Vfb1 is considered as the feedback voltage, (c) and (d) are diagrams when considering Vfb1 and Vfb1 as the feedback voltage.
This is a diagram when both fb2 are considered.

[0015] The time axes in (a), (b), (c), and (d) of the same figure are on the same scale. Moreover, the waveform shown by a solid line is a case where the load on the motor is a normal load, and the waveform shown by a dotted line is a case where the load is increased.

In (a), when Vo is at the L level, the control transistor Tr1 is turned on, and power is supplied to the motor M. The time width is between time t1 and t2 indicated by PW, and at time t2 Vo becomes H.
level, and the transistor Tr1 turns off. In other words, the power supply is stopped.

From then on, similarly, the transistor Tr1 is turned on.
/OFF is repeated, and electric power determined by the duty ratio is supplied to the motor M, and a predetermined rotational speed is obtained.

Note that the time width PW is determined by the resistance R4 and the capacitor C.
It is determined by the time constant defined by 1, and is a substantially constant value. That is, in the circuit shown in FIG. 4, the timer function of IC1 is not used, but only the comparator function is used.

(b) shows how the timing to turn on the transistor Tr1 is determined.

That is, a back electromotive force is generated in the motor according to its rotational speed, but when power supply to the motor M is stopped at time t2, the electromotive force gradually decreases as the rotational speed decreases, and the electromotive force gradually decreases as the rotational speed decreases. It matches Vref at t3.

When Vfb1 matches Vref, Vo becomes L level and transistor Tr1 is turned on. That is, between times t3 and t4, Vo is the transistor Tr1.
becomes active, and power is supplied to the motor M. Thereafter, it operates repeatedly in the same manner, and power is supplied to maintain the predetermined rotational speed.

However, when the load on motor M increases,
The speed at which the rotational speed of the motor M decreases when the power supply is stopped becomes faster, and at time t11, Vfb1 becomes equal to Vref, and the time at which power supply to the motor M starts is brought forward.

That is, the ON/OFF state of the transistor Tr1 is
The OFF duty ratio increases, the power supplied to the motor M increases, and power is supplied to maintain the predetermined rotational speed.

By the way, the voltage Vfb generated across the resistor R6
2 is also fed back, so the voltage that gives the speed command is actually Vref+Vfb2.

Therefore, as shown in (c) and (d), when the load increases, the current Im flowing through the motor M increases and the reference voltage Vref+Vfb2 also increases. Therefore, at time t21 earlier than the time t11, Vfb1
matches Vref+Vfb2, and the start time of power supply to motor M is further advanced. In other words, the control characteristics of the rotational speed are improved.

In the case of the circuit shown in FIG. 4, Vfb2 acts extremely effectively when the rotation speed of the motor M is low, and maintains good control characteristics of the rotation speed.

[0027]

The current feedback resistor R6 inserted in series with the motor M is normally designed and selected to have a value of about 1/3 of the armature resistance of the motor M.

By the way, it is common for the motor M to be overloaded, even temporarily. In that case, the current flowing through the motor M reaches more than 10 times the rated current.

Therefore, the allowable power of the resistor R6 must also be selected to a value that can sufficiently withstand overload.

However, there is a problem in that the resistor R6, which has a sufficient margin against overload, has a large external size and the speed control circuit for the motor M has to be large. Moreover, the resistance R6
The electric power consumed in this process is not only a loss that cannot be converted into power, but also has the problem of generating heat.

[0031] Also, the time width P for supplying electric power to the motor M
W is determined by a time constant circuit including a resistor R4 and a capacitor C1, and the time constant circuit serves as a feedback circuit for the feedback voltage Vfb1.

Therefore, when the rotational speed of motor M decreases, the voltage of capacitor C1 fluctuates greatly, and resistor R
The time width PW determined by the time constant circuit of 4 and capacitor C1
This tends to cause fluctuations in the rotation of the motor M, causing uneven rotation of the motor M.

The technical problem of the present invention is to solve the above-mentioned problems in a motor speed control circuit, and to realize a speed control circuit that is small, has high power efficiency, and does not cause uneven rotation even at low speed rotation. be.

[0034]

[Means for Solving the Problems] FIG. 1 is a circuit diagram illustrating the basic configuration of the present invention.

In the present invention, the controlled motor M is connected to the power source Vcc.
A control transistor TR connected to the hot end side of the motor M and connected to the cold end side controls the power supplied to the motor M to control the speed, and when the transistor TR is
The feature is that the time width PW for N is defined by comparison/timer 1.

That is, in a circuit that controls the rotational speed of the DC motor M based on the reference voltage Vref that commands the rotational speed of the motor M, at least one of the reference voltages Vr
When ef and the other comparison voltage Vfb match, the output voltage Vo is activated for a predetermined time period PW, and the comparison timer 1 is driven by the output voltage Vo of the comparison timer 1. It consists of a switching transistor TR.

Then, one end of the controlled motor M is connected to a power source Vc.
The comparison/timer 1 Comparison voltage Vfb
This is a speed control circuit for a DC motor that is fed back.

[0038]

[Operation] FIG. 2 is a waveform diagram illustrating the operation. still,(
The time axes in a) and (b) are on the same scale. Moreover, the waveform shown by a solid line is a case where the load on the motor is a normal load, and the waveform shown by a dotted line is a case where the load is increased.

In (a), when Vo is at H level, control transistor TR is turned on, and power is supplied to motor M.

[0040]The time width is the time t1 indicated by PW.
- t2, and at time t2 Vo becomes L level and transistor TR is turned off. In other words, the power supply is stopped. Note that the time width PW is set to a value predetermined by the comparison/timer 1.

[0041] Similarly, the transistor TR is turned ON/O.
By repeating FF, electric power determined by the duty ratio is supplied to the motor M, and a predetermined rotational speed is obtained.

(b) shows how the timing to turn on the transistor TR is determined.

In other words, a counter electromotive force is generated in the motor according to its rotational speed, but the direction of the electromotive force is dependent on the power supply Vcc.
The direction is opposite to that of .

Further, when the power supply to the motor M is stopped, the rotational speed of the motor M gradually decreases, and the back electromotive force generated by the motor also gradually decreases.

Therefore, the voltage at the connection point a between the motor M and the transistor TR, ie, the feedback voltage Vfb, gradually increases and becomes equal to Vref at time t3.

When Vfb matches Vref, Vo becomes H level, that is, active, and transistor TR is turned on. That is, power is supplied to the motor M at PW between times t3 and t4.

[0047] Thereafter, the operation is repeated in the same manner, and power is supplied to maintain the predetermined rotational speed.

However, when the load on motor M increases,
The speed at which the rotational speed of the motor M decreases when the power supply is stopped increases, and at time t31, Vfb coincides with Vref, and the time at which power supply to the motor M starts is brought forward.

That is, ON/OF of transistor TR
The F duty ratio increases, the power supplied to the motor M increases, and power is supplied to maintain the predetermined rotational speed.

Note that since the time width PW is set to a fixed value by comparison/timer 1, even if the motor M is rotating at a low speed and the feedback voltage Vfb fluctuates greatly, the time width PW
There is no fluctuation in the rotation speed, and therefore no uneven rotation occurs.

[0051]

EXAMPLES Next, examples will explain how the present invention can be practically implemented.

FIG. 3 is a diagram illustrating an example, in which (a)
is a circuit diagram, and (b) and (c) are waveform diagrams explaining the operation. Note that the time axes in (b) and (c) are on the same scale.

(1) Speed control circuit The circuit of this embodiment is fundamentally different from the conventional circuit shown in FIG.
The positions of M are reversed between the hot end and cold end of the power supply, secondly, there is no current feedback resistor connected in series to the motor M, and thirdly, IC
1, and the timer function is regulated by the resistor R10 and the capacitor C4.

Further, the transistor Tr2 is in a Hi active operation, which is turned on when the No. 2 terminal of the IC1 is at the H level.

Incidentally, the transistor Tr1 in FIG. 4 was in Lo active operation. Therefore, this embodiment is different in this respect as well.

(2) Operation The operation of the circuit in (a) will be explained with reference to FIGS. 3(b) and 3(c).

The operation of this embodiment is basically the same as that shown in FIG. However, since an integrating circuit consisting of R11 and C3 is inserted into the feedback circuit for the feedback voltage Vfb, the Vfb is smoothed as determined by the time constant of the integrating circuit.

The integration circuit is inserted to prevent IC1 from malfunctioning due to noise whose cycle is shorter than the switching cycle of Tr2. Specifically, brush noise of the motor M can be exemplified.

Now, in (b), when Vo is at H level, the control transistor Tr2 is turned on, and power is supplied to the motor M.

[0060]The time width is the time t1 indicated by PW.
- t2, and at time t2 Vo becomes L level and transistor Tr2 is turned off. In other words, the power supply is stopped.

Note that the time width PW is set to a value predetermined by the comparison/timer 1. For example, R10 is 10
When KΩ and C4 are set to 1μF, PW≒10ms
becomes.

Thereafter, similarly, the transistor Tr2 is turned on.
/OFF is repeated, and electric power determined by the duty ratio is supplied to the motor M, and a predetermined rotational speed is obtained.

(c) shows how the timing to turn on the transistor Tr2 is determined.

That is, when the power supply to the motor M is stopped at time t2, the rotational speed of the motor M gradually decreases, and the back electromotive force generated by the motor M also gradually decreases. Therefore, Vfb gradually increases, and at time t3, Vfb gradually increases.
Matches ref.

When Vfb matches Vref, Vo becomes H level, that is, active, and transistor Tr2
turns on. That is, power is supplied to the motor M at PW between times t3 and t4.

[0066] Thereafter, the operation is repeated in the same manner, and power is supplied to maintain the predetermined rotational speed.

However, when the load on motor M increases,
The speed at which the rotational speed of the motor M decreases when power supply is stopped increases, and at time t41, Vfb coincides with Vref, and the time at which power supply to the motor M starts is brought forward.

That is, the ON/OFF state of the transistor Tr2 is
The OFF duty ratio increases, the power supplied to the motor M increases, and power is supplied to maintain the predetermined rotational speed.

(3) Results of Implementation In this example, since a resistor for motor current feedback is not used, a control circuit that generates little heat can be realized despite its small size.

Furthermore, there was no rotational unevenness during low-speed rotation, and good speed control was possible over a wide rotational range.

Incidentally, the fluctuation amplitude of the feedback voltage when the motor M is rotating at low speed, that is, Vfb shown in FIG.
is smaller than the fluctuation amplitude of Vfb1 shown in FIG.
From this, it can be seen that the characteristics at low speed rotation have been improved.

[0072]

As described above, according to the present invention, it is possible to realize a speed control circuit for a DC motor that is small in size, generates little heat, and has excellent speed controllability.

Therefore, it is possible to provide a speed control circuit suitable for DC motors, which are becoming increasingly smaller and more powerful, and it is possible to promote the miniaturization and higher performance of devices that require speed control.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram illustrating the basic configuration of the present invention.

FIG. 2 is a waveform diagram illustrating the operation. Furthermore, (a) (b)
) have the same time scale. Moreover, the waveform shown by a solid line is a case where the load on the motor is a normal load, and the waveform shown by a dotted line is a case where the load is increased.

FIG. 3 is a diagram explaining the embodiment, (a) is a circuit diagram; (a) is a circuit diagram;
b) and (c) are waveform diagrams illustrating the operation. Furthermore, (b
) The time axes in (c) are on the same scale.

FIG. 4 is a conventional example of a circuit that controls the rotational speed of a motor.

[Fig. 5] Waveform diagrams explaining the operation state, (a) and (b)
is a diagram when only Vfb1 is considered as the feedback voltage, (
c) (d) are diagrams when both Vfb1 and Vfb2 are considered as feedback voltages.

[Explanation of symbols]

1 Comparison/timer IC1
555IC (timer IC with built-in comparator) R1~R12 Resistor

Claims (1)

[Claims] [Claim 1] Based on a reference voltage (Vref) that commands the rotational speed of the DC motor (M),
A circuit for controlling the rotation speed of at least one reference voltage (Vref) and the other comparison voltage (Vfb).
A comparison method in which the output voltage (Vo) is active for a predetermined period of time (PW) when the
It consists of a timer (1) and a transistor (TR) that switches when driven by the output voltage (Vo) of the comparison/timer (1), and connects one end of the controlled motor (M) to the power source (Vcc). The controlled motor (M) is connected to the transistor (TR) whose other end is connected to the cold end side of the power supply (Vcc)
The voltage at the point (a) connecting the transistor (TR) and the comparison voltage (Vfb) of the timer (1) is
A speed control circuit for a DC motor, characterized by: