JPS5912218B2 - variable frequency oscillation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a variable frequency oscillation circuit used for acceleration/deceleration control of pulse motors in printers, machine tools, etc.

Pulse motors are widely used in printers and other applications because the speed and angle of rotation can be controlled externally without using feedback signals, but when accelerating or decelerating at high speeds, control is required to match the output from the motor. .

That is, if the inertia on the motor shaft is J, the friction of each part is Tf, and the motor output torque is TM, then the acceleration θ that can be accelerated by the motor is as follows, and it is necessary to make the acceleration/deceleration commensurate with the output of the motor.

When performing the above-mentioned control, a method is used in which acceleration and deceleration are performed gradually as shown in FIG.

According to this method, since the acceleration is constant, the operation is proportional to the output of the motor, and disturbances, which are the most important problem when using a pulse motor, do not occur.

FIG. 2 shows the general configuration of a control system that performs such control.

The variable frequency oscillation circuit 2 generates pulses whose oscillation frequency becomes higher as time passes (lower during deceleration) in response to a signal from the control circuit 1 that operates in response to a drive command.

The pulse motor driver 3 drives the pulse motor 4 by the output pulse of the variable frequency oscillation circuit 2, and rotates it as shown in FIG.

Although this is a commonly used pulse motor control method, the following problems arise when attempting to realize the variable frequency oscillation circuit described above.

That is, the variable frequency oscillator circuit 2 can be roughly divided into one in which a reference clock of a constant period is divided by a frequency division counter and the frequency division ratio is variable, an oscillator whose oscillation frequency can be changed by voltage, current, etc. For example, some use an astable multivibrator.

The former configuration has good frequency accuracy, but in order to change the frequency smoothly, it is necessary to increase the number of bits of the frequency division counter, which increases the circuit scale.

Furthermore, when the operating frequency is high, it may not be possible to implement it with a normal TTL element.

For example, in order to obtain an oscillation output of 50KHz with a resolution of 12 bits, the reference clock frequency must be 200MHz.
z or higher, and a very expensive ultra-high-speed IC must be used.

Although the circuit configuration in which the circuit is implemented digitally operates reliably, it is often disadvantageous in terms of the cost and scale of the circuit.

For this reason, the latter circuit configuration is usually adopted for general purposes.

However, although this configuration can significantly reduce the amount of circuitry, it takes time and effort to set the oscillation frequency, and the frequency stability tends to deteriorate.

That is, since the variable frequency oscillation circuit controls its oscillation frequency using an externally applied voltage or current, setting the frequency requires adjusting the circuit voltage.

In addition, there are few problems with expensive oscillators that are made into dedicated ICs, but oscillation circuits implemented using ordinary ICs, ) transistors, etc. have poor stability, especially at the lower limit of the oscillation frequency, and cannot be stabilized by adjusting the input voltage, etc. conversion is often difficult.

This will be specifically explained using a conventional variable frequency oscillation circuit shown in FIG.

In FIG. 3, transistor 6 forms a constant current source whose current value is controlled by a control voltage, and capacitor 7 is gradually charged by this current to a charging voltage V.

is the upper threshold voltage vTH1 as shown in FIG.
When this happens, the output of comparator 9 is inverted.

This causes the base of the discharge transistor 8 to connect to the resistor 1.
0 and the charge on capacitor 7 is discharged through transistor 8.

This discharge takes 31 hours to occur instantaneously.

This period becomes the inversion time of the output pulse of the comparator 9, as shown in FIG.

Discharging continues until the voltage on capacitor 7 reaches the lower threshold voltage VTH2, at which point comparator 9
The output of is restored.

Then, it is charged again until it becomes VTRt.

The period T of this oscillator is now determined by the capacitor 7, the control voltage and the emitter resistor 5 of the transistor 6.

The problem here is that the closer you get to the lower limit frequency of oscillation, the more
Since the charging current becomes small, the voltage drop across the emitter resistor 5 becomes relatively small with respect to the base-emitter voltage of the transistor 6 near the lower limit frequency, and the temperature change of the base-emitter voltage of the transistor 6 and the parameter The charge current becomes unstable due to the influence of variations in the charge current.

In other words, the stability of the oscillation frequency is no longer maintained.

Conventionally, to solve this problem, constant current circuits were designed to be stable even at low currents by using high-speed operational amplifiers.

However, using an expensive high-speed operational amplifier increases the cost and is not desirable.

An object of the present invention is to provide a variable frequency oscillation circuit that is inexpensive and has a stable lower limit oscillation frequency.

The variable frequency oscillation circuit according to the present invention is characterized by a free-running oscillation circuit, a circuit that controls the free-running oscillation frequency of the free-running oscillation circuit according to a control signal, and a circuit that controls the free-running oscillation frequency and its reference frequency. a circuit that compares the free-running oscillation frequency and outputs an error signal according to the frequency difference when the free-running oscillation frequency becomes lower than the reference frequency, and controls the error signal with a polarity that increases the free-running oscillation frequency. and a circuit for superimposing the signal on the signal.

It will become clear from the following description of the embodiments that both the free-running oscillation circuit and the oscillation frequency comparison circuit can be realized using an inexpensive TTL-IC monostable multivibrator or the like.

FIG. 5 shows and explains the variable frequency oscillation circuit according to the present invention.

Monostable multivibrator (commercially available (7) TTL-I
C is fine) 15, capacitor 16, delay line 17
and the resistor 14 constitute a free-running oscillation circuit.The transistor 13 is connected in series between the +5V line and the resistor 14, and the resistor 11 and capacitor 12 are the frequency control circuit of the free-running oscillation circuit. It consists of

The higher the control voltage input to the transistor 13, the higher the free-running oscillation frequency of the free-running oscillation circuit.

Retriggerable monostable multivibrator (commercially available TT
(L-IC is fine) 18, capacitor 19, resistor 20, 2
2, 23 and the oven collector type inverter 21 compare the free-running oscillation frequency of the free-running oscillation circuit with a preset reference frequency, and when the free-running oscillation frequency falls below this reference frequency, both A frequency comparison circuit is configured to output an error signal according to the frequency difference.

The diode 24 cooperates with the frequency control circuit to feed back the error signal to the free-running oscillation circuit, and superimposes the error signal on the control signal.

Next, the operation will be explained with reference to the time chart shown in FIG.

When the free-running oscillation frequency is higher than the reference frequency, the monostable multivibrator 18 of the frequency comparator circuit changes to the monostable multivibrator 1 of the free-running oscillation circuit before returning to a stable state.
Since it is retriggered by the output A of 5, it remains in a metastable state.

Therefore, the output B of the monostable multivibrator 18 is fixed at a high level, and the error signal C is not output.

When the free-running oscillation frequency becomes lower than the reference frequency, that is, the repetition period of the output A of the monostable multivibrator 15 becomes longer than the metastable time of the monostable multivibrator 18.As shown in FIG. 4, the monostable multivibrator 18 transits to a stable state for a time of Δt.

It is clear that the length of this time Δt is proportional to the frequency difference between the free-running oscillation frequency and the reference frequency. Therefore, a pulse with a width proportional to the frequency difference is applied as an error signal C to the capacitor 12 via the diode 24. It will be done.

Note that the reference frequency, in other words, the metastable time of the monostable multivibrator 18 is determined by the capacitor 19 and the resistor 20.

As the pulse width of the error signal C, that is, the magnitude of the frequency difference, increases, the capacitor voltage Vo is charged to a higher value, and the free-running oscillation frequency tends to increase more strongly.

Due to such a feed buffer effect, when the control voltage drops below the minimum value, the free-running oscillation frequency is stabilized at a certain lower limit frequency determined by the reference frequency and the gain of the feedback loop.

Generally, this lower limit frequency approximately coincides with the reference frequency.

In this way, the lower limit oscillation frequency of the variable frequency oscillation circuit of the present invention is stabilized by the feedback action, so that the influence of variations in parameters of the transistor 13 of the frequency control circuit and temperature changes in the base-emitter voltage is almost eliminated. Completely avoidable.

Another feature of the present invention is that the error signal is superimposed on the control signal, and the effect thereof will be explained with reference to FIGS. 7 and 8.

FIG. 7 shows a variable frequency oscillation circuit that is almost the same as the combination of the free-running oscillation circuit and the frequency control circuit shown in FIG.

The difference from the above embodiment is that the frequency difference signal is not fed back to the base of the transistor 13, and the resistor 30 is replaced by a monostable monomulti 150 to enable free-running oscillation when the transistor 13 is off. This point is connected between the control terminal and the power supply.

In this circuit as well, if the resistor 30 and capacitor 16 are appropriately selected, the lowest free-running oscillation frequency when the transistor 13 is off can be defined to be substantially constant.

However, if the frequency control voltage, that is, the base voltage of the transistor 13, drops pointlessly below the threshold voltage of the transistor, even if the control voltage starts to rise next time, the control voltage will not reach the threshold voltage. The transistor 13 remains off, and the frequency does not follow the control voltage during this period.

In other words, dead time occurs.

In contrast, according to the present invention, since an error signal is superimposed on the control voltage, when the oscillation frequency becomes approximately equal to the lower limit oscillation frequency, the control voltage does not decrease any further, and the transistor 1
The voltage does not fall significantly below the threshold voltage of 3.

Therefore, as shown in FIG. 8, when the control voltage increases, the oscillation frequency follows and increases with virtually no dead time.

The dotted line in FIG. 8 shows the change characteristics of the oscillation frequency in the circuit of FIG. 7, and there is a dead time td between the control voltage and the time when the oscillation frequency starts rising.

As described above, the present invention uses an inexpensive monostable multivibrator (individual component circuit or integrated circuit), etc., without using an expensive high-speed operational amplifier, etc., to generate a stable control signal and oscillation frequency at the lower limit oscillation frequency. It is possible to provide a variable frequency oscillation circuit that has no dead time between the response of

It should be noted that the present invention is not limited to the configuration of the embodiment described above, but can be modified in a wide range of ways.

For example, it will be clear that the frequency comparison circuit can be constructed using a crystal oscillator and a counter.

[Brief explanation of the drawing]

Figure 1 is a timing diagram necessary for controlling the pulse motor, Figure 2 is a block diagram of the control system for controlling the pulse motor, Figure 3 is a circuit diagram of a conventional variable frequency oscillation circuit, and Figure 4 is a diagram of the control system for controlling the pulse motor. FIG. 3 is an operating waveform diagram of the circuit, FIG. 5 is a circuit diagram showing the structure of the variable frequency oscillation circuit according to the present invention, and FIG.
Figure 7 is a circuit diagram showing a circuit for explaining certain features and effects of the present invention.
The figure is a control characteristic diagram of the circuit of FIG. 5 and the circuit of FIG. 7 according to the present invention. 13...Transistor constituting the frequency control circuit, 1
5.17... Monostable multivibrator and delay line forming one running oscillation circuit, 18.21... Monostable multivibrator and inverter forming frequency comparison circuit, 24... Diode.

Claims (1)

[Claims] 1. A free-running oscillation circuit, a control circuit that controls the oscillation frequency of the free-running oscillation circuit according to a control signal, and a control circuit that oscillates at a reference frequency corresponding to the lower limit frequency to be output from the free-running oscillation circuit. A monostable multivibrator circuit that is triggered by the output of a running oscillation circuit and outputs an error signal corresponding to the frequency difference when the output frequency of the free-running oscillation circuit becomes lower than the reference frequency, and the frequency difference is reduced. and a circuit for superimposing the error signal on the control signal with a polarity of.