JPS627794B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a failure detection device for a DC motor control circuit, and more particularly to a failure detection device for a DC motor control circuit whose rotational speed is controlled by a pulse width modulation method.

Speed control using a pulse width modulation method is widely used in applications where the speed needs to be accurately controlled, such as in a document feeder of a copying machine. FIG. 1 shows an example of such a conventional control circuit. The DC motor 1 and the power transistor 3 are connected in series to a power source 4 . The rotation speed of the motor 1 is converted into a voltage V by the tachometer generator 2, and is applied to the control circuit 6 together with the reference voltage V ₀ of the rotation speed setting section 5. The control circuit 6 supplies a control voltage according to the deviation of the voltage V from V ₀ to the pulse width modulation section 7, and the pulse width modulation section 7 supplies a base voltage with a pulse width corresponding to this to the transistor 3. Control this on/off. Through such feedback control, the rotation speed of the motor 1 is automatically controlled to the value set by the rotation speed setting section. As mentioned above, in the conventional DC motor speed control circuit, the rotational speed of the motor 1 is controlled by changing the duty ratio of the pulse signal applied to the base of the transistor 3, so the duty ratio increases and reaches 1. When this point is reached, there is no period during which the transistor 3 is turned off, and this state becomes equivalent to an uncontrolled state. In order to avoid this, it is common practice to allow a margin in the DC power supply so that the duty ratio does not become 1 even at the maximum rotational speed of the motor 1. For this reason, if a failure occurs in which the power transistor 3 becomes conductive, the DC motor 1 will rotate at a high speed higher than a specified value, resulting in runaway or damage to related devices. When used in a document conveying device of a copying machine, it damages the document and degrades image fidelity, making it impossible to operate normally.

The present invention immediately detects when the motor begins to rotate at a high speed higher than a specified value, and
This makes it possible to take appropriate measures such as issuing an alarm or shutting off the power.

FIG. 2 is a block diagram of one embodiment of the present invention.
The same symbols as in FIG. 1 represent the same parts. 8 is a photocoupler consisting of a light emitting element 81 to which the terminal voltage of the motor 1 is applied and a light receiving element placed opposite thereto, that is, a photosensitive switching element 82; 9 is a comparator; and 10 is a relay. When the power transistor 3 is on and the motor 1 is rotating, the light emitting element 81 to which the terminal voltage is applied
is excited, the light receiving element 82 is turned on, and the transistor
Tr is turned off, capacitor C is charged via resistor R, and its terminal voltage Vc rises. On the other hand, when the transistor 3 is turned off and current is no longer supplied to the motor 1, the light emitting element 81 is also deenergized, the light receiving element 82 is turned off, and the transistor Tr is turned on, so the capacitor C is charged and the terminal voltage Vc decreases. do. When the speed of the motor 1 is being controlled normally, the power transistor 3 is turned on and off repeatedly by the pulse width modulation section 7.
Therefore, since the current to the motor 1 is also interrupted, the capacitor C is repeatedly charged and discharged. For this reason, the time constant CR is changed to the power transistor 3
If you choose a value larger than the switching period of
The terminal voltage Vc of the capacitor C does not rise above the specified value Vs. However, if the transistor 3 continues to be on due to a failure in the control circuit 6 or the power transistor 3 itself, and the rotational speed of the motor 1 attempts to increase beyond the limit, the light emitting element 81 remains excited, and as a result, the transistor
The Tr continues to be off and the capacitor C remains charged, and the terminal voltage Vc of the capacitor gradually increases.
Finally, it becomes equal to the reference value Vs. At this time, the comparator 9 generates a trigger pulse to make the SCR conductive,
The relay inserted in that circuit is energized to motor 1.
Turn off the power switch S. Furthermore, it is of course possible to issue an alarm by conducting the SCR.

As described above, according to the present invention, it is possible to quickly detect when the motor 1 is about to reach a high speed higher than a specified value and issue an alarm, power off, or both at the same time. Inconveniences and accidents that may occur can be prevented. If the present invention is applied to a document conveying device of a copying machine, damage to the document can be prevented.

Further, according to the present invention, the circuit on the DC motor 1 side and the circuit on the failure detection side (the light receiving element, that is, the light sensitive switching element 82, the capacitor C, and the comparator 9)
), it becomes easy and reliable to electrically insulate the circuit on the DC motor side, making it possible to lower the voltage and make it constant voltage regardless of the voltage on the DC motor side, and as a result, standardize the design. It becomes possible to reduce costs.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a conventional failure detection device for a DC motor control circuit, and FIG. 2 is a block diagram of an embodiment of the present invention. 1...DC motor, 2...Tachometer generator, 3...
Power transistor, 5... Rotation speed setting section, 6... Control circuit, 7... Pulse width modulation section, 8... Photocoupler, 9... Comparison circuit, 10... Relay.

Claims (1)

[Claims] 1. A DC motor driven by a pulse width modulated current, a light emitting element connected to the circuit of the DC motor and whose light emission is controlled according to on/off of the pulse width modulated current, and a circuit of the DC motor. a light-sensitive switching element that is electrically insulated from the light-sensitive switching element and conducts in response to light emission from the light-emitting element; A DC motor control circuit comprising a capacitor to be discharged and a comparator for comparing the charging voltage of the capacitor with a reference voltage, and detecting a failure in the DC motor control circuit based on the output of the comparator. Failure detection device.