JPH09331629A - Current limiting structure during motor control by dc chopper circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a current limiting structure during control of a motor by a DC chopper circuit which can control the motor current by a pulse signal of the same frequency as in a normal operation even when a trouble occurs. SOLUTION: An operational amplifier 8 and external resistors R1 , R2 , R3 constitute a hysteresis comparator II. The hysteresis comparator II generates a pulse signal P2 which changes its state each time the motor current I detected by a current detector 6 when a CPU 7 has a trouble passes an upper limit value and a lower limit value of the hysteresis comparator II. The pulse signal P2 and a pulse signal P1 which is an output signal of the CPU 7 which has a 100% duty when the CPU 7 has a trouble are logic-operated by an AND circuit 9 and an operation result is output as a pulse signal P3 . By this method, the conduction state of an IGBT 5 can be controlled by the pulse signal P3 which has the same waveform as the pulse signal P2 .

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current limiting structure for controlling a motor by means of a DC chopper circuit, and more particularly to a structure using a battery as a power source and a DC motor as a drive source, for example, an electric vehicle such as a battery / forklift truck. It is useful.

[0002]

2. Description of the Related Art Some forklifts are electrically driven by a battery as a power source and a motor as a drive source. In this type of electric forklift truck, speed control by a DC chopper circuit is generally used. In this speed control, for example, by using a pulse signal of a predetermined frequency, which is an output signal of a control unit that is preferably formed by a CPU, the IGB
The conduction of the switch means of the DC chopper circuit, which is preferably formed by T or the like, is controlled.

At this time, the control means inputs a speed command value to the motor and changes the duty of the pulse signal which is an output signal corresponding to the speed command value to control the conduction time of the switch means and apply it to the motor. The average voltage is controlled to control the speed.

In such speed control, hardware current limiting means is provided in order to prevent an excessive current from flowing to the motor due to an abnormality in the software of the control means. This type of current control means according to the prior art,
At the time of abnormality, a pulse signal, which is an output signal of the control means, which should be a signal of a predetermined frequency as shown in FIG.
Thinning out as shown in FIG.
By (indicated by a dotted line in (b)), the conduction time of the switch means is shortened to limit the motor current.

[0005]

In the speed control according to the prior art as described above by the digital current feedback, the abnormalities of the control means are dealt with by thinning out the pulse of the pulse signal so that the excessive current does not flow to the motor. Is limited to. Therefore, when there is an abnormality, the frequency of the pulse signal becomes low, and the ripple current flowing through the smoothing capacitor of the DC chopper circuit increases. On the other hand, if the capacity of the smoothing capacitor is increased to avoid such a problem, there is a problem that not only the space occupied by the smoothing capacitor increases but also the cost increases.

In view of the above-mentioned prior art, the present invention provides a current limiting structure at the time of motor control by a DC chopper circuit capable of limiting the motor current with a pulse signal having a frequency similar to that in a normal state even in an abnormal state. With the goal.

[0007]

The structure of the present invention for achieving the above object is characterized by the following points.

1) In a motor control circuit using a DC chopper circuit, which is configured to change and control the conduction time of the switch means of the DC chopper circuit by changing the duty of the first pulse signal sent by the control means, It has a comparing means for sending out a second pulse signal whose state changes when the motor current detected when the control means is abnormal passes the upper limit value and when it passes the lower limit value, and the output of the comparing means is provided. By passing the second pulse signal and the first pulse signal, which are signals, through a logic circuit, a third pulse signal having a predetermined frequency is obtained as an output signal of the logic circuit, and the third pulse signal is used for the switching means. It is configured to control continuity.

2) In 1), the comparing means is a hysteresis comparator formed by selecting the upper and lower limit values and the width of both by the combination of an operational amplifier and its external resistance.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of the present invention together with a DC chopper circuit. As shown in the figure, D
The C-chopper circuit I has a well-known general configuration, and detects a battery 1 which is a power source, a smoothing capacitor 2, a freewheeling diode 3, a motor 4 which is a load, an IGBI 5 which is a switch means, and a motor current I. It has a current detector 6 that operates.

The CPU 7 as the control means controls the speed command value S _C.
And the motor current I detected by the current detector 6 are compared, and a pulse signal P ₁ of a predetermined frequency whose duty is controlled according to the deviation between the two is sent. This pulse signal P ₁ is input to the AND circuit 9 together with the output signal P ₂ of the operational amplifier 8. The AND circuit 9 takes the AND logic of the pulse signal P ₁ and the output signal P _2, and the result is used as the pulse signal P ₃ in I.
Send to GBT5.

As will be described in detail later, the operational amplifier 8 is normally used.
The output signal P _{2 of} is in the "1" state. Therefore, at this time, the pulse signal P ₃ coincides with the pulse signal P ₁ .
As a result, in the normal state, the conduction time of the IGBT ₅ is uniquely determined by the pulse signal P ₁ , and the predetermined motor current I can be passed.

The operational amplifier 8 has an external resistor R₁,
R₂, R_ThreeTogether with Hysteresis Comparator II
doing. That is, the non-inverting input terminal of the operational amplifier 8
Is resistance R ₁Limit current setting value I via_refCorresponding to
A constant voltage is applied and a resistance R is applied to the inverting input terminal._Three
The detection voltage corresponding to the motor current I is applied via
You. Thus resistance R₁, R₂Select the voltage division ratio by
The predetermined width W that is uniquely determined by this division ratio
It is possible to give the operational amplifier 8 the hysteresis characteristic of
At the same time, the limit current setting value I_refDepending on the value of
The upper and lower limit values U and L of the motor current I corresponding to
The inter-value is uniquely determined.

Here, the limit current setting value I _ref is set in correspondence with the value of the motor current I to be limited, and the width W of the hysteresis characteristic is selected in correspondence with the frequency of the pulse signal P _{1 in} the normal state. Keep it.

As a result, as shown in FIG. 2, the control means 7
When the value of the motor current I rises with the increase in the motor current I, it falls at the time when it passes the upper limit value U corresponding to the hysteresis characteristic. The pulse signal P ₂ that rises at the time when the lower limit value L is exceeded is obtained as the output signal of the operational amplifier 8. Therefore, when the change rate of the motor current I is the same, the pulse signal P _{2 at} this time has a constant frequency determined by the change rate of the motor current I and the width W of the hysteresis characteristic, that is, the pulse signal P 2 in the normal state. The pulse signal P _{2 has the} same frequency as that of ₁ .

On the other hand, when the CPU 7 is abnormal, the pulse signal P ₁ which is its output has a duty of 100 in most cases.
It is considered to be%. In this way, when the duty of the pulse signal P ₁ reaches 100%, the pulse signal P ₂ is supplied to one input terminal of the AND circuit 9, so that the pulse signal P which is the output signal of the AND circuit 9 is supplied. ₃ matches the pulse signal P ₂ . Therefore, the values of the resistors R ₁ and R ₂ are determined so that the frequency of the pulse signal P ₂ at this time matches the frequency of the pulse signal P ₁ at the normal time or has a frequency in the vicinity thereof.

The operation in this embodiment will be described together with its operation mode. First, in a normal state, the pulse signal P ₂ which is the output signal of the operational amplifier 8 is a signal in which the state of “1” is maintained. Therefore, in this case, the pulse signal P ₃ and the pulse signal P ₁ match, and the conduction control of the IGBT 5 is performed so that the motor current I corresponding to the speed command value S _C flows.

If an abnormality occurs in the CPU 7 during operation in such a state and the duty of the pulse signal P ₁ becomes 100%, the IGBT 5 accordingly becomes conductive and the motor current I rapidly increases. As a result, the motor current I
Pulse signal P ₂ falls at a time point when has passed the upper limit value U,
The pulse signal P ₃ which is the output signal of the AND circuit 9 also falls. As a result, the IGBT 5 is cut off and the motor current I gradually decreases. As a result, when the motor current I passes the lower limit L, the pulse signal P ₂ rises again, the pulse signal P ₃ which is the output signal of the AND circuit 9 rises, and the IGBT 5 is rendered conductive. Thus, the motor current I is gradually increased again, and the same operation is repeated at regular intervals.

[0020]

As described above in detail with the embodiments, according to the present invention, when the abnormality of the control means occurs and the motor current increases beyond the predetermined value, D
Since the pulse signal for controlling the switching means of the C-chopper circuit can have a predetermined duty of a predetermined frequency similar to that of the pulse signal at the normal time, it is possible to change the components of the DC chopper circuit such as the smoothing capacitor. Good control characteristics can be obtained even when the control means is abnormal.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention together with a DC chopper circuit.

FIG. 2 is a waveform diagram for explaining the operation of the circuit of FIG.

FIG. 3 is a waveform diagram showing a waveform of a pulse signal in the conventional technique.

[Explanation of symbols]

I DC chopper circuit II Hysteresis comparator 1 Battery 4 Motor 5 IGBT 6 Current detector 7 CPU 8 Operational amplifier 9 AND circuit S _C Speed command value I _ref Limit current setting value P ₁ , P ₂ , P ₃ Pulse signal I Motor current U Upper limit Value V Lower limit value W Width

Claims (2)

[Claims] 1. A motor control circuit using a DC chopper circuit configured to change and control the conduction time of the switch means of the DC chopper circuit by changing the duty of the first pulse signal sent by the control means, When the control means is abnormal, there is a comparison means for sending a second pulse signal whose state changes at the time when the motor current passes the upper limit value and when the motor current passes the lower limit value, which is the output signal of this comparison means. By passing the second pulse signal and the first pulse signal through a logic circuit, a third pulse signal having a predetermined frequency is obtained as an output signal of the logic circuit, and the conduction of the switch means is controlled by the third pulse signal. A current limiting structure at the time of motor control by a DC chopper circuit characterized by being configured to perform. 2. The DC chopper according to claim 1, wherein the comparing means is a hysteresis comparator formed by selecting an upper limit value / lower limit value and a width of both of them by a combination of an operational amplifier and an external resistance thereof. Current limiting structure for motor control by circuit.