CIRCUIT FOR DRIVING LOAD

Abstract: A circuit for driving a load wherein the breaking mechanism for breaking a main power supply when a failure occurs is a fail-safe one. The circuit can drive an inductive load, saving power. The circuit also is improved in delay of its operation stop. The breaking mechanism is of a contactless one. In a feeding circuit for fedding power to the load, interposed is a means for sensing failure of semiconductor switching elements which perform the ON/OFF control of the power-feed to the load. The breaking mechanism is operated by the output of the failure sensing means. When driving an inductive load, two power supplies for feeding power to the load are interposed in the feeding circuit. Upon generating a signal for commanding the feeding circuit to drive the load, a high voltage is applied to the load by the two power supplies; and after a predetermined time, one of the two power supplies is stopped. In a stationary operation, a low voltage is applied to the load, feeding the power by one power supply, utilizing the signal for commanding the feeding circuit to drive the load, a pulse width modulation output is created. Using the output, power is fed to the load via a transformer. Thereby, when the load is driven stationarily, fed is a voltage lower than the voltage when starting the driving operation. Power for driving a load is saved, and the delay of operation stop is improved.

Description: SPECIFICATION Technical Field The invention relates to a load driving circuit for driving a hysteresis load, employing a technique of saving electricity when driving the load. Background Art Devices such as press controllers must provide a high degree of safety and must be fail-safe so that they are switched to a safety side when failures, short circuits, disconnections, etc., occur. Load driving circuits for driving loads such as motors and solenoids that are controlled must also be fail-safe. One of the conventional load driving circuits directly connects a semiconductor switch such as a thyristor, a solid-state relay (hereinafter referred to as SSR), or an electromagnetic relay having contacts to a load in series and provides a load driving instruction signal to turn ON and OFF the switch or the relay, to thereby control the operation of the load. If the semiconductor switch short-circuits or if the relay contact melts, a current will flow to the load even if there is no input signal (load driving instruction signal). Namely, the conventional circuit has a danger that it may erroneously provide an output to the load although there is no input. Such circuit is not fail-safe, and therefore, is unemployable for devices that require a high degree of safety. To be fail-safe, the load driving circuits may employ an electromagnetic relay having special contacts (for example, carbon contacts) that never melt. This sort of contacts, however, is short in service life. To secure fail-safe characteristics, another type of load driving circuits has been proposed (Japanese Unexamined Patent Publication Nos. 60-223445. and 60-227326 and U.S. Patent No. 4,661,880). These circuits directly control a load driving switch circuit with an input signal (load driving instruction signal) and monitor the ON/OFF status of the switch circuit through a fail-safe monitor circuit. Upon detecting electricity supplied to a load with no input signal, the monitor circuit forcibly breaks a primary power source, to surely prevent the most serious accident during the operation of the load. Another type of load driving circuits connects an input signal to a power supply circuit of a load via an electrically isolated signal receiving system involving a transformer. According to this type, an AC input signal (load driving instruction signal) is amplified by an amplifier, and the amplified signal is supplied to a primary winding of the transformer so that a secondary winding thereof may generate an alternating current. The alternating current is converted by a rectifier diode into a direct current, which is supplied to the power supply circuit of the load. This arrangement involves no semiconductor switches that may cause short-circuit failures nor has the problem of short service lives of electromagnetic relays, thereby ensuring fail-safe characteristics. Even of this type, load driving circuits of large capacity for, for example, presses usually employ contact breaking mechanisms having relays for breaking a primary power source that supplies electricity to a load. Since the contact breaking mechanisms always have the problem of melt and wear, they are unsatisfactory in reliability. According to the technique of indirectly driving a load through a transformer in response to an input signal, the load will generate a counter-electromotive force when the input signal is turned OFF, if the load is a DC electromagnetic valve or relay that is inductive. The counter-electromotive force produces a discharge current, which flows to a power supply circuit of the load through a rectifier diode. This results in causing a delay in stopping the load after the turning OFF of the input signal. Some loads such as electromagnetic valves and relays show hysteresis that an input level for starting the loads differs from an input level for stopping the loads. These hysteresis loads continuously operate if an input level sufficient for maintaining the operation is supplied thereto after the start thereof. In spite of this phenomenon, the prior art continuously supplies the starting input level as it is to the loads, thereby wasting electricity. An object of the invention is to provide a load driving circuit that is capable of saving electricity when driving a hysteresis load. Disclosure of Invention The invention provides a load driving circuit for driving a load showing hysteresis that an operation start level of the load is higher than an operation stop level of the load. The load driving circuit rectifies an AC signal prepared from a load driving instruction signal and supplies the rectified signal to the load, to thereby drive the load. The load driving circuit includes a fail-safe load driving signal generator for providing a load driving instruction signal of logical value 1 representing a high energy state in response to a load driving enable signal, an output signal of logical value 0 representing a low energy state when not receiving the load driving enable signal, and an output signal of logical value 0 representing a low level state if the generator itself becomes out of order; a signal oscillator for generating a periodic oscillation output with the output of the load driving instruction signal generator serving as a power source, the oscillation output temporally inclining; a signal comparator for receiving the output of the load driving instruction signal generator as a power source, comparing the oscillation output of the signal oscillator with a threshold value that gradually rises with a predetermined time constant, and generating a pulse width modulated output that is at high level while the oscillation output is higher than the threshold value; an amplified AC output supply unit for amplifying the pulse width modulated output of the signal comparator through a transformer and supplying the amplified AC output to a power supply circuit of the hysteresis load; and a rectifier for rectifying the amplified AC output of the amplified AC output supply unit and supplying the rectified output to the load. In this arrangement, the transformer provides the maximum output energy when the duty ratio of the pulse width modulated output is 50%. The output energy of the transformer decreases as the duty ratio becomes larger or smaller than 50%. Accordingly, the output energy supplied to the load gradually increases at first and exceeds the operation start level of the load. Thereafter, the output energy to the load decreases below the operation start level, and after a predetermined time, settles to a level that is slightly higher than the operation stop level. This arrangement is advantageous in reducing power supply after the start of the operation of the load, thereby saving electricity. Since the output of the load driving instruction signal generator is used as a power source for the signal oscillator and signal comparator, the signal oscillator and signal comparator will never be activated if the load driving instruction signal generator provides no output signal. In addition, the load driving instruction signal generator has a fail-safe structure that never erroneously provides an output of logical value 1 representing a high energy state. A load driving output prepared from the load driving instruction signal is supplied to the load through the transformer. This arrangement enhances the fail-safe characteristics. An embodiment of a load driving circuit according to the present invention will be explained in detail with reference to the drawings. Fig. 1 is a circuit diagram showing a load driving circuit according to an embodiment of the invention; and Fig. 2 is a time chart showing outputs of essential parts of the above embodiment. Best Mode for Carrying Out the Invention Figure 1 shows an arrangement of the load driving circuit according to an embodiment of the invention. A signal processor 71 serves as a load driving instruction signal generator and is formed of a known fail-safe AND oscillator. When receiving a load driving enable signal from a sensor (not shown) for monitoring a safety state, the signal processor 71 provides an output (a load driving instruction signal I N ) of logical value 1 representing a high energy state. When receiving no load driving enable signal from the sensor, the signal processor 71 provides an output of logical value 0 representing a low energy state. When the signal processor is out of order, it never erroneously provides an output of logical value 1. Instead, it provides an output of logical value 0 representing a low level state. A triangular wave generator 72 serves as a signal oscillator and uses the load driving instruction signal I N from the signal processor 71 as a power source, to generate a triangular signal u shown in Fig. 2. A level comparator 73 serves as a signal comparator and uses the load driving instruction signal I N from the signal processor 71 as a power source. The level comparator 73 compares the triangular signal u of the triangular wave generator 72 with a threshold value p that gradually rises with a predetermined time constant, and provides a pulse width modulated (hereinafter referred to as PWM) output s that maintains high level while the triangular signal u is higher than the threshold value p. The time constant of the threshold value p is determined by a resistor R1 and a capacitor C1. When the capacitor C1 is saturated after a predetermined time, the threshold value p is kept at a value (= R2 * V/(R1 + R2)) obtained by dividing the voltage V of the load driving instruction signal I N by resistors R1 and R2. The PWM output s of the level comparator 73 is applied to a gate G of a semiconductor switch such as a MOSFET 74. The MOSFET 74 is connected to a power source Vcc through a primary winding of a transformer 75. The source of the MOSFET 74 is grounded. According to the ON/OFF period of the PWM output s, a current of the power source Vcc is supplied to the primary winding of the transformer 75, so that a secondary winding of the transformer 75 generates an amplified AC output due to the transformer coupling amplification. The AC output is supplied to a power supply circuit for driving a load 77. Namely, the AC output is rectified by a rectifier 76, which provides a rectified output of energy E shown in Fig. 2 to the load 77 such as an electromagnetic valve or an electromagnetic relay showing hysteresis. The operation of the load driving circuit of this embodiment will be explained. The signal processor 71 provides the load driving instruction signal I N , which drives the triangular wave generator 72 and level comparator 73. The triangular wave generator 72 generates the periodic triangular signal u as shown in Fig. 2. In response to the load driving instruction signal I N , the threshold value p is provided to the level comparator 73. The threshold value p gradually rises as shown in Fig. 2 according to the time constant determined by the resistor R1 and capacitor C1. The level comparator 73 compares the threshold value p with the triangular signal u, and generates the PWM output s, which keeps a high level while the triangular signal u is higher than the threshold value p. As shown in Fig. 2, the pulse width of the PWM output s narrows as the threshold value p gradually rises. When the capacitor C1 is saturated and the threshold value p is kept at a constant value determined by the voltage dividing ratio of the resistors R1 and R2, the pulse width of the PWM output s becomes constant. In response to the PWM output s, the MOSFET 74 periodically turns ON and OFF. According to the ON and OFF operations of the MOSFET 74, the secondary winding of the transformer 75 provides an amplified AC output, which is rectified by the rectifier 76. The energy E of the rectified output of the rectifier 76 becomes maximum when the duty ratio of the PWM output s is at about 50% as shown in Fig. 2. When the duty ratio is lower or higher than 50%, the energy E decreases, and when the capacitor C1 is saturated, the energy E keeps a constant level. In Fig. 2, the load 77 starts to operate at an input level of E1 and stops to operate at an input level of E2. The output energy E gradually increases after the generation of the load driving instruction signal I N , and when it exceeds the operation start level E1, the load 77 is turned ON. Thereafter, the output energy E decreases and then maintains a constant level. If the constant level is set to be higher than the operation stop level, the load 77 may keep an ON state at the constant level that is lower than those of prior arts. Accordingly, the power consumption of the load 77 becomes smaller after the load is started. Compared with the conventional load driving circuits, the circuit of this embodiment is capable of greatly reducing power consumption. The triangular wave generator 72 and level comparator 73 use the load driving instruction signal I N from the signal processor 71 as a power source, so that they will never operate if there is no load driving instruction signal I N . Since the output of the MOSFET 74 is extracted through transformer coupling, the output of the level comparator 73 or of the power source Vcc is not transferred to the secondary winding of the transformer 75, i.e., to the load 77, if the MOSFET 74 is short-circuited or broken. In this way, the load driving circuit of this embodiment will provide no rectified output for driving the load 77 if the signal processor 71 provides no load driving instruction signal I N . The signal processor 71 will never erroneously provide an output of logical value 1 if it becomes out of order. Namely, it always provides an output of logical value 0 representing a low energy state, if it is out of order. With these arrangements, the load driving circuit of this embodiment is fail-safe to never erroneously provide load driving output E if there is no load driving instruction signal I N . According to this embodiment, the oscillation signal provided to the level comparator 73 is triangular. Instead, a signal of any shape such as a sawtooth signal or a sine wave signal is employable if the signal is capable of providing a temporally inclining output. The invention provides a load driving circuit for driving a load that shows hysteresis that an operation start level of the load is higher than an operation stop level of the load. To start the load, the load driving circuit applies an input level to sufficiently start the load, and once the load is started, applies an input level that is lower than the operation start level but within a range to sufficiently maintain the operation of the load. Compared with the conventional load driving circuits, this circuit is able to reduce power consumption. In addition, this arrangement is fail-safe so that it never erroneously drives the load if there is no load driving instruction output, thereby greatly improving the safety and reliability of the circuit. Capability of Exploitation in Industry This invention safely and efficiently drives a load that is a final controlled object of industrial equipment that requires a high degree of safety. The present invention, therefore, has a great capability of exploitation in industry.

Claims: A load driving circuit for driving a load (77) characterised in that it shows hysteresis that an operation start level of the load (77) is higher than an operation stop level of the load (77), the load driving circuit rectifying an AC signal prepared from a load driving instruction signal (IN) and supplying the rectified signal to the load (77) to thereby drive the load (77), the load driving circuit comprising fail-safe load driving instruction signal generation means (71) for providing a load driving instruction signal (I N ) of logical value 1 representing a high energy state when receiving a load driving enable signal, an output signal of logical value 0 representing a low energy state when not receiving, the load driving enable signal, and an output signal of logical value 0 representing a low level state if the generation means (71) itself is out of order; signal oscillation means (72) for providing a periodic oscillation output (u) with the output of the load driving instruction signal generation means (71) serving as a power source, the oscillation output (u) temporally inclining; signal comparison means (73) for receiving the output (I N ) of the load driving instruction signal generation means(71) as a power source, comparing the oscillation output (u) of the oscillation means (72) with a threshold value (P) that gradually increases with a predetermined time constant, and generating a pulse width modulated output (S) that is at high level while the oscillation output (u) is higher than the threshold value; amplified AC output supply means (75) for amplifying the pulse width modulated output (S) of the comparison means (73) through a transformer (75) and supplying an amplified AC output to a power supply circuit of the hysteresis load (77); and a rectifier (76) for rectifying the amplified AC output provided by the amplified AC output supply means (75) and supplying the rectified output to the load (77). The load driving circuit according to claim 1, characterised in that the amplified AC output supply means includes a MOSFET (74) and the transformer (75), the MOSFET (74) having a gate (G) for receiving the pulse width modulated signal from the signal comparison means (73), a drain (D) connected to a power source (Vcc) through a primary winding of the transformer (75), and a source grounded.

Inventors: FUTSUHARA KOICHI, SAKAI MASAYOSHI, FUTSUHARA, KOICHI, SAKAI, MASAYOSHI, FUTSUHARA, KOICHI YONO OFFICE THE, SAKAI, MASAYOSHI YONO OFFICE THE
Applicants: NIPPON SIGNAL CO LTD, THE NIPPON SIGNAL CO. LTD.
Application year: 1993
Main classification: H01F7
IPC codes: H01H47, H01F7