Load driving circuit

Abstract: This invention relates to a load driving circuit having a fail-safe breaking mechanism for breaking a primary power source when a failure occurs. This invention also relates to a load driving circuit capable of saving electricity in driving an inductive load and reducing a delay in stopping the load. The breaking mechanism for breaking the primary power source has no contact. The load driving circuit includes a power supply circuit involving a semiconductor switching element that turns ON and OFF the supply of power to the load. There is arranged a detector for detecting a failure in the semiconductor switching element. When detecting a failure, the detector provides an output signal to activate the breaking mechanism. To drive the inductive load, the power supply circuit may have two power supply sources. In response to a load driving instruction signal, the two power supply sources together apply a high voltage to the load. After a predetermined period, one of the power supply sources is stopped, and during a steady-state operation of the load, the remaining power source applies a low voltage to the load. The load driving instruction signal may be used to provide a pulse width modulated output, which is used to supply power to the load through a transformer. During a steady-state operation of the load, this arrangement supplies a voltage lower than an operation start voltage to the load, to thereby reduce power consumption and a delay in stopping the load.

Description: Technical Field The invention relates to a load driving circuit for driving an inductive load that shows hysteresis involving different start and stop levels, employing a technique of lessening a delay in stopping 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 for supplying a high voltage to start an inductive load showing hysteresis that an operation stop voltage is lower than an operation start voltage and supplying a voltage that is slightly higher than the operation stop voltage during a steady-state operation, thereby lessening a delay in stopping the load after the turning OFF of an input signal. Disclosure of Invention The invention provides a load driving circuit for driving an inductive load showing hysteresis that an operation stop voltage is lower than an operation start voltage. The load driving circuit rectifies an AC signal prepared from a load driving AC instruction signal and supplies the rectified signal to the load, to thereby drive the load. The load driving circuit includes a first output supply unit for supplying a first rectified output to the load in response to the load driving instruction signal, the level of the first rectified output being higher than the operation stop voltage and lower than the operation start voltage; a second output supply unit for supplying a second rectified output to the load for a predetermined period in response to the load driving instruction signal, the second rectified output overlapping the first rectified output and being supplied to the load, the level of the overlapping first and second rectified outputs being higher than the operation start voltage. The invention supplies a high voltage to the load only to start the load, and thereafter, supplies a lower voltage than the operation start voltage to the load, to achieve a steady-state operation. This technique reduces energy accumulated in the load, to thereby shorten a period from the stoppage of the load driving instruction signal until the voltage to the load drops below the operation stop voltage and lessen a delay in stopping the load. It is possible to arrange a zener diode in the power supply circuit of the load and a unit for monitoring a failure in the zener diode. As soon as a counter-electromotive force produced by the load decreases below a zener voltage after the load driving instruction signal is stopped, the power supply circuit of the load is opened. Accordingly, a delay in stopping the load is further reduced. When the failure monitoring unit detects a failure in the zener diode, the load driving instruction signal is stopped to secure fail-safe characteristics. Brief Description of the Drawings Fig. 1 is a circuit diagram showing a load driving circuit according to a first embodiment of the invention; Fig. 2 is a view explaining the voltage hysteresis characteristics of a load of the above embodiment at the start and stop of the operation of the load; Fig. 3 is a time chart showing the states of power supplied to the load of the above embodiment; Fig. 4 is a circuit diagram showing a load driving circuit according to a second embodiment of the invention; Fig. 5 is a circuit diagram showing a load driving circuit according to a third embodiment of the invention; Best Mode for Carrying Out the Invention Embodiments of the present invention will be explained in detail with reference to the drawings. Load driving circuits according to the invention will be explained with reference to Figs. 1 to 5. Figure 1 shows a load driving circuit according to a first embodiment of the invention. In Fig. 1, an AC input signal corresponding to a load driving instruction signal is amplified by an AC amplifier 31. The amplified input is supplied to a primary winding of a transformer 32. A secondary winding of the transformer 32 generates an AC voltage accordingly. The AC voltage is rectified by a rectifier 33 involving four diodes, and the rectifier 33 provides a first rectified output to an inductive load 34 such as a solenoid. As shown in Fig. 2, the load 34 shows hysteresis that an operation stop voltage V OFF of the load 34 is lower than an operation start voltage V ON thereof. The amplified signal from the AC amplifier 31 is supplied to a second rectifier 35 too. The rectifier 35 provides a rectified signal to a differential circuit 36 having a predetermined time constant. An output of the differential circuit 36 is supplied to a fail-safe AND oscillator 37. An oscillation output of the AND oscillator 37 is amplified by a second AC amplifier 38. The amplified signal is supplied to a primary winding of a second transformer 39. A secondary winding of the transformer 39 generates an AC voltage accordingly for a predetermined period that is determined by the time constant of the differential circuit 36. The generated AC voltage is rectified by a third rectifier 40, which provides a second rectified output to the load 34. As shown in Fig. 2, the rectified output voltage V 1 of the rectifier 33 is higher than the operation stop voltage V OFF of the load 34 and lower than the operation start voltage V ON thereof. The rectified output voltage V 2 of the rectifier 40 is set such that, when it overlaps the output voltage V 1 of the rectifier 33, the sum of the overlapping voltages V 1 plus V 2 is higher than the operation start voltage V ON of the load 34. The transformer 32 and rectifier 33 form a first output supply unit, and the rectifier 35, differential circuit 36, AND oscillator 37, AC amplifier 38, transformer 39, and rectifier 40 form a second output supply unit. The operation of the load driving circuit of this embodiment will be explained with reference to Fig. 3. The input signal, i.e., the load driving instruction signal becomes ON and is amplified by the AC amplifier 31. The amplified signal is supplied to the primary winding of the transformer 32. The secondary winding of the transformer 32 generates an AC voltage, which is rectified by the rectifier 33 into the rectified output V 1 . At the same time, the amplified output of the AC amplifier 31 is rectified by the rectifier 35 and is differentiated by the differential circuit 36. According to the differentiated signal, the AND oscillator 37 provides an AC output, which is amplified by the AC amplifier 38. The amplified signal is supplied to the primary winding of the transformer 39. The secondary winding of the transformer 39 generates an AC voltage accordingly, which is rectified by the rectifier 40 into the rectified output V 2 . To start the load, the rectified voltages V 1 and V 2 overlap each other to form a voltage (V 1 + V 2 ) that is higher than the operation start voltage V ON of the load 34. The overlapping voltages are supplied to the load 34. After the predetermined period from the reception of the input signal, the differentiated signal disappears to stop the AC output of the AND oscillator 37. Accordingly, the rectified output V 2 of the rectifier 40 disappears. Thereafter, only the rectified voltage V 1 of the rectifier 33, which is slightly higher than the operation stop voltage V OFF of the load 34, continuously drives the load 34. When the input signal becomes OFF, the rectified output V 1 of the rectifier 33 stops, and similar to the prior art, the load 34 generates a counter-electromotive force, which causes a discharge current. Since the driving voltage (current) supplied to the load 34 is lower than that of the prior art, energy accumulated in the load 34 at the time of stoppage is smaller. This results in shortening a period from the turning OFF of the input signal to a moment when the counter-electromotive force produced by the load becomes lower than the operation stop voltage V OFF , thereby lessening a delay in stopping the load after the issuance of a load stopping instruction signal. A resistor may be interposed in series with the power supply line to the load, to further lessen the delay. A load driving circuit according to a second embodiment of the invention will be explained with reference to Fig. 4. The same parts as those of the first embodiment of Fig. 1 will be represented with like reference marks and their explanations will not be repeated. In Fig. 4, a power supply circuit of a load 34 has a zener diode 41 having a zener voltage Vz. The zener diode 41 is oriented to block a discharge current due to a counter electromotive force produced by the load 34 when an input signal (load driving instruction signal) is stopped. A monitor circuit 50 serving as a zener diode status monitoring unit monitors whether or not the zener diode 41 is normal. When the zener diode is abnormal, the monitor circuit stops the load driving. instruction signal. The monitor circuit 50 includes a fourth rectifier 51 for rectifying a load driving instruction signal; a fail-safe window comparator 53 having an input terminal for receiving an output of the rectifier 51 and another input terminal for receiving a voltage from anode between the load 34 and the cathode of the zener diode 41 through a resistor 52; and ON delay circuit 54 for receiving an AC output of the window comparator 53 and providing an output to an AC amplifier 31; a third transformer 55 for generating an AC output on a secondary winding thereof according to the input signal provided to a primary winding thereof; and a forth rectifier 56 for rectifying the AC output of the transformer 55 and providing a rectified output V 3 . A constant voltage Vcc is applied to anode between the anode of the zener diode 41 and the rectifier 56. The window comparator 53 may be the fail-safe AND oscillator explained above. The window comparator has upper and lower threshold values with respect to an input signal. The window comparator provides an AC output only when a voltage (potential Vx) at an intermediate point X between the load 34 and the zener diode 41 is within a range of "Vcc < Vx < = Vcc + Vz" and there is an input signal. The operation of this load driving circuit will be explained. To start the load, an input signal is supplied to the monitor circuit 50. The input signal is rectified by the rectifier 51, which provides a rectified output. The rectified output is supplied to one input terminal of the window comparator 53. The input signal is also supplied to the primary winding of the third transformer 55. The secondary winding of the transformer produces an AC output, which is rectified by the rectifier 56. The rectifier 56 provides the rectified output V 3 . When the zener diode 41 is normal, a voltage at the point X in Fig. 4 in the power supply circuit of the load becomes higher than Vcc, due to the rectified output V 3 . The voltage at the point X is supplied to the other input terminal of the window comparator 53. The window comparator 53 provides an AC output, which is delayed by the ON delay circuit 54 for a predetermined time after the generation of the input signal. The output signal of the ON delay circuit is supplied as a signal for driving the load 34, to the AC amplifier 31. The amplifier provides an amplified driving signal according to which the rectified outputs V 1 and V 2 are generated through transformers 32 and 39 and rectifiers 33 and 40, similar to the first embodiment. The outputs V 1 and V 2 overlap each other and are supplied to start the load 34. After a while, the rectified output V 2 disappears, and the steady operation of the load is maintained with the voltage V 1 that is lower than the start voltage. If the zener diode 41 is normal, the voltage at the point X will be Vcc + Vz during the operation of the load 34, so that the window comparator 53 continuously provides an output. When the input signal is stopped to stop the electricity to the load 34, the load 34 generates a counter-electromotive force that produces a discharge current. According to this embodiment, the power supply circuit of the load is opened to stop the load 34 by the zener diode 41 when the counter-electromotive force of the load 34 becomes lower than the zener voltage Vz. This arrangement further shortens a delay in stopping the operation of the load 34. Since the rectified output V 3 generated substantially at the same time as the reception of the input signal is lower than the operation stop voltage V OFF of the load 34, the rectified output V 3 will not start the load 34. Even if the constant voltage Vcc is applied to the load 34, the load 34 will not start if the resistor 52 has high resistance to cause only a fine current to flow to the load 34. An operation when the zener diode is out of order will be explained. When the zener diode 41 is short-circuited, a potential difference between ends of the zener diode 41 disappears, and the voltage at the point X becomes Vcc. As a result, an input to the window comparator 53 becomes lower than the power source voltage Vcc of the window comparator 53, to cause the window comparator 53 to provide no output. Accordingly, the rectified output V 1 will not be generated even if there is an input signal. The load 34, therefore, receives no voltage to maintain the operation thereof. As a result, the load 34 stops. If the zener diode 41 causes an open failure, the rectified output V 3 increases because the zener diode 41, which is usually connected, is open. As a result, the voltage at the point X exceeds the upper threshold value of the window comparator 53. Then, the window comparator 53 provides no output, to thereby stop the load 34. In this way, the operation of the load is stopped irrespective of an input signal, if the zener diode 41 becomes out of order. This results in securing fail-safe characteristics. Figure 5 shows another monitor circuit 50 for monitoring the zener diode 41. The resistance of a resistor 57 is set according to a current value that stops the operation of the load 34. An oscillator 58 is driving through the resistor 57. An output of the oscillator 58 is provided to a forth transformer 59. An output of the transformer is rectified by a sixth rectifier 60. An output of the rectifier 60 is added to a constant voltage Vcc, which is equal to a power source voltage Vcc of a window comparator 53. An added rectified output V 4 is supplied to the window comparator 53. Similar to the second embodiment, the oscillator 58 of this third embodiment provides no output if the zener diode 41 is short-circuited. In this case, the rectified output V 4 becomes equal to the constant voltage Vcc, so that the window comparator 53 provides no output. If the zener diode 41 causes an open failure, the voltage at a point X of Fig. 5 increases, so that the rectified output V 4 exceeds an upper threshold value of the window comparator 53. This results in stopping the output of the window comparator 53. In this way, this embodiment is also fail-safe because the operation of the load 34 is stopped against any failure in the zener diode 41. The invention provides a load driving circuit that produces a high voltage to start a load, and thereafter, a voltage lower than the start voltage, to maintain a steady-state operation of the load. This technique shortens a delay in stopping the load, the delay being caused by a counter-electromotive force generated by the load when the load is stopped. A zener diode may be inserted in a power supply circuit of the load, to further shorten the delay in stopping the load. The status of the zener diode is always monitored, and if the zener diode fails, the supply of power to the load is stopped to ensure fail-safe characteristics. 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 an inductive load (34) that shows hysteresis that an operation stop voltage of the load is lower than an operation start voltage of the load (34), the load driving circuit rectifying an AC signal prepared from a load driving AC instruction signal and supplying the rectified signal to the load (34) to thereby drive the load (34), characterized in that the load driving circuit comprising first output supply means (32, 33) for supplying a first rectified output (V 1 ) to the load (34) in response to the load driving instruction signal, the level of the first rectified output (V 1 ) being higher than the operation stop voltage and lower than the operation start voltage; and second output supply means (35, 36, 37, 38, 39, 40) for supplying a second rectified output (V 2 ) only for a predetermined period in response to the load driving instruction signal, the second rectified output (V 2 ) overlapping the first rectified output (V 1 ) and being supplied to the load (34), the level of the overlapping first and second rectified outputs (V 1 , V 2 ) being higher than the operation start voltage of the load. The load driving circuit according to claim 1, wherein the second output supply means (35-40) includes a second rectifier (35) for rectifying the amplified input signal; a differential circuit (36) having a predetermined time constant and differentiating the rectified output of the second rectifier (35); a fail-safe AND oscillator (37) for providing an AND of the differentiated output of the differential circuit (36) and providing no oscillation output if the AND oscillator (37) itself is out of order; a second amplifier (38) for amplifying the oscillation output of the AND oscillator (37); a second transformer (39) for generating an AC output from a secondary winding thereof according to the amplified output of the second amplifier (38) provided to a primary winding thereof; and a third rectifier (40) for rectifying the AC output of the second transformer (39) and providing the second rectified output (V 2 ) to the load. The load driving circuit according to claim 1, further comprising a zener diode (41) disposed in the power supply circuit of the load (34), oriented in a direction to block a discharge current due to a counter-electromotive force generated by the load (34) when the load driving instruction signal is stopped; and zener diode status monitor means (50) for monitoring whether or not the zener diode (41) is normal, and if it is abnormal, stopping the supply of the load driving instruction signal to the first output supply means (32, 33). The load driving circuit according to claim 3, wherein the monitor means (50) includes a fourth rectifier (51) for rectifying the load driving instruction signal; a fail-safe window comparator (53) having an input terminal for receiving a rectified output voltage from the fourth rectifier (51) and another input terminal for receiving a voltage from a node (X) between the load (34) and the cathode of the zener diode (41) in the power supply circuit of the load through a resistor (52), providing an output only when the rectified output voltage is present and the voltage from the power supply circuit of the load is within a predetermined range, and stopping the output if the window comparator (53) itself is out of order; an ON delay circuit (54) for providing an output to the first output supply circuit (32, 33) a predetermined delay time after receiving the output of the window comparator (53); a third transformer (55) for generating an AC output from a secondary winding thereof according to the load driving instruction signal provided to a primary winding thereof; and a fifth rectifier (56) for rectifying the AC output of the third transformer (55) and providing a third rectified output (V 3 ), which is lower than the operation stop voltage of the load, to a node (X) between the load (34) and the anode of the zener diode (41) in the power supply circuit, the same voltage as the power source voltage (V CC ) of the window comparator (53) being applied to a node between the anode of the zener diode (41) and the fifth rectifier (56). The load driving circuit according to claim 4, wherein the zener diode status monitor means includes, instead of applying the same voltage as the power source voltage of the window comparator (53) to the node between the anode of the zener diode (41) and the fifth rectifier (51), an oscillator (58) to be driven according to a terminal voltage of the zener diode (41); a fourth transformer (51) for generating an AC output from a secondary winding thereof according to the oscillation output of the oscillator (51) applied to a primary winding thereof; and a sixth rectifier (60) for rectifying the AC output of the fifth transformer (59), a rectified output of the sixth rectifier (60) being applied to the window comparator (53).

Inventors: FUTSUHARA KOICHI, SAKAI MASAYOSHI, FUTSUHARA, KOICHI, SAKAI, MASAYOSHI, Futsuhara, Koichi, The Nippon Sugnal Co. Ltd., Sakai, Masayoshi, The Nippon Signal Co. Ltd.
Applicants: NIPPON SIGNAL CO LTD, THE NIPPON SIGNAL CO. LTD.
Application year: 1993
Main classification: H01F7
IPC codes: H01F7, H01H47