Emergency stop circuit

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to emergency stop circuits, in various machines such as robots or machine tools, for stopping operations thereof in an emergency.

2. Description of the Related Art

In a robot system, a safety measure is taken by surrounding the robot operating range with a fence so as not to let a person come into the robot operating area within the fence. The fence is provided with a door or the like, and when the door is opened, an emergency stop signal is output so as to stop the operation of the robot. Further, when an operation is taught to the robot, a teaching pendant is controlled to operate the robot, so the teaching pendant is provided with an emergency stop command button or the like, whereby the operation of the robot is stopped in an emergency by inputting an emergency stop signal through the button (see, for example, Japanese Patent Application Laid-open No. 10-217180).

Further, machine tools, injection molders or the like are also so configured that when a door of a processing unit or the like is opened, an emergency stop signal is output so as to stop the operation of the machine. That is, driving of the motor for driving an operable unit of the machine is stopped in an emergency to thereby stop the operation of the machine.

Fig. 7 shows an example of an emergency stop circuit used for a robot system or the like. In order not to damage the safety when one element of the emergency stop circuit is failed, the emergency stop circuit of the robot system is configured to detect emergency stop factors through independent two systems of emergency stop lines, composed of components with contacts such as relays, respectively. The machine is so configured that through a safety relay circuit 13 connected with the emergency stop circuit, power supply contacts Ca and Cb are controlled so as to interrupt power supply to the servo motor 12 for driving the machine to thereby cause the machine to be in the emergency stop state.

There are various matters serving as factors for stopping machines in an emergency, depending on machines. They include an emergency stop button and a door switch. Fig. 7 shows two emergency stop factors 14 and 15. The emergency stop factor 14 interrupts power supply to the relays R1a and R1b for the two systems of emergency stop lines A and B when the emergency stop button is manipulated. On the other hand, the emergency stop factor 15 opens a contact thereof by a relay, not shown, so as to stop power supply to the relays R2a and R2b. These relays or the like are provided as many as emergency stop factors. In Fig. 7, two emergency stop factors 14 and 15 are shown as examples.

In each of the two systems of the emergency stop lines A and B, normally-open contacts of the relays for respective emergency stop factors are connected in series. In the example shown in Fig. 7, on the line A, a normally-open contact r1a of the relay R1a, a normally-open contact r2a of the relay R2a, a normally-open contact r3a of a relay R3a operable by a command from the CPU 10, a normally-open contact k1a of a safety relay K1 in the safety relay circuit 13, and a safety relay K2 are connected in series, and a voltage is applied to either end of the series circuit. A normally-open contact k2a of the safety relay K2 is connected in parallel with the normally-open contact k1a of the safety relay K1.

Similarly, on the line B of the other system, a normally-open contact r1b of the relay R1b, a normally-open contact r2b of the relay R2b, a normally-open contact r3b of a relay R3b operable by a command from the CPU 10, a normally-open contact k1b of a safety relay K1 on the safety relay circuit 13, and a safety relay K3 are connected in series, and a voltage is applied to either end of the series circuit. A normally-open contact k3a of the safety relay K3 is connected in parallel with the normally-open contact k1b of the safety relay K1.

Relating to the contactor Ca, a normally-close contact k1c of the safety relay K1, a normally-open contact k2c of the safety relay K2, and a normally-open contact k3c of the safety relay K3 are connected in series, and a voltage is applied to the series circuit. Similarly, relating to the contactor Cb, a normally-close contact k1d of the safety relay K1, a normally-open contact k2d of the safety relay K2, and a normally-open contact k3d of the safety relay K3 are connected in series, and a voltage is applied to the series circuit.

The servo amplifier 11 is connected with a three-phase power source via contacts Ca1 and Cb1; Ca2 and Cb2; and Ca3 and Cb3, which are connected in series for respective phases. The contacts ca1, ca2 and ca3 are normally-open contacts, for respective phases, of the contactor Ca, and the contacts cb1, cb2 and cb3 are normally-open contacts, for respective phases, of the contactor Cb. Further, the normally-close contacts ca4 and cb4 of the contactors Ca and Cb, the normally-close contacts k2b and k3b of the safety relays K2 and K3, and the safety relay K1 are connected in series, and a voltage is applied to either end of the series circuit.

In Fig. 7, "DI" indicates a digital input element, and "DO" indicates a digital output element. The digital input elements DI constitutes a detecting means for detecting the states of respective contacts of the relays R1a to R3a and R1b to R3b operable by the emergency stop factors 14 and 15 and commands from the CPU 10.

At the time of power being supplied, the safety relay K1 operates to close the normally-open contacts k1a and k1b thereof, and to open the normally-close contacts k1c and k1d. If no emergency stop command is inputted from an emergency stop factor, the contacts r1a to r3a on the line A and the contacts r1b to r3b on the line B are closed so that the safety relays K2 and K3 are excited, and the safety relays K2 and K3 are self held via the contacts k2a and k3a. Due to the safety relays K2 and K3 being excited, the normally-close contacts k2b and k3b are opened, so that the safety relay K1 is non-excited. Thereby, the contact k1c to k3c are closed, and the contactor Ca is excited. Similarly, the contact k1d to k3d are closed, and the contactor Cb is excited. Consequently, the contacts of the contactors Ca and Cb are closed, so that the power is supplied to the servo amplifier 11 from the power source, whereby the servo motor 12 becomes operable.

If an emergency stop command is inputted due to any one of the emergency stop factors 14 and 15, or an emergency stop command is outputted from the CPU 10, and the contacts r1a to r3a or the contacts r1b to rb3 on either emergency stop line A or B are opened, the safety relay K2 and/or the safety relay K3 is non-excited, whereby the contactors k2c, k2d, k3c and k3d are opened and the contactors Ca and Cb are non-excited, whereby the contacts ca1 to ca3 and cb1 to cb3 are opened to thereby interrupt power supply to the servo motor 12. Consequently, operation of the servomotor 12 is stopped, and the machine is stopped in an emergency.

The conventional emergency stop circuit uses a safety relay circuit composed of safety relays in which operations of the contacts are assured, whereby specially-designed, expensive components must be used. Further, the circuit is complicated and a number of general components must be used as well. This causes an adverse effect on the cost and reliability.

SUMMARY OF THE INVENTION

An emergency stop circuit according to the present invention comprises: two emergency stop lines, each of which is connected with a contactor; and a power supply circuit for supplying power to a motor for driving a machine from a power source via series circuits composed of contacts of respective contactors. In each emergency stop line, a contact which is opened when an emergency stop command signal is inputted from an emergency stop factor, and a contact which is opened by a command from a CPU provided to each emergency stop line, are connected in series to thereby connect the contactor with the power source. The states of these contacts are detected by a detecting means.

In one mode of the emergency stop circuit of the present invention, each CPU outputs a command to open a contact on the self emergency stop line when information about the states of the contacts on the self emergency stop line, detected by the detecting means, and information about the states of the contacts on the other emergency stop line, transmitted from the other CPU, do not coincide with each other.

In a second mode of the emergency stop circuit of the present invention, each CPU determines whether the states of the contacts on the self emergency stop line, detected by the detecting means, are normal to conduct the contactor, and if they are not normal, the CPU transmits an abnormality signal to the other CPU, and the CPU which receives the abnormality signal outputs a command to open a contact on the self emergency stop line.

A CPU, determining that the states of the contacts of the self emergency stop line are not normal to conduct the contactor, may also outputs a command to a contact on the self emergency stop line to open the contact.

The first and second modes of the emergency stop circuit according to the present invention can take the following aspects.

The CPUs transmit and receive a watchdog signal between them so as to check, with each other, whether an operation of the other CPU is normal, and when either CPU detects an abnormal operation of the other CPU through the check, the CPU outputs a command to open a contact on the self emergency stop line to thereby open the self emergency stop line.

Each contactor is provided with a detecting contact operable with the contacts of the contactor so as to detect contact states thereof, and a contact state detecting means for detecting a contact state of the detecting contact, and detected information from the contact state detecting means is included in the states of the contacts of the emergency stop line.

An additional CPU is provided besides the respective CPUs corresponding to the respective emergency stop lines, and contacts, which are opened by a command from the additional CPU, are provided on respective emergency stop lines. The respective CPUs corresponding to the respective emergency stop lines and the additional CPU transmit and receive watchdog signals between them so as to check whether operations of the CPUs are normal. When the additional CPU detects an abnormal operation in either of the CPUs corresponding to the respective emergency stop lines, the additional CPU outputs at least a command to open a contact provided on the emergency stop line of the CPU in which the abnormal operation is detected.

An additional CPU is provided besides the respective CPUs corresponding to the respective emergency stop lines. The respective CPUs corresponding to the respective emergency stop lines and the additional CPU transmit and receive watchdog signals between them so as to check whether operations of the CPUs are normal. When the additional CPU detects an abnormal operation in either of the CPUs corresponding to the respective emergency stop lines, the additional CPU outputs at least an emergency stop command to a CPU in which the abnormal operation is not detected, and the CPU receiving the emergency stop command outputs a command to open a contact provided on the emergency stop line.

The emergency stop circuit of the present invention does not require a safety relay circuit composed of expensive safety relays with contact operation assurance. Further, an emergency stop is performed by outputting emergency stop commands doubly by the hardware and the software, whereby the emergency stop can be performed more securely. Moreover, a CPU, on the emergency stop line in which an abnormality is detected, transmits an abnormality signal to the other CPU on the other emergency stop line so as to open the emergency stop line on the other CPU side, whereby the emergency stop can be performed more securely.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention will become apparent from the following description of embodiments with reference to the accompanying drawings, in which: <ul id="ul0001" list-style="none" compact="compact"><li>Fig. 1 is a circuit diagram showing a first embodiment of an emergency stop circuit according to the present invention;</li><li>Fig. 2 is a flowchart showing emergency stop processing performed by a first CPU in the emergency stop circuit shown in Fig. 1;</li><li>Fig. 3 shows a variation of the emergency stop processing shown in Fig. 2;</li><li>Fig. 4 is a flowchart showing emergency stop processing, using a watchdog signal, performed by the first CPU in the emergency stop circuit shown in Fig. 1;</li><li>Fig. 5 is a flowchart showing emergency stop processing, using a watchdog signal, performed by a second CPU in the emergency stop circuit shown in Fig. 1;</li><li>Fig. 6 is a circuit diagram showing a second embodiment of an emergency stop circuit according to the present invention; and</li><li>Fig. 7 is a circuit diagram for a conventional emergency stop.</li></ul>

DESCRIPTION OF THE EMBODIMENTS

Fig. 1 is a circuit diagram of a first embodiment of an emergency stop circuit according to the present invention, the circuit being applied to a driving motor in a robot, a machine tool or various industrial machinery.

Comparing with the conventional emergency stop circuit shown in Fig. 7, the present embodiment is characterized in that the safety relay circuit 13 is omitted while another CPU is added so as to have two CPUs (a first CPU 10a and a second CPU 10b).

In the emergency stop circuit of Fig. 1, emergency stop factors are detected by independent two systems of circuits, as same as the emergency stop circuit of Fig. 7. The circuit has relays, and contacts to stop power supply to the relays, for the two systems of the emergency stop lines A and B, respectively. The emergency stop factors and the number thereof are different depending on machines to apply. In the example shown in Fig. 1, two emergency stop factors 14 and 15 are indicated. In this example, power supply to relays R1a, R1b, R2a and R2b is interrupted by a push button for one emergency stop factor 14, or by relay contacts for the other emergency stop factor 15, whereby an emergency stop command is transmitted.

On the line A, a contact r1a of the relay R1a for the emergency stop factor 14, a contact r2a of the relay R2a for the emergency stop factor 15, a contact r3a of a relay R3a operable by a command from the first CPU 10a, and a contactor Ca are connected in series, and a voltage is applied to either end of the series circuit. Similarly, on the line B, a contact r1b of the relay R1b for the emergency stop factor 14, a contact r2b of the relay R2b for the emergency stop factor 15, a contact r3b of a relay R3b operable by a command from the second CPU 10b, and a contactor Cb are connected in series, and a voltage is applied to either end of the series circuit.

Further, the line A includes digital input elements DIla to DI3a constituting a detection means with which the first CPU 10a detects the states of the contacts r1a to r3a. Similarly, the line B includes digital input elements DI1b to DI3b constituting a detection means with which the second CPU 10b detects the states of the contacts r1b to r3b.

On the line A, when each contact r1a to r3a is closed, a high level is detected from each digital input element DIla to DI3a. When the contact r1a is closed but the contact r2a is opened, a high level ("1") is detected from the digital input element DIla for the contact r1a, and a low level ("0") is detected from the digital input element DI2a for the contact r2a. Similarly, on the line B, when each contact r1b to r3b is closed, a high level ("1") is detected from each digital input element DI1b to DI3b.

Reference numerals DOa and DOb indicate digital output elements. The first CPU 10a and the second CPU 10b drive the relays R3a and R3b via the digital output elements Doa and Dob, respectively.

A servo amplifier 11 for driving a servo motor 12 connects with a three-phase power source via contacts ca1, cb1; ca2, cb2; and ca3, cb3, connected in series for each phase. The contacts ca1, ca2 and ca3 are normally-open contacts for respective phases of the contactor Ca, and the contacts cb1, cb2 and cb3 are normally-open contacts for respective phases of the contactor Cb. One end of a normally-close contact ca4 of the contactor Ca connects with a direct-current power source, and the other end thereof connects with a digital input element DIca, whereby the first CPU 10a monitors the states of the contacts of the contactor Ca. Similarly, one end of a normally-close contact cb4 of the contactor Cb connects with the direct-current power source, and the other end thereof connects with a digital input element DIcb, whereby the second CPU 10b monitors the states of the contacts of the contactor Cb.

When the power is supplied, the relays R1a, R1b, R2a and R2b for the emergency stop factors 14 and 15 are excited, and the normally-open contacts r1a, r1b, r2a and r2b thereof are closed. Further, since no emergency stop command is output from the first CPU 10a or the second CPU 10b, the relays R3a and R3b are excited, and the contacts r3a and r3b thereof are closed. Consequently, the contactors Ca and Cb are excited so as to close the normally-open contacts ca1 to ca3 and cb1 to cb3 thereof, whereby the power is supplied to the servo amplifier 11 so that the servo motor 12 is in the operable state. In this normal operable state, the first CPU 10a receives signals of "1, 1, 1, 0" from the digital input elements DI1a, DI2a, DI3a, and DIca. Similarly, the second CPU 10b receives signals of "1, 1, 1, 0" from the digital input elements DI1b, DI2b, DI3b, and DIcb.

Now, if any emergency stop factor is operated, for example, if the emergency stop factor 15 is operated, the relays R2a and R2b thereof are operated and the respective contacts r2a and r2b thereof are opened, so that the power supply to the contactors Ca and Cb stops. Thereby, the contactors Ca and Cb stop their operations and open the normally-open contacts ca1 to ca3 and cb1 to cb3 so as to stop the power supply to the servo amplifier 11. Here, even if one relay of the emergency stop factor 15 or one of the contactors Ca and Cb is failed, it is possible to stop the power supply to the servo motor 12 and to perform an emergency stop securely, if the other relay or contactor works normally. For example, even in a case where the relay R2a is failed and the contact r2a thereof is not opened, the relay R2b operates to cause the contactor Cb to be non-excited, so that the normally-open contacts cb1, cb2 and cb3 are opened. Thereby, the power supply to the servomotor is stopped securely. Similarly, if the contactor Cb is operationally failed, for example, the contactor Ca operates to interrupt the power supply to the servomotor 12.

As described above, by the relays operated by the emergency stop factors, the power supply to the servomotor 12 is stopped so as to perform an emergency stop securely by the dual-system hardware. Further, in the present embodiment, two CPUs, that is, the first CPU 10a and the second CPU 10b, execute an emergency stop by software, which provides a more secured emergency stop.

As methods for performing an emergency stop by software, the present embodiment uses two methods. One is a method in which emergency stop processing is performed when the operational states of respective contacts on respective emergency stop lines A and B, inputted from the digital input elements, do not coincide with each other. The other one is a method in which a watchdog signal is exchanged so as to check whether each CPU works normally, and if either CPU does not work normally, emergency stop processing is also performed by the other CPU.

Fig. 2 is a flowchart showing processing in which the first CPU 10a in Fig. 1 monitors the operational states of the contacts so as to detect unconformity in the operational states of the respective contacts on the emergency stop lines A and B to thereby perform emergency stop processing. The first CPU 10a performs this processing in prescribed cycles.

First, the first CPU 10a reads signals from the digital input elements DI1a, DI2a, DI3a and DIca, constituting the detecting means for detecting the contact states, of the line A (Step a1), and transmits information indicating the contact states to the second CPU 10b (Step a2). Further, the first CPU 10a receives information indicating the contact states of the line B, detected by the digital input elements DI1b, DI2b, DI3b and DIcb and transmitted from the second CPU 10b (Step a3), and determines whether the contact states of the line A and the contact states of the line B coincide with each other (Step a4). If they coincide, the first CPU 10a ends the processing here.

On the other hand, if the contact states of the line A and the contact states of the line B do not coincide with each other, the first CPU 10a outputs an emergency stop signal. The first CPU 10a outputs an emergency stop signal of the line A of itself so as to cause the relay R3a to be non-excited via the digital output element DOa to thereby open the contact r3a thereof (Step a5). When the contact r3a is opened, the power supply to the contactor Ca is stopped, causing the contactor Ca to be non-excited. Thereby, the normally-open contact ca1 to ca3 are opened and the power supply to the servo amplifier 11 is interrupted, so that the operation of the servomotor 12 is stopped.

The second CPU 10b also performs processing similar to that shown in Fig. 2. In the similar processing of Steps a1 and a2, the second CPU 10b reads signals from the digital input elements DI1b, DI2b, DI3b and DIcb, and transmits them to the first CPU 10a. When the information indicating the operational states of the contacts transmitted from the first CPU 10a and the information indicating the operational states of the contacts read out by the second CPU 10b do not coincide with each other, the second CPU 10b causes the relay R3b to be non-excited so as to open the contact r3b thereof to thereby stop the operation of the servo motor 12. In this way, when the operational states of the contacts detected by the first CPU 10a and those detected by the second CPU 10b do not coincide, the contacts r3a and r3b are caused to be opened so as to cause the contactors Ca and Cb to be non-excited, whereby an operation to stop the operation of the servo motor 12 is performed.

Fig. 3 is a flowchart showing a processing method, other than the one shown in Fig. 2, for performing an emergency stop on the basis of operational states of respective contacts of the emergency stop lines A and B, inputted from the digital input elements. In this method, the operational states of respective contacts inputted from the digital input elements are determined to be normal or not, and if not, an emergency stop signal is transmitted to the other CPU so as to cause emergency stop processing to be performed by the other CPU as well.

First, the first CPU 10a reads signals from the digital input elements DI1a, DI2a, DI3a and DIca constituting the detecting means for detecting the contact states (Step a'1), and determines whether or not the output pattern of the digital input elements DI1a, DI2a, DI3a and DIca shows "1, 1, 1, 0" which indicates the normal state (Step a'2). If the output pattern shows the normal state, the first CPU 10a transmits a normal state signal to the second CPU 10b (Step a'3). Further, the first CPU 10a reads signals transmitted from the second CPU 10b (Step a'4), and determines whether the signals read show the normal state (Step a'5), and if they show the normal state, ends the processing of this cycle. If, on the other hand, the first CPU 10a determines that the output pattern of the normal state cannot be read in Step a'2, the first CPU 10a transmits an abnormality signal to the second CPU 10b (Step a'7), and outputs an emergency stop signal so as to cause the relay R3a to be non-excited via the digital output element DOa to thereby open the contact r3a thereof (Step a'6). Further, if the first CPU 10a receives an abnormality signal transmitted from the second CPU 10b (Step a'5), the first CPU 10a also outputs an emergency stop signal so as to cause the relay R3a to be non-excited to thereby open the contact r3a thereof. With the contact r3a being opened, the power supply to the contactor Ca is stopped, so that the contact Ca is to be non-excited, and the normally-open contacts ca1 to ca3 thereof are opened to thereby interrupt the power supply to the servo amplifier 11 and stop the operation of the servo motor 12.

In other words, the first CPU 10a outputs an emergency stop signal when a signal pattern of the contact states detected from the line A of itself is abnormal and also when a signal pattern of the contact states in the other line B, transmitted from the second CPU 10b, is abnormal, so as to cause the contactor Ca to be non-excited to thereby stop the power supply to the servo amplifier 11.

The second CPU 10b also performs processing similar to that shown in Fig. 3. The second CPU 10b performs processing similar to that of the CPU 10a except that, in the processing of Steps a'1 and a'2, the second CPU 10b reads signals from the digital input elements DI1b, DI2b, DI3b and DIcb, and determines whether the output pattern of the digital input elements DI1b, DI2b, DI3b and the DIcb is "1, 1, 1, 0" which shows the normal state, and that, in Step a6, the second CPU 10b outputs an emergency stop command to the digital output DOb to thereby cause the relay R3b to be non-excited.

As described above, when a pattern of the contact state signals is abnormal, the contactor of the line of itself is caused to be non-excited and also caused the other contactor to be non-excited. With both of the two contactors being non-excited, an emergency stop can be performed further securely. Note that even in this case, an emergency stop command may be output only when an abnormality signal is transmitted from the other CPU (Steps a'4, a'5 and a'6).

If one emergency stop element is failed in each of the two emergency stop lines A and B in the emergency stop circuit, for example, when the contactor Ca is failed in the emergency stop line A whereby the contacts ca1 to ca3 cannot be opened, and further the relay R1b is failed in the emergency stop line B whereby the contact r1b cannot be opened, the servo motor cannot be stopped if the emergency stop means consists solely of hardware such as relays. That is, although the relay contact r1a is opened when an emergency stop signal due to the emergency stop factor 14 is inputted and the power supply to the relays R1a and R1b is released, the contacts ca1 to ca3 are not opened due to the failure of the contactor Ca, and further the relay contact r1b is not opened, whereby the contactor Cb is in the excited state, so that the contacts cb1 to cb3 are remained to be closed.

However, according to the present embodiment, when the relay r1a is opened, a pattern detected by the detecting means of the digital input elements DI1a, DI2a, DI3a and DIca of the first CPU 10a becomes "0, 0, 0, 0", which is different from the pattern "1, 1, 1, 0" showing the normal state. In the method shown in Fig. 3, the first CPU 10a detects the abnormality and transmits an abnormality signal to the second CPU 10b. Upon receipt of the abnormality signal, the second CPU 10b causes the relay R3b to be non-excited to thereby open the contact r3b thereof. Consequently, the contactor Cb working normally is caused to be non-excited so as to open the contacts cb1 to cb3 thereof to thereby stop the power supply to the servo motor 12 and perform an emergency stop.

Further, according to the method shown in Fig. 2, a pattern detected by the detecting means of the digital input elements DI1b, DI2b DI3b and DIcb on the side of the second CPU 10b is "1, 1, 1, 0" showing the normal state, so the contact states do not coincide with each other. Thereby, an emergency stop signal is outputted from each of the first CPU 10a and the second CPU 10b so as to cause the relays R3a and R3b to be non-excited to thereby open the contacts r3a and r3b and stop conducting to the contactors Ca and Cb. This enables to cause the contactor Cb working normally to be non-excited.

Next, an explanation will be given for emergency stop processing performed based on a watchdog signal. Figs. 4 and 5 show an example, among others, of emergency stop processing based on a watchdog signal. Fig. 4 shows processing performed by one CPU (the first CPU 10a) to cause an emergency stop by a watchdog signal, and Fig. 5 shows processing performed by the other CPU (the second CPU 10b). These two CPUs perform the processing in synchronization.

The first CPU 10a performs processing shown in Fig. 4 every prescribed cycles, and determines whether the first CPU 10a itself operates normally (Step b1). If it operates normally, the first CPU 10a outputs a watchdog signal WDS to the second CPU 10b, and resets a timer T and starts it (Steps b2, b3). If the watchdog signal WDS is sent back from the second CPU 10b before the timer T completes timing (Steps b4, b5), the first CPU10a ends the processing as no abnormality is found.

On the other hand, if the first CPU 10a determines that the operation of itself is abnormal in Step b1, or if the timer T completes timing before the first CPU 10a receives the watchdog signal WDS, the first CPU 10a outputs an emergency stop command to the digital output DOa (Step b6) so as to cause the relay R3a to be non-excited to thereby open the contact r3a thereof, and to cause the contactor Ca to be non-excited to thereby open the contactors ca1 to ca3 thereof, and to interrupt the power supply to the servo motor 12 and perform an emergency stop.

The second CPU 10b performs the processing shown in Fig. 5 in synchronization with the performing cycles of the processing in Fig. 4 performed by the first CPU 10a. First, the second CPU 10b determines whether the second CPU 10b itself operates normally (Step c1), and if it operates normally, the second CPU 10b resets a timer T and starts it (Step c2, c3). If the second CPU 10b receives a watchdog signal WDS from the first CPU 10a before the timer T completes timing (Step c4), it sends the watchdog signal WDS back to the first CPU 10a (Step c5), and ends the processing of this processing cycle. On the other hand, if the second CPU 10b determines that the operation of itself is abnormal in Step c1, or the timer T completes the timing before the second CPU 10b receives the watchdog signal WDS, the second CPU 10b outputs an emergency stop command to the digital output DOb (Step c6) so as to cause the relay R3b to be non-excited to thereby open the contact r3b thereof, and to cause the contactor Cb to be non-excited to thereby open the contacts cb1-cb3 thereof, and to interrupt power supply to the servo motor and perform an emergency stop.

As described above, when one of the two CPUs does not operate normally, the contactor of the line on the side of the CPU is caused to be non-excited to thereby interrupt power supply to the servo amplifier 11, while a watchdog signal WDS is not sent to the other CPU. Thereby, the other CPU detects the fact that it does not receive the watchdog signal, and causes the contactor of itself to be non-excited to thereby interrupt power supply to the servo amplifier 11. In this way, an emergency stop is performed securely.

In the aforementioned embodiment, if one CPU does not operate normally, the relay R3a or R3b of the line on the side of the CPU is caused to be non-excited and the contactor is also caused to be non-excited to thereby interrupt power supply to the servo amplifier 11. However, since this CPU does not operate normally, it may be acceptable to perform only operation to cause the relay R3a or R3b of the line on the side of the other CPU to be non-excited and to cause the contactor to be non-excited.

The aforementioned first embodiment is provided with two systems for performing an emergency stop, whereby even when the hardware such as a relay in one system is abnormal, an emergency stop can be performed by the hardware such as a relay in the other system. Further, since the first embodiment uses commands from the CPUs, if an abnormality is detected in one system, an emergency stop command is outputted from the CPU of the system, while an emergency stop command is also outputted from the CPU of the other system, whereby an emergency stop can be performed further securely.

Fig. 6 is an emergency stop circuit diagram according to a second embodiment of the present invention. The second embodiment is characterized in that a third CPU 10c is added to the first embodiment shown in Fig. 1. In this embodiment, normally-open contacts r4a and r4b of relays R4a and R4b driven by the third CPU 10c via digital output elements DO2a and DO2b are added to the lines A and B of the respective systems, and are connected in series with respective contacts of the relays for the emergency stop factors.

In the second embodiment, the only difference from the first embodiment is that watchdog signals are transmitted and received between the first CPU 10a and the third CPU 10c, and between the second CPU 10b and the third CPU 10c to thereby detect abnormal operations in the first CPU 10a and the second CPU 10b. When an abnormal operation is detected in either the first CPU 10a or the second CPU 10b, the relay R4a or R4b is caused to be non-excited so as to open the normally-open contact r4a or r4b to thereby perform an emergency stop.

That is, the third CPU 10c sends watchdog signals WDS to the first CPU 10a and to the second CPU 10b, and if the first or the second CPUs 10a or 10b does not sent the watchdog signal WDS back to itself (the third CPU 10c), the third CPU 10c causes the relay R4a or R4b to be non-excited so as to open the normally-open contacts r4a or r4b to thereby perform an emergency stop.

The third CPU 10c performs processing similar to that of Steps b2 to b6 in Fig. 4 every prescribed cycles. The third CPU 10b sends watchdog signals WDS to the first CPU 10a and to the second CPU 10b, and if the watchdog signals WDS are sent back to the third CPU 10c from the first CPU 10a and from the second CPU 10b, respectively, before the timer T completes timing, the third CPU 10b ends the processing of the present cycle. On the other hand, if the watchdog signals WDS are not sent back to the third CPU 10c before the timer T completes timing, the third CPU 10c outputs an emergency stop command to the digital output elements DO2a and DO2b so as to cause the relays R4a and R4b to be non-excited to thereby open the normally-open contacts r4a and r4b and to perform an emergency stop.

On the sides of the first CPU 10a and the second CPU 10b, they perform processing similar to the processing shown in Fig. 5 except Steps c2 and c3. If the first CPU 10a and the second CPU 10b operate normally and they receive watchdog signals from the third CPU 10c within the prescribed time period, they send the watchdog signals WDS back to the third CPU 10c. On the other hand, if the first CPU 10a and the second CPU 10b do not operate normally, so that they do not send the watchdog signals WDS back to the third CPU 10c within the prescribed time period, the third CPU 10c outputs an emergency stop command to the relays R4a and R4b to thereby perform an emergency stop.

Although, in the second embodiment, the digital output elements DO2a and DO2b and the relays R4a and R4b are provided, it may be acceptable that these digital output elements and the relays are not to be provided, and the third CPU 10c transmits an emergency stop command to the first CPU 10a and to the second CPU 10b as shown by the dashed lines in Fig. 6, and the first and second CPUs 10a and 10b, when received the emergency stop command, cause the contacts r3a and r3b of the relays R3a and R3b in their systems to be opened. Further, although, in Fig. 6, a watchdog signal WDS is also transmitted and received between the first CPU 10a and the second CPU 10b, abnormal operations in the first CPU 10a and the second CPU 10b can be detected due to the transmission and reception of the watchdog signals WDS performed between the first CPU 10a and the third CPU 10c and between the second CPU 10b and the third CPU 10c. Therefore, a detection of abnormality through the transmission and reception of the watchdog signal WDS between the first CPU 10a and the second CPU 10b may not be performed. However, if this detection of abnormality is performed, the emergency stop operation becomes more accurate.