A multiple-optical-axis photoelectric sensor is a sensor in which a projector and an optical receiver having a plurality of optical elements are arranged to face each other to set a detection area configured by a plurality of optical axes. This sensor executes a detection process by a method that sequentially enables the optical axes to check the light incidence state of each of the optical axes. In a multiple-optical-axis photoelectric sensor used for safety, an output is set in an on state (high level) while the detection process determines that the detection area is not light-shielded, and the output is turned off when the detection area is light-shielded.
The projector and the optical receiver of the multiple-optical-axis photoelectric sensor, in the above detection process, communicate with each other to synchronize light projection/reception, and can also communicate with a projector and an optical receiver of another sensor. There is some system that is configured such that, by using the communication function, projectors and optical receivers of a plurality of sensors are connected to each other through a communication line to cause detection processes in the respective sensors to sequentially proceed on the basis of communication between the sensors.
As a concrete conventional technique, Patent Document 1 describes that a sensor at one terminal of a plurality of connected sensors is used as a master sensor, after the master sensor executes the first detection process, an instruction to start detect is transmitted to a sensor having an order lower than that of the master sensor by one, and the sensors sequentially perform the same operations to cause the detection processes to sequentially proceed from an upper order to a lower order.
Patent Document 1 also describes that the projectors and the optical receivers of the sensors exchange signals between upper-order and lower-order devices on activation and determine whether their own devices are master devices or slave devices on the basis of the presence/absence of inputs of the signals.
FIG. 8 shows a sequence of a conventional detection process similar to the method described in Patent Document 1. In this example, in a system including a sensor Sa including a projector 1a and an optical receiver 2a, a sensor Sb including a projector 1b and an optical receiver 2b, and a sensor Sc including a projector 1c and an optical receiver 2c that are connected to each other in a state in which the projectors and the optical receivers can communicate with each other, communication to transfer detection processes are performed between the optical receivers 2a, 2b, and 2c of the sensors Sa, Sb, and Sc (this communication will be called “inter-sensor communication” hereinafter). Also between the optical receivers and the projectors configuring each of the sensors Sa, Sb, and Sc, communication to synchronize operation timings of detection processes is performed (this communication will be called “in-sensor communication” hereinafter).
More specifically, a detection process is executed after the sensor Sa executes the in-sensor communication and instructs the second sensor Sb to start a detection process. After the sensor Sb that receives the instruction returns a response signal to the sensor Sa, the sensor Sb executes in-sensor communication and a detection process to instruct the third sensor Sc to start a detection process. After the sensor Sc that receives the instruction returns a response signal to the sensor Sb, the sensor Sc executes in-sensor communication and a detection process to instruct the third sensor Sa to start a detection process. The processes executed up to now are set as one cycle, and the same sequence is repeated to circulate the detection processes by the sensors Sa, Sb, and Sc.