Abstract:
A module with isolated analog inputs for a programmable controller, the module including plural input pathways that are each isolated with aid of at least one optical isolation static relay controlled selectively with the aid of a controller. The module further includes a multiplexer including plural channels each controlled to open or to close, a channel of the multiplexer being placed in series with an isolated input pathway of the module.

Description:
FIELD OF THE INVENTION 
     The present invention pertains to a module with analogue inputs that is used in a programmable controller. 
     BACKGROUND 
     A programmable controller or PLC (“Programmable Logical Controller”) is an automatic control facility capable of driving, controlling and/or monitoring one or more processes to be controlled. 
     Of generally modular design, a PLC programmable controller is composed of various modules which intercommunicate through a transmission bus, called a “backplane” bus in this field. The modules are fixed mechanically in a rack, which comprises a printed circuit which also supports the backplane bus as well as the connection elements intended to cooperate with connectors generally present on the rear part of the modules so as to effect the necessary link between the modules and the bus. The number of modules depends of course on the size and the type of process to be automated. 
     Typically, a programmable controller can comprise:
         A power supply module providing the various voltages to the other modules through the backplane bus.   A central unit module UC which comprises embedded software (“firmware”) integrating a real-time operating system OS, and an application program, or user program, containing the instructions to be executed by the embedded software to perform the desired control operations. The UC module also generally comprises a connection on the front face to programming tools of personal computer PC type.   Input/output I/O modules of various types as a function of the process to be controlled, such as digital I/Os or analogue TORs for counting, etc. These I/O modules are linked to sensors and actuators participating in the automated management of the process.   One or more modules for communicating with communication networks (Ethernet, CAN, etc.) or man-machine interfaces (screen, keyboard, etc.).       

     By way of example, an input/output module can comprise between 1 and 32 I/O pathways, a PLC controller that may be capable, depending on the model, of managing several hundred I/O pathways. If required, several racks are therefore connected together in one and the same PLC. Thus, as a function of the application and the process to be automated, a PLC controller can comprise a large number of modules. 
     Currently, analogue input modules comprise several pathways isolated from one another with the aid of optical isolation static relays also called “OptoMos” (trademark). Each pathway of the module is controlled by two static relays, one of the poles of whose MOSs is common. These components have an almost unlimited lifetime but each exhibit a not inconsiderable leakage current. Thus, when the module comprises more than four pathways, the unit leakage currents of each of the static relays add together thus giving rise to consequent errors in the measurements. 
     SUMMARY 
     The aim of the invention is to propose a module with analogue inputs comprising several pathways isolated from one another, in which the leakage currents are particularly reduced so as to always be able to guarantee the precision of the measurements. 
     This aim is achieved by a module with isolated analogue inputs for programmable controller, the said module comprising several input pathways that are each isolated from one another with the aid of at least one optical isolation static relay controlled selectively with the aid of control means, the said module being characterized in that it comprises a multiplexer furnished with several channels each controlled to open or to close, a channel of the multiplexer being placed in series with an isolated input pathway of the module. 
     According to one feature, each input pathway is isolated with the aid of two optical isolation static relays arranged in series. 
     According to another feature, the multiplexer is of low-voltage analogue type. 
     According to another feature, the multiplexer comprises control means intended to selectively control each of its channels. 
     According to another feature, the control means of the multiplexer are coupled to the control means of the optical isolation static relays. 
     According to another feature, the multiplexer comprises eight channels. It comprises at least as many channels as pathways in the module. If it comprises more channels than pathways in the module, some of its channels can be employed for the autocalibration of the measurement chain. 
     The module according to the invention is adapted to be employed in a modular programmable controller. 
     Other characteristics and advantages will appear in the detailed description which follows while referring to an embodiment given by way of example and represented by the appended drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically represents the structure of the invention. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     A module with analogue inputs comprises several pathways  1 ,  2 ,  3 ,  4  isolated with the aid of optical isolation static relays, also called “OptoMos” (trademark). This type of component is in particular described in U.S. Pat. No. 4,390,790. More precisely, this type of optical isolation static relay  1   a ,  1   b ,  2   a ,  2   b ,  3   a ,  3   b ,  4   a ,  4   b  (hereinafter  1   a - 4   b ) comprises an input circuit composed of a light-emitting diode able to convert an input control signal (i in  FIG. 1 ) into infrared light. The light-emitting diode is optically coupled to a conversion circuit formed of an assembly of photovoltaic cells and of associated drive circuits. The photovoltaic cell generates the voltage necessary for the control of a MOSFET circuit (represented in a simplified manner by a simple breaker) which switches the output load. 
     With reference to  FIG. 1 , in a module with isolated analogue inputs, each pathway comprises two optical isolation static relays (hereinafter static relay). For each input pathway, the two static relays are linked in series by their input circuit. The inputs of the MOSFET circuits are linked to a terminal block which defines hardware-wise the various pathways to which the sensors will connect. Control means  6  make it possible to selectively control each input pathway  1 - 4  by dispatching the control signal into the input circuits of the two static relays in series. The optical isolation static relays support voltages of possibly as much as 600 volts for switching times of less than 1 ms. 
     The optical isolation static relays  1   a - 4   b  make it possible to isolate the pathways from one another. For the unselected input pathways, these static relays each allow through a not inconsiderable leakage current. The addition of these leakage currents gives rise to errors in the measurements which may turn out to be crippling when the number of pathways is greater than four. 
     According to the invention, a low-voltage analogue multiplexer  5  (DC supply voltage lying between +30 VDC and −30 VDC approximately) is therefore placed between the analogue-digital converter ADC and the static relays  1   a - 4   b . This multiplexer  5  exhibits a number of channels that is at least equal to the number of pathways of the input module. In  FIG. 1 , each channel  5   a - 5   h  of the multiplexer  5  is controlled by a distinct breaker connected on the one hand to the MOSFET circuit of the first optical isolation static relay  1   a ,  2   a ,  3   a ,  4   a  of a pathway  1 - 4  and on the other hand to the analogue-digital converter ADC. The output of the MOSFET circuit of the second static relay  1   b ,  2   b ,  3   b ,  4   b  of each pathway is for its part linked to the analogue zero volts. The breakers of the multiplexer  5  can for example exhibit a structure of MOSFET type and they make it possible to limit the leakage currents destined for the analogue-digital converter ADC to a few nanoamperes. 
     The multiplexer  5  comprises for example eight channels  5   a - 5   h  and control means  7  making it possible to selectively control each of the pathways. The multiplexer  5  is for example available under the reference ADG508F. The control means  7  of the multiplexer are for example coupled to the control means  6  of the static relays  1   a - 4   b  so as to synchronize the selective open/close control of each of the pathways  1 - 4  and the selective open/close control of the corresponding channel  5   a - 5   h  of the multiplexer  5 . 
     When its number of channels is sufficient, the multiplexer  5  can carry out two functions. The first function already described above is to reduce the leakage currents of the static relays  1   a - 4   b  by connecting each input pathway to a channel  5   a - 5   d  of the multiplexer  5 . The multiplexer can furthermore be involved in the autocalibration of the measurement chain. Specifically, so as to avoid procedures for periodic calibration so as to guarantee the precision of the measurements, it is known to embed an autocalibration function carried out by virtue of the microprocessor of the card of the input module. The autocalibration of the measurement chain is carried out with the aid of a reference circuit of precise voltage and the multiplexer. In this case, the multiplexer  5  makes it possible to route the voltage reference in order to measure it and to short-circuit the input of the bus in order to measure the offset voltage. These two measurements carried out cyclically make it possible to compensate for the acquisition chain measurement errors. 
     Thus, when, as represented in  FIG. 1 , the module comprises four input pathways  1 - 4  and the multiplexer comprises eight channels  5   a - 5   h , four channels  5   a - 5   d  of the multiplexer  5  are placed in series with the optical isolation static relays so as to limit the leakage currents and the other four channels  5   e - 5   h  can be employed in the following manner:
         one channel routes the voltage reference,   one channel routes the ground,   one channel routes the power supply (5 volts) intended to detect a reference voltage fault,   one channel is unused and is grounded.       

     It is of course understood that it is possible, without departing from the scope of the invention, to contemplate other variants and refinements of detail and even to envisage the use of equivalent means.