Module with isolated analogue inputs having low leakage current

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DESCRIPTION

Isolated analog input module with small leakage current

The present invention refers to an analog input module used in a programmable controller.

A programmable automaton or CLP (Programmable Logic Controller) is an automation equipment capable of directing, controlling and / or supervising one or more processes to control.

Generally modular in construction, a PLC is made up of different modules that communicate with each other through a transmission bus, referred to in this field as the backplane bus (or backplane). The modules are mechanically fixed to a frame, which comprises a printed circuit that also supports the motherboard backplane bus as well as the connection elements that are intended to cooperate with connectors generally present at the rear of the modules to make the necessary connection. between the modules and the bus. The number of modules depends, of course, on the size and type of processes to be automated.

Typically, a programmable controller can comprise:

- a power supply module that provides the different voltages to the rest of the modules through the motherboard backplane bus,

- a UC central unit module comprising an integrated computer program, called a builder program (firmware), which integrates a real-time OS operating system, and an application program, or user program, which contains the Instructions to be executed by the integrated computer program to carry out the desired automation operations. Generally, the UC module also comprises a connection on the front face for programming tools of the personal computer type PC.

- I / O input / output modules of various types, depending on the process to be controlled, such as digital I / O or TON (all or nothing), analog, counters, etc. These I / O modules are attached to sensors and actuators that participate in the automated management of the process.

- one or more communication modules with communication networks (Ethernet, CAN ...) or human-machine interfaces (display, keyboard ...).

By way of example, an input / output module can comprise between 1 and 32 I / O channels, a PLC being capable, depending on the models, of managing several hundred I / O channels. Therefore, in case of need, several racks are connected to each other in the same PLC. In this way, depending on the application and the process to be automated, a PLC automaton can comprise a large number of modules.

Currently, analog input modules comprise various channels isolated from each other with the help of static optically isolated relays, also known as "OptoMos" (registered trademark). Each channel of the module is controlled by two static optical isolation relays, in which one of the MOS poles is common. These components have an almost unlimited lifespan, but each one presents a not inconsiderable exhaust current. In this way, when the module comprises more than four paths, the unit leakage currents of each of the static relays are added together, which leads to consequent errors in the measurements.

Document US 4 821 167 discloses a typical input module for programmable controllers in which the analog inputs are multiplexed to distribute them to a single A / D converter.

The object of the invention is to propose an analog input module comprising several mutually isolated channels, in which the leakage currents are particularly reduced in order to always guarantee the precision of the measurements.

This object is achieved with a module of isolated analog inputs for a programmable automaton, said module comprising several input channels, isolated from each other, each one with the help of at least one static relay of optical isolation selectively controlled with the help of control means. , said module being characterized by comprising a multiplexer provided with several channels, each controlled to open or close, placing a channel of the multiplexer in series to an isolated input channel of the module.

According to one particular feature, each input channel is isolated with the help of two static optical isolation relays mounted in series.

According to another feature, the multiplexer is of the low voltage analog type.

According to another feature, the multiplexer comprises control means whose object is 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 static optical isolation relays.

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According to another feature, the multiplexer comprises eight channels. It comprises at least as many channels as there are channels in the module. If you have more channels than channels in the module, some of these channels can be used for self-calibration of the measurement system.

The module according to the invention is adapted to be used in a modular programmable controller.

Other characteristics and advantages will appear in the following detailed description which refers to an embodiment provided as an example and represented by the attached drawings in which:

- figure 1 schematically represents the structure of the invention.

An analog input module comprises several channels, 1, 2, 3, 4 isolated with the help of static relays of optical isolation, also called OptoMos (registered trademark). This type of component is described in particular in United States Patent US 4,390,790. More specifically, this type of optical isolation static relay 1a, 1b, 2a, 2b, 3a, 3b, 4a, 4b (hereinafter 1a-4b) comprises an input circuit consisting of a light-emitting diode capable of converting an input control signal (i in figure 1) into infrared light. The light-emitting diode is optically coupled to a conversion circuit made up of a set of photovoltaic cells and associated steering circuits. The photovoltaic cell generates the necessary voltage to control a MOSFET circuit (represented by m simplified by a simple switch) that switches the output load.

With reference to Figure 1, in an isolated analog input module, each channel comprises two static optical isolation relays (hereinafter static relay). For each input path, the two static relays are linked in series by their input circuit. The inputs of the MOSFET circuits are linked to a terminal that materially defines the different paths on which the sensors are connected. The control means 6 allow to selectively control each input channel 1-4 by sending the control signal to the input circuits of the two static relays in series. Optically isolated static relays withstand voltages that can reach up to 600 Volts for switching times of less than 1 ms.

The static relays of optical isolation 1a-4b allow to isolate the tracks from each other. For the non-selected input paths, each of these optically isolating static relays allows a non-negligible leakage current to pass. The sum of these exhaust currents leads to errors in the measurements, which can be insurmountable when the number of paths is greater than four.

According to the invention, a low-voltage analog multiplexer 5 (direct supply voltage between approximately + 30VDC and -30VDC) is therefore placed between the analog-digital converter CAD and the static relays 1a-4b. This multiplexer 5 has a number of channels at least equal to the number of channels of the input module. In figure 1, each channel 5a-5h of multiplexer 5 is controlled by a different switch that is connected on the one hand to the MOSFET circuit of the first static relay of optical isolation 1a, 2a, 3a, 4a of a channel 1-4 and on the other side to the analog-digital CAD converter. The output of the MOSFET circuit of the second static relay 1b, 2b, 3b, 4b of each channel, in turn, is connected to zero analog volts. The switches of the multiplexer 5 can have, for example, a MOSFET type structure and make it possible to limit the leakage currents to the analog-digital converter CAD to a few nanoamps.

The multiplexer 5 comprises, for example, eight channels 5a-5h and control means 7 that make it possible to selectively control each of the channels. Multiplexer 5 is available, for example, under the reference ADG508F. The multiplexer control means 7, for example, are coupled to the control means 6 of the static relays 1a-4b to synchronize the selective opening / closing control of each of the channels 1-4 and the selective opening / closing control. closing of the corresponding channel 5a-5h of multiplexer 5.

When its number of channels is sufficient, the multiplexer 5 can perform two functions. The first function, already described above, is to reduce the leakage currents of the static relays 1a-4b by connecting each input channel to a channel 5a-5d of multiplexer 5. The multiplexer can also participate in the self-calibration of the measurement system. . In fact, in order to avoid periodic calibration procedures to guarantee the precision of the measurements, it is customary to integrate a self-calibration function that is carried out thanks to the microprocessor of the input module card. The self-calibration of the measurement system is carried out with the help of a precise reference voltage circuit and the multiplexer. In this case, the multiplexer 5 makes it possible to distribute the reference voltage to measure it and to bridge the input to the bus to measure the phase shift voltage. These two measurements taken cyclically make it possible to compensate for the measurement errors of the acquisition system.

In this way, when the module comprises four input channels 1-4 and the multiplexer comprises eight channels 5a-5h, as shown in figure 1, four channels 5a-5d of multiplexer 5 are put in series with the relays Optical isolation statics to limit exhaust currents and the remaining four channels 5e-5h can be used as follows:

-one channel distributes the voltage reference,

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-a channel distributes the mass,

-one channel distributes the power supply (5 Volts) to detect failures in the reference voltage, -one channel does not work and is connected to the ground.