GB2046476A - Programmable logic controllers - Google Patents

Abstract

A programmable logic controller system has programmable logic control modules (2) to (8) each connected in use to input and output functions (MC1 to MC4, F1 to F5) in a machine for injection moulding, stage lighting or packaging. Each module is connected to a common data highway (16) and to a master control unit (10) which supplies each module with its programmed logic upon power-up. Each module transmits, in sequence, a map of the state of the input/output functions to which it is connected upon the highway and, in response to this, each module stores a map of the complete input/output conditions. Highway data transmission is in bit-serial, character-serial format. This is stated to reduce cabling costs yet enables reprogramming easily to take place by reprogramming only of the unit 10. <IMAGE>

Description

SPECIFICATION Programmable logic controllers For some years, electrical relay logic in capital machinery has had a cost-effective alternative in the programmable logic controller (PLC). A PLC usually incorporates a microprocessor and typically performs the logic timing and sequencing functions required to interface, for instance, control panels, limit-switches, level detectors, thermostats, to the clutches, solenoids and motors which drive the machinery.

PLCs feature solid state reliability and programmed logic. Programmable logic means that the logic and timing operations can be programmed and tested by means of, for example, a console with a keyboard and VDU. When the program has been tested it can be committed to low-cost memories which are installed in the PLC. The expensive console can then be removed.

PLCs have some disadvantages. For instance, every contact, or switch must be wired from its physical position on the machine to the PLC, and similarly all contactor, clutch and lamp wiring must be routed from the PLC to the actual device site. This wiring may have to be broken down in the factory and re-made on site. Moreover, in large installations many miles of wiring may be required. Another disadvantage is that the marginal cost of PLCs tends to be high. Because the logic is central, the cost tends to increase in large steps. In many applications of PLCs the capacity of the PLC is under-employed and it suffers a cost penalty accordingly. Afurther disadvantage is that PLCs do not handle analogue information easily.

We have now devised an approach to PLCs which minimises these disadvantages. This is accomplished by distributing the logic and corresponding inputs and outputs throughout the machine installation, typically in modular form. The interconnection between such modules is by means of a simple data and power highway. The wiring is thus minimised and the modules are of relatively low cost. It is also a simple and not expensive matter to extend the system by the connection of further modules to the highway as needs dictate.

According to the invention we provide a programmable logic controller system, for installation in a capital machine, which comprises a plurality of separate non-hierarchical programmable logic control units (herein termed PLCU's) each connectable to one or more input and/or output functions in the installation and containing, in use, programmed logic for controlling output functions in response to input functions, a common highwayfortransmitting data between the units, and a master control unit connected to said highway for providing said programmed logic to each PLCU during power-up.

The invention also relates to a programmable logic control unit and to a control unit for use in an installation specified in the preceding paragraph.

Preferably each unit is in modular form and contains a microprocessor which in use is at least partially controlled by the program supplied by the control unit.

Preferred features of the invention will now be described with reference to the accompanying drawings, given by way of example, wherein Figure 1 shows schematically an embodiment of the invention, Figure 2 shows schematically the visual appearance of a PLC module employed in the invention, and Figure 3 shows schematically the electrical circuit configuration for the module of Figure 2.

Referring to Figure 1, there is shown a plurality of PLC modules 2,4,6 and 8 installed within a piece of capital machinery and controlling functions F1 to F5 on the machine. The control of the functions is in response to machine conditions MC1 to MC4 represented as inputs to each respective PLC module.

Each module contains input isolators and output drivers as in a conventional PLC, but only a few of each (typically a maximum of sixteen) rather than the larger number in a conventional PLC. Modules 2 to 6 are shown as controlling single functions F1 to F3 whereas module 8 is shown as controlling two functions, F4 and F5 via the input isolators and output drivers. The installation also includes a BOOT module 10, a matrix module 12, and a display module 14.

All modules are interconnected by a four line highway 16 consisting of a two conductor power line and a two conductor bi-directional data line.

The BOOT module controls and organises the installation upon power-up (BOOT techniques are known in the computer art). Each module contains a protocol to enable transmission and reception of data upon the highway 16 to take place. The matrix module 12 (which is optional) is for connection to a keyboard and/or display for controlling the operation of the installation. The display module 14 (also optional) includes display panels showing the status of all inputs and outputs and is employed in servicing of the installation.

Although the BOOT module 10 is shown as a discrete module having no direct input/output interface with the machine functions, it could equally well do so as illustrated by the dashed arrows and machine condition MC5 and function F6. Thus F1 to F6 are controlled by a logical combination of inputs MC1 to MCs in a manner determined by a program which is distributed amongst the modules by the BOOT module upon power-up.

Atypical PLC module is illustrated schematically in Figure 2. The highway 16 is shown as a fourconductor ribbon cable to which the module is connected by means of a highway connector 18. By employing a clamp-type connector as illustrated, additional PLC modules can be easily added to the installation simply by clamping a connection to the highway at the appropriate position. The module encloses the electronic circuitry necessary for its function (described below with reference to Figure 3) and additionally includes a transformer 20 providing power from the highway for the input/output functions e.g. MC1, F1 etc. and the logic within the PLC module. The transformer 20 supplies a connector rail 22 which enables electrical connection of the module to the various input and output devices with which the module is associated.

Referring to Figure 3, each PLC module comprises a CPU 24, ROM 26, RAM 28, input/output ports 30, logic clock 32, a universal asynchrous receiver/ transmitter (UART) 34, and isolators 36. The transformer 20 is shown supplying power to the logic circuitry (line 38), for the input/output devices (line 40) and for the data lines on the highway 16 (line 42).

The ROM and RAM are coupled to the CPU by the usual address, control and data buses (44,45 and 46 respectively). The ROM 26 holds an appropriate program for line and input/output handling and for BOOT protocol. The actual program for the system is stored in RAM, 28, which is loaded via the data lines of the highway 16 from the BOOT module 10 during power-up of the installations. The BOOT module contains the program for the system in a separate memory which is preferably non-volatile. Data is transmitted upon the highway in bit serial, character serial fashion, and the UART 34 and isolators 36 are capable of handling such data to provide the necessary interface with other components within the PLC module. Decode logic 48 controlled by the address and control buses 44 and 47 is employed to enable the relevant components of the module.

The power supplied on the power part of the highway is typically 110 volts AC and will require rectification before supply upon lines 38 and 42.

Each module is provided with an address code for identification purposes and each input and output on each module a corresponding identifying code. The choice of how the inputs and outputs are allocated between the modules is determined by the physical position of the input and output devices on the machine. Preferably each input is of the contact closure type and each output either a triac or transistor drive.

Since data transmission upon the highway 16 is in serial fashion, it will normally be desirable to employ a data priority system whereby different types of data are allocated different priorities and consequently given different priorities for access to the highway. Any operation which must be carried out at high speed (e.g. as a consequence of an emergency condition) can be handled either within the module itself (without transmission on the highway) or communicated to another module via the input/ output functions directly. The organisation of the installation as described facilitates the handling of analog information by digitising such information for transmission between modules.

A system according to the invention operates as follows.

Upon power-up, the BOOT module (which holds the program for all modules in non-volatile form) proceeds to transmit the program upon the data highway. Each module stores the part of the transmission it recognises as its own and, at the end of transmission, the modules acknowledge that their data has been received intact. If no acknowledgement is received, the BOOT module retransmits the program.

Once the full program has been successfully distributed the modules proceed to follow their own logic program and transmit upon the data highway in sequence the contents (or change in contents) of their own I/O map, which is stored in its own RAM.

These module maps include status bits such as flags as well as the state of the actual I/O pins.

Each module receives all transmissions and uses them to update a map of the entire system I/O held in its individual RAM. Each module thus maintains a map in its RAM of the states of all inputs and outputs in the system and, entirely autonomously, executes its own logic program based upon the contents of the map of the state of the machine.

The most likely distribution of logic is that those outputs physically connected to a module will be operated by logic within that module's program.

Thus each module will execute a series of logical combinations of entries in its machine state map and will use the answers to operate its outputs according to the logic of the combination. This program will then be repeated continually.

Each module thus acts as an independent logic controller. Each has the same status as any other module and shares time on the highway in an equal way: the system is not hierarchial. The data highway is used by a module to inform all other modules of the state of its inputs and outputs in order that they can keep their maps up-to-date. Any module may operate peripheral devices as slaves to itself but all modules have equal status.

Since transmissions upon the data highway are in digital format, some or all of the modules may include A to D and/or D to A converters so that analog input or output functions may be monitored.

It may be desirable to have the BOOT module re-transmit the full program from time to tirne to maintain the integrity of the program at each module. It is solely for practical convenience that initially all program data emanates from a single point. Thus, during power-up, the data highway is used to transmit the full program from a module which is more accessible to the engineer than other modules (which may be inaccessible). Alterations to the program in remote inaccessible modules thus becomes straightforward.

Systems according to the invention can be employed in a wide variety of applicatons. Typical applications, for example, are the control of injection moulding machines, packaging machinery, machine tools and stage lighting.

Claims (10)

1. A programmable logic controllersystem, for installation in a capital machine which comprises a plurality of separate non-hierarchical programmable logic control units (herein termed PLCU's) each connectable to one or more input and/or output functions in the installation and containing, in use, programmed logic for controlling output functions in response to input functions, a common highway for transmitting data between the units, and a master control unit connected to said highway for providing said programmed logic to each PLCU during powerup.

2. A system according to claim 1 wherein each PLCU includes a non-volatile memory holding at least the address of the unit and a memory for receiving said programmed logic upon power-up.

3. A system according to claim 1 or 2 wherein each PLCU transmits in sequence onto the highway a map of the states of the inputs and/or outputs with which it is associated in the installation, and each PLCU, upon receiving the transmission, maintains a map of the states of all of the machine inputs and outputs being monitored.

4. A system according to any of claims 1 to 3 wherein the highway includes means for supplying power to the PLCU's.

5. A system according to any of claims 1 to 4 wherein the master control unit is additionally a PLCU.

6. A system according to any of claims 1 to 5 wherein transmission of data upon the highway is in digital form.

7. A system according to claim 6 wherein the transmission of data upon the highway is in bit serial, character serial form.

8. A programmable logic controller system substantially as herein described with reference to the accompanying drawings.

9. A PLCU for use in a system as claimed in any of claims 1 to 8.

10. A master control unit for use in a system as claimed in any of claims 1 to 8.