Actuators and Control, Part 4 Grant Agreement No LLP UK-LEONARDO-LMP Project acronym: CLEM Project title: Cloud services for E-Learning in Mechatronics Technology

PLC The programmable logic controller (PLC) is a microprocessor-based system that accepts input data from switches and sensors, processes the data by making decisions in accordance with the logic of the stored program, generates output signals to devices that perform a particular function. The purpose of a PLC is replace electromechanical relays as logic elements, by a solid-state digital computer with a stored program, able to emulate the interconnection of many relays to perform certain logical tasks. The logic in a ladder diagram typically flows from left to right. The diagram can be divided into sections called rungs, analogous to the rungs on a ladder. Each rung typically consists of a combination of input instructions. These instructions lead to a single output instruction, right-most on a rung.

PLC Fig.39 Correspondence of relay circuits to Ladder diagram for PLC. Ladder diagram programming allows PLCs to perform several different types of tasks, including Boolean logic, timing, counting, arithmetic, and special functions. The contacts and coil functions are not real; The flow of current from one contact to the other is not real but virtual.

PLC PLC is an electronic control relay with built-in logic, timer, and counter functions. Control algorithms are developed by 12 input; 8 output. The algorithm is fed in the form of circuit diagram using ladder logic, to the PLC. Every contact in the circuit diagram can be defined as either a make or break contact. Make contact carry signal 1 and break contact carry signal 0. To replace relays, they execute: Boolean (bit, logical) operations; timer/counter functions (a finite state automaton). Analog I/O, integer or even floating point arithmetic; PWM outputs; and RTC are implemented in up-to-date PLCs.

Fig. 40 Block diagram representing input and output signals connected to the PLC. Fig.41 Example of PLC Ladder diagram. A PLC works by continually scanning a program. The scantime Is the time it takes to check the input status; execute all branches of the program using internal bit variables if an; update the output status. Programming languages for PLCs are described in IEC nomenclature: LD—ladder diagram; IL—instruction list (an assembler); SFC—sequential function chart (usually called by the proprietary name GRAFCET); ST—structured text (similar to a high level language); FBD—function block diagram. PLC

PLC: Example n.3 The automatic cycle has been programmed in Ladder and implemented in a PLC controller of type Festo FPC 404 in order to test the proposed on/off automatic cycle. Fig. 29 Displacement-set diagram. Fig.42 The electro-pneumatic circuit.

PLC: Example n.3 The program is composed by five basic parts: 1. A Left vertical line; 2. A Right vertical line; 3. One horizontal line, with two symbols: The symbol -| |- which indicates a switch, called I:0/1 The symbol -( )- which indicates the output relay, called O:0/1 Every ladder program has -a single left-side vertical line, which indicates an electrical line that is connected to high voltage; - a right-side vertical line, which indicates an electrical line that is connected to ground (zero volts); - an horizontal line represents one instruction. Instruction can be considered as an “IF (condition(s)) THEN (action)” statement. IF (input switch I:0/1 is ON) THEN (output relay O:0/1 is ON). --| |-- represents a switch, and is called an “EXAMINE IF ON’ instruction or “normally open” switch. --( )-- represents an Output relay (or a Coil). The left side is directly connected to the left vertical line, and is therefore at high voltage. When the switch is OFF, the right side of this switch will be a ZERO voltage.

Fig.43 Ladder logic Diagram for the electro-pneumatic circuit in Fig.42. Sensor s Input Comman d Output a0a0 I1.0A−A− O1.0 a1a1 I1.1A+A+ O1.2 b0b0 I1.2B−B− O1.3 b1b1 I1.3B+B+ O1.4 mI1.4 Table 2 - Correspondence of command / sensors used in Fig.43. PLC: Example n.3

Referring to Fig.43 and Table 2, command –( S )– is set coil. If any rung path passes power, output is energized and remains energized, even when no rung path passes power. Output signals are O1.0 e O1.3 related to commands A − and B −, respectively; command –( R )– is a reset coil. If any rung path passes power, output is de-energized and remains de-energized, even when no rung path passes power. Output signals are O1.2 e O1.4 related to commands A + e B +, respectively.

The program in Fig. 43 can be explained as: 1)In Rung n.1, symbol -] [- shows the signals of transition (m, a0, b0) which are necessary for the start the cycle, while - (R) - and - (S) - reset and are set the signals related to commands A- and A +, to move forward the piston of the cylinder A. It also indicates the setting of F0.0.1, which corresponds to the interruption of the same phase and the preparation of the next. 2) In rung n.2 are taken into account the transition signal a1 (I1.1) and the control signals that move forward the piston of the cylinder B (O1.3 O1.4 reset and set), while the setting of F0.0.2 allows you to turn off that phase and to prepare the next one. 3) In rung n.3 there is the activation signal of b1 which corresponds to the internal variable I1.3, In this phase is disactivated O1.4 and activated O1.3, thus cylinder B retracts. By setting F0.0.3 the next step is activated and disabled the current phase. 4) In the last step is considered the enabling signal b0 (corresponding to the signal I1.2) with which it can begin the fourth phase. Resets the variable O1.2 and enables variable O1.0 for the return of the piston of the cylinder A. The setting of F0.0.4, finally, allows to deactivate the phase and simultaneously to re-establish the initial conditions to cycle completed. 5) An additional press of the m button causes a new beginning of the cycle resulting in repetition of the previous four phases. PLC: Example n.3