PLC Control Systems

// HAMK Control Systems Course · TwinCAT 3 / Beckhoff EtherCAT · Group 7

Process State Machine — Pick & Place with Cooling

Live Variables (GVL_IO)

Variable	Type	Value	Description

LED Indicators

🟢 Green
Idle

🔵 Blue
Cooling

🟡 Yellow
Wait Robot

🔴 Red
Fault

Temperature Sensor (LM35)

24.2 °C

Setpoint: 8.0°C

Monitoring…

State Transition Log

Refrigerator Control Visualization

System Parameters

Variable	Value

Temperature History (TwinCAT YT Chart)

E_ProcessState — Enumeration (DUT)

// Data Unit Type — State machine enumeration
{attribute 'qualified_only'}
{attribute 'strict'}
TYPE E_ProcessState :
(
    stIdle,               // Waiting for StartCycle
    stRequestMoveToP2,    // PLC sets RobotCmd := TRUE
    stWaitRobotAtP2,      // Wait for RobotAck = TRUE (at cooling tank)
    stCooling,            // Temperature supervision
    stRequestMoveToP3,    // PLC sets RobotCmd := FALSE
    stWaitRobotAtP3,      // Wait for RobotAck = FALSE (at output)
    stDone,               // Cycle complete
    stFault               // Timeout or error
);
END_TYPE

MAIN — State Machine in Structured Text (IEC 61131-3)

CASE eState OF
    E_ProcessState.stIdle:
        bRobotCmd  := FALSE;
        bCycleDone := FALSE;
        IF bStartCycle THEN
            eState := E_ProcessState.stRequestMoveToP2;
        END_IF;

    E_ProcessState.stRequestMoveToP2:
        bRobotCmd := TRUE;               // MG400 DI8 goes HIGH
        eState    := E_ProcessState.stWaitRobotAtP2;

    E_ProcessState.stWaitRobotAtP2:
        tonStepTimeout(IN := TRUE, PT := T#30S);
        IF bRobotAck THEN             // MG400 DO8 → EL1008 DI8
            eState := E_ProcessState.stCooling;
        ELSIF tonStepTimeout.Q THEN
            eState := E_ProcessState.stFault;
        END_IF;

    E_ProcessState.stCooling:
        // LM35 → EL3074 analog input → rTempValue (scaled to °C)
        IF rTempValue <= rTempSetpoint THEN
            eState := E_ProcessState.stRequestMoveToP3;
        END_IF;

    E_ProcessState.stRequestMoveToP3:
        bRobotCmd := FALSE;              // Signal: move P2 → P3
        eState    := E_ProcessState.stWaitRobotAtP3;

    E_ProcessState.stWaitRobotAtP3:
        tonStepTimeout(IN := TRUE, PT := T#30S);
        IF NOT bRobotAck THEN
            eState := E_ProcessState.stDone;
        ELSIF tonStepTimeout.Q THEN
            eState := E_ProcessState.stFault;
        END_IF;

    E_ProcessState.stDone:
        bCycleDone   := TRUE;
        bFaultActive := FALSE;
        IF NOT bStartCycle THEN
            eState := E_ProcessState.stIdle;
        END_IF;

    E_ProcessState.stFault:
        bRobotCmd    := FALSE;
        bFaultActive := TRUE;
        IF bResetFault THEN
            eState := E_ProcessState.stIdle;
        END_IF;
END_CASE;

Beckhoff EtherCAT I/O Architecture

IPC (Industrial PC)  ─── EtherCAT ───┐
  TwinCAT Runtime 4024                │
  IP: 192.168.1.10                    ▼
                              EL1008  Digital Input  8-ch
                              EL2008  Digital Output 8-ch
                              EL3074  Analog Input  4-ch (0-10V / Real32)
                              EL4074  Analog Output 4-ch

── Wiring to Dobot MG400 ──
EL2008 DO4 ──────────────── MG400 DI8  (PLC → Robot CMD)
MG400 DO8  ──────────────── EL1008 DI8 (Robot → PLC ACK)
GND (PLC)  ──────────────── GND (MG400)

── Temperature Sensor LM35 ──
Brown  → +24V
Black  → 0V (GND)
Blue   → EL3074 Channel 1 (analog input)
Scale: 10 mV/°C  →  22°C = 0.22V input

Robot–PLC Handshake Protocol

Single-bit bidirectional handshake:

1. PLC sets CMD = TRUE
   → Robot picks at Position 1
   → Robot moves to Position 2 (cooling tank)

2. Robot sets ACK = TRUE
   → PLC enters stCooling
   → Waits: rTempValue ≤ 8.0°C

3. PLC sets CMD = FALSE
   → Robot moves from P2 to P3 (output)

4. Robot sets ACK = FALSE
   → PLC enters stDone

Timeout: 30s per step → stFault
Reset: momentary switch bResetFault

Physical I/O Mapping

Variable	Qualifier	Terminal	Channel	Description
bDoorOpen	AT %I*	EL1008	CH1	Toggle switch → door sensor
bResetFault	AT %I*	EL1008	CH2	Momentary button → fault reset
bRobotAck	AT %I*	EL1008	CH8	MG400 DO8 → robot acknowledge
bLight	AT %Q*	EL2008	CH1	LED → door open indicator
bAlarm	AT %Q*	EL2008	CH2	Red LED → blinking alarm
bRobotCmd	AT %Q*	EL2008	CH4	MG400 DI8 → robot command
rTempValue	AT %I*	EL3074	CH1	LM35 analog → Real32 °C