Pick & Place PLC Control · Anderson Uceta

The Problem

In manufacturing lines with robotic pick-and-place systems, unplanned downtime is one of the most costly issues. Cycle time degradation — when a machine gradually slows down — often goes unnoticed until a full stop occurs. By the time maintenance is called, production has already been lost.

The Solution

A complete PLC program that controls a Pick & Place cartesian robot and simultaneously monitors production KPIs in real time. The system measures every cycle, tracks statistical trends (average, min, max), and lays the groundwork for early degradation detection.

Program Structure

FC1 — Control

Ladder (LAD)

Safety and master control logic — Start/Stop latching, Emergency Stop, fault detection, reset verification, and forced output shutdown.

FC2 — Sequence

SCL

State machine controlling the pick and place cycle — 10 main steps with two-phase motion pattern, automatic repetition, conveyor coordination.

FC3 — Monitoring

SCL

Real-time production analytics — cycle time measurement, statistical tracking (last, avg, min, max), machine state and fault counting.

Sequence Logic — FC2

The sequence is implemented as a CASE state machine in SCL. Each step controls physical outputs and waits for the appropriate sensor feedback before advancing. Motion axes use a two-phase pattern to avoid false transitions within the same scan cycle.

Step	Action	Transition
0	Idle — waiting	Running = TRUE
1	Start entry conveyor	Sensor_Entry = TRUE
2	Stop entry conveyor	Immediate
3 / 31	Lower Z axis	Moving_Z start → end
4	Activate gripper	Item_Detected = TRUE
5 / 51	Raise Z axis	Moving_Z start → end
6 / 61	Move X to exit position	Moving_X start → end
7 / 71	Lower Z at exit	Moving_Z start → end
8	Release gripper + raise Z	Item_Detected = FALSE
9 / 91	Return to home	All axes stopped

Production Monitoring — FC3

Every completed cycle is measured using a TON timer as a stopwatch. FC3 computes a running average and tracks min/max cycle times — giving a real-time baseline for detecting performance degradation before it causes downtime.

CycleTime_Last

Last cycle duration (ms)

CycleTime_Avg

Moving average (ms)

CycleTime_Min

Best cycle recorded (ms)

CycleTime_Max

Worst cycle recorded (ms)

CycleCount

Total pieces completed

MachineState

0=Idle · 1=Run · 2=Fault

Key Technical Decisions

Two-phase motion pattern: Factory I/O axis signals are TRUE during motion, requiring a start-detection sub-step before the completion check — prevents same-scan false transitions
Separate safety layer: FC1 handles all safety independently from the sequence. If FC2 has a bug, FC1 still forces all outputs OFF on emergency stop
TON as stopwatch: Used a TON timer with a long PT as a cycle chronometer instead of system clock reads — simpler and reliable in PLCSIM Standard
Exit conveyor always running: Prevents product accumulation at the output — a real production concern, not just a simulation detail

Phase 2 — Python Data Layer (planned)