Full control system re-engineering of a 1.5-ton tea blending plant
A tea processor's blending line had been out of service since its original control system failed. The line includes a rotating blending drum, bucket elevators, conveyors, a vibratory sifter, dust extraction and pneumatic gates. The original Mitsubishi FX3U-based controls were dead, and neither the manufacturer nor the original integrator offered support.
We took full engineering responsibility. That covered root-cause investigation, a new control philosophy, a PLC and HMI system engineered from scratch, removal of the old installation, and commissioning on site with product running through the line.
Safety & process interlocks
- Reverse-flow safe start: no feeder runs until dust extraction is proven by run feedback.
- Zero-speed drum interlock: the drum can't reverse without 0 RPM confirmed and a 15-second dwell.
- Discharge protection: the gates close at once if a delivery elevator trips, preventing spillage.
- Level control: capacitive high-level sensors stop feeding automatically when the hopper is full.
- Hardwired emergency stop: isolates every motor independently of the PLC.
- Motor supervision: every motor is monitored for run feedback and overload, with fault alarms on the HMI.
Investigation
Site survey, fault-finding on the failed system, review of OEM documentation, and mapping the process into five control phases.
Engineering design
Control philosophy to IEC 60204-1 and IEC 61131, I/O schedule and tag database, schematics, and specification of the PLC, HMI, motor starters, sensors and pneumatics.
Software
Siemens TIA Portal: a library of reusable motor-control blocks with auto, manual and service modes, feedback timeouts and start/stop delays, plus sequence logic and a multi-screen HMI.
Installation
Decommissioning of the old system under a written method statement. Installation of an IP65 control enclosure for the tea-dust environment, power and control wiring, and metering.
Commissioning
I/O checks, interlock proving, sequence trials and load commissioning with product. Handed over in August 2026.
