# SCADA & MES: the control loop and its memory

Why living processes demand closed-loop control, how cascade strategies spend ordered actuators against drift, why the electronic batch record is part of the loop rather than paperwork, and why measurement sets the ceiling.

A bioreactor cannot be set and forgotten because the cells keep changing the plant they live in — and the batch record exists because a controlled process must be reconstructible.

Source: https://en.bioecon.ru/docs/bioproduction-equipment/automation-robotics/scada-mes-for-bioproduction/
Updated: 2026-09-07



An ordinary factory process can be tuned once and left alone; a bioreactor cannot, because its contents are alive and keep editing their own environment. Cells consume oxygen, excrete CO₂, acidify or alkalise the medium, generate heat, and change viscosity as they grow — the plant is non-stationary by construction, while the quality of the product depends on holding temperature, pH and dissolved oxygen inside narrow bands for days. Holding a setpoint against a drifting plant, around the clock, with consistent response time, is precisely what a control system does and a human cannot. That is the whole justification for the SCADA layer: not monitoring as a courtesy, but feedback as a necessity.

## Cascades: spending actuators in order

Dissolved oxygen illustrates the standard strategy. The controller cannot push a variable directly; it must choose actuators, and they are not equivalent. Raising agitator speed improves gas transfer without changing the gas; enriching with oxygen costs money and complicates safety; sparging harder strips CO₂ and foams the medium. So the loop is a cascade: agitation is spent first, and only when the cheap actuator saturates does the expensive one engage. pH control shows the same asymmetry with a worse complication: the actuators have long dead time — base pumped in takes minutes to reach the pH probe, mix, and register — and control loops with dead time oscillate if tuned aggressively. Tuning is therefore not a commissioning detail but a quality attribute: an hour-long pH excursion is invisible to the vessel and legible in the product's glycosylation pattern.

## The record is part of the loop

The electronic batch record is usually described as compliance; mechanically it is the process's memory. A controlled run must be reconstructible — every setpoint change, alarm and operator intervention timestamped and attributed — for two reasons that are engineering, not bureaucracy. First, diagnosis: when a batch drifts, the reconstruction is how the drift is traced to a cause. Second, detection: review-by-exception is a comparison of the current run's trajectories against the family of previous runs, and only a complete, structured record makes the comparison automatic. The recipe standards in this space (ISA-88 for batch control) encode the same idea structurally: a process defined as named phases and parameters is portable between vessels and comparable between batches. The record turns each run into training data for controlling the next one.

## Measurement is the ceiling

Both the loop and the record are limited by what can be measured, when. Probes drift and require calibration; critical variables like cell density and product quality are historically off-line assays with hours of lag, so the controller runs partially blind on those axes and interpolates. The real content of process-analytical technology is moving measurements inside the loop — in-line spectroscopy estimating composition continuously so that feeding reacts to the culture rather than to the calendar. What the software layer can never compensate for is a variable nobody measures; the honest boundary of bioprocess automation is the boundary of its sensor suite.