SUDS — Soap Usage & Dispensing System

Problem

Electrostatic-dissipating soap in the glass washer was causing static-shock incidents on the line. Early conductivity sensing was unusable: the same supply that drove the probes also collapsed under load, so readings looked like process swings when they were really voltage drop and coupled noise.

Approach

I treated it as a measurement problem before a software problem. Star grounding came first. Then Atlas Scientific conductivity modules got their own isolated supply and differential wiring into a Revolution Pi, with Python normalizing tank data for an Ignition Perspective dosing view.

Architecture

Four washer tanks feed isolated Atlas Scientific conductivity modules. Independent 5 V rails and differential pairs land on the RevPi; Perspective only sees cleaned engineering units and dose commands.

Implementation

The edge script never trusts a raw UART line. It rejects probe chatter, converts microsiemens to a tank state, and only then writes tags the Perspective application uses for alarming and dose control.

# Isolated 5 V rail + differential pair into the RevPi UART
PROBE_ERROR_PREFIXES = ("*", "ERROR")

def read_conductivity(uart):
    uart.write(b"R\r")
    line = uart.readline().decode("ascii", errors="ignore").strip()
    if not line or line.startswith(PROBE_ERROR_PREFIXES):
        return None
    us_cm = float(line)
    if us_cm < 0:
        return None
    return us_cm

def tank_state(us_cm, low, high):
    if us_cm is None:
        return "fault"
    if us_cm < low:
        return "dose"
    if us_cm > high:
        return "high"
    return "ok"

Results