EPICS Fridge Control

A learning project for developing a simulated refrigerator control system using EPICS.

The project was originally started during EPICS training and is now being extended into an interactive control-system simulation.

Status: Work in progress.
The temperature-control/thermostat logic is currently implemented. Automatic temperature dynamics, door control, lighting, alarms, and the final Phoebus GUI are still to be added.

Current architecture

The refrigerator is currently implemented using a ready-made EPICS softIocPVA and an EPICS database.

fridge.db
    |
    v
st.cmd
    |
    v
softIocPVA
    |
    v
EPICS PVs

No external hardware or StreamDevice support is required because the refrigerator is currently simulated entirely within the EPICS environment.

Project structure

fridge-control/
├── .gitignore
├── README.md
├── fridge.db
└── st.cmd

Reusable PV prefix

The database uses the macro:

$(P)

rather than hard-coding FRIDGE: into every record.

The startup file loads the database with:

P=FRIDGE:

For example:

$(P)TEMP:ACTUAL

becomes:

FRIDGE:TEMP:ACTUAL

This allows the same database structure to be reused later with different equipment prefixes.

Current PVs

The current implementation contains:

FRIDGE:TEMP:ACTUAL
FRIDGE:TEMP:SET
FRIDGE:TEMP:HYST
FRIDGE:TEMP:WARM
FRIDGE:TEMP:COLD
FRIDGE:THERMOSTAT
FRIDGE:PUMP

FRIDGE:TEMP:ACTUAL

Represents the current fridge temperature.

It is implemented as an analog input (ai) because it represents temperature information coming into the control system.

At the current development stage, the value is manually changed using pvput to simulate different fridge temperatures.

Automatic temperature dynamics will be added later.

FRIDGE:TEMP:SET

Represents the desired fridge temperature selected by the operator.

It is implemented as an analog output (ao).

The current configuration uses:

Minimum setpoint: 1 degC
Maximum setpoint: 8 degC
Default setpoint: 4 degC

FRIDGE:TEMP:HYST

Defines the thermostat hysteresis/deadband.

The default value is:

1 degC

The operator can adjust this value.

For example:

TEMP:SET  = 4 degC
TEMP:HYST = 1 degC

produces:

Upper threshold = 5 degC
Lower threshold = 3 degC

FRIDGE:TEMP:WARM

A calc record that determines whether the actual temperature is above the upper thermostat threshold.

The calculation is:

ACTUAL > SET + HYST

The result is:

1 = temperature above upper threshold
0 = otherwise

The record uses CP (Process on Change) links so that changes to the actual temperature, setpoint, or hysteresis automatically cause the calculation to be processed.

FRIDGE:TEMP:COLD

A calc record that determines whether the actual temperature is below the lower thermostat threshold.

The calculation is:

ACTUAL < SET - HYST

The result is:

1 = temperature below lower threshold
0 = otherwise

This record also uses CP links for automatic processing.

FRIDGE:THERMOSTAT

A calcout record implementing the thermostat state logic.

It reads:

TEMP:WARM
TEMP:COLD
current PUMP state

and evaluates:

A ? 1 : (B ? 0 : C)

where:

A = TEMP:WARM
B = TEMP:COLD
C = current PUMP state

This means:

WARM = 1
    -> Pump ON

COLD = 1
    -> Pump OFF

WARM = 0 and COLD = 0
    -> Keep previous pump state

The calculated result is sent to:

FRIDGE:PUMP

using the OUT link of the calcout record.

FRIDGE:PUMP

Represents the simulated cooling pump.

It is implemented as a binary output (bo):

0 = OFF
1 = ON

The pump is now controlled automatically by FRIDGE:THERMOSTAT.

Thermostat hysteresis

The thermostat deliberately uses two different switching thresholds.

For example:

SET  = 4 degC
HYST = 1 degC

gives:

ACTUAL > 5 degC
    -> Pump turns ON

3 degC <= ACTUAL <= 5 degC
    -> Pump keeps its previous state

ACTUAL < 3 degC
    -> Pump turns OFF

This prevents rapid ON/OFF switching around the temperature setpoint.

Example cooling cycle:

Actual temperature = 6.0
        |
        v
WARM = 1
        |
        v
PUMP ON
        |
        v
temperature falls

Actual temperature = 4.0
        |
        v
WARM = 0
COLD = 0
        |
        v
PUMP remains ON

Actual temperature = 2.8
        |
        v
COLD = 1
        |
        v
PUMP OFF

Current control flow

                  TEMP:ACTUAL
                       |
             +---------+---------+
             |                   |
             v                   v
        TEMP:WARM           TEMP:COLD
     ACTUAL > SET+HYST   ACTUAL < SET-HYST
             |                   |
             +---------+---------+
                       |
                       v
                  THERMOSTAT
               A ? 1 : (B ? 0 : C)
                       |
                       | OUT
                       v
                     PUMP

CP links cause the temperature calculations to process automatically when their input values change.

The thermostat uses the current pump state inside the hysteresis band so that the pump retains its previous state.

Running the project

Activate the Mamba EPICS environment:

mamba activate epics

Go to the project directory:

cd ~/fridge-control

Start the IOC:

softIocPVA st.cmd

The IOC shell should appear:

epics>

List the loaded records with:

dbl

Testing from an EPICS client

Open another terminal and activate the EPICS environment:

mamba activate epics

Check the main settings:

pvget FRIDGE:TEMP:ACTUAL
pvget FRIDGE:TEMP:SET
pvget FRIDGE:TEMP:HYST
pvget FRIDGE:PUMP

For the current development stage, the actual temperature can be changed manually to simulate the fridge becoming warmer or colder:

pvput FRIDGE:TEMP:ACTUAL 6

With a setpoint of 4 degC and hysteresis of 1 degC, the pump should turn ON.

Move the temperature into the hysteresis band:

pvput FRIDGE:TEMP:ACTUAL 4

The pump should remain ON.

Then simulate a sufficiently cold fridge:

pvput FRIDGE:TEMP:ACTUAL 2

The pump should turn OFF.

The control state can be inspected using:

pvget FRIDGE:TEMP:WARM
pvget FRIDGE:TEMP:COLD
pvget FRIDGE:THERMOSTAT
pvget FRIDGE:PUMP

EPICS concepts demonstrated

The project currently demonstrates:

• EPICS database records
• Reusable PV prefixes using macros
• Analog input (ai)
• Analog output (ao)
• Binary output (bo)
• calc records
• calcout records
• Record links
• CP (Process on Change)
• Automatic record processing
• Configurable hysteresis
• Stateful thermostat control
• Soft IOC operation
• PV Access clients (pvget, pvput, pvmonitor)

Planned development

The project is not complete.

Next steps include:

• Simulate automatic temperature evolution
• Make the pump gradually cool the fridge
• Simulate temperature rise when cooling is off
• Add a door sensor
• Add automatic interior-light control
• Add door-open timing/alarm logic
• Add high-temperature alarms
• Test the complete closed-loop simulation
• Build a Phoebus Display Builder GUI
• Add live temperature/status visualization
• Add temperature plotting/trending

Phoebus

Basic communication between Phoebus on Windows and the EPICS IOC running in WSL has already been tested successfully.

A Phoebus Text Update widget was able to read an EPICS PV from the WSL IOC and update automatically when the PV changed.

The complete fridge operator interface will be developed after the IOC/control logic is finished.

Status

Work in progress — automatic thermostat checkpoint.

The project currently contains a working configurable thermostat with hysteresis. The next major step is to simulate the physical temperature response of the fridge so that the system becomes a complete closed-loop simulation.