Small Hydro Power Plant with Elixir

Part 2

Nerves Conf EU

2025/09/10

1

Outline

• System map, network topology and devices
• Communication between nodes
• Generator Control and fault monitoring
• Off-site & On-site working

2

System map

3

MODBUS devices

MODBUS devices

Hub

Hub

Erlang Cluster

SDC

PGC

DIO

Temp

Volt

RPM

DIO

Volt

SoC

optical fiber

turbidity, water level

SoC: battery’s State of Charge [%]

Control Plant

Collects and pushes data to WEB

4

PGC

Hub

Bus line Multimeter

Gen line Multimeter

Rotation meter

DIOs

DIOs, temps

Modbus TCP

Device

Modbus TCP/RTU Converter

RS-485 bus

Why we chose MODBUS TCP

• CONS
• The data are not pushed from the MODBUS device
• We must poll the data periodically

• PROS
• Everything is IP based, independent from the `devices`
• This plant may be used for decades�we may need to change the Nerves device itself
• Many devices support MODBUS TCP/RTU. So they're interchangeable.

5

Momentary switches without interrupts

The switches on panel can work with 100ms data polling loop.

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For this system, this duration is sufficient.

6

Which MODBUS TCP library we use

We choose our own way.

Make things under control, this is the core library of our system.

If there is a bug, that’s our bug. ��We can immediately fix it ourselves.

7

Modbuzz

• Support both TCP/RTU
• use :gen_tcp for TCP
• use Circuits.UART for RTU

• Can make gateway

• Support both client/server

Give it a try�and give us feedback

8

Communication between nodes

9

MODBUS devices

MODBUS devices

Erlang Cluster

SDC

PGC

DIO

Temp

Volt

RPM

DIO

Volt

SOC

GenServer.call with {:global, term}

name registration

:erpc.call(node, module, function, args)

Let’s dig in about control

We don't use any PLCs to control. �So, how do we control the plant?

10

Input Valve

Needle Valve

Writing `Sequence` with GenServer

11

part of Run sequence

12

part of Run sequence

How it works

13

Generator

Power

Bus

Power

Adjust to sync with the bus line

• Voltage
• adjusted by AVR device

• Frequency
• adjusted by actuator

• Phase
• just wait for a match

14

Fault monitor checks ex.

• Node connectivity
• Head tank water level, turbidity
• Generator coil temps, bearing temps, over rev
• Protection relays like OCR, OVR-UVR, UFR, …
• C: Current, V: Voltage, F: Frequency

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Check their values and run the fault sequence if needed.

15

But what if the fault monitor crashes?

Stop safely even if what happens 1

• When the fault monitor crashes.

16

FM

FMM

Fault Monitor Monitor

FM

FMM

FMM detects it and

invokes `EmergencyStop` sequence

Stop safely even if what happens 2

• When our app fails and a Nerves device fails.

17

Make heartbeats and monitors them with primitive devices.

When the beats stopped, invoke `EmergencyStop` circuit.

Off-site working

• Test programs using Mox and Mock

• Design system ⇔ read devices spec

• Confirm devices behavior and write the program

18

On-site working

• Integration tests

• We faced several issues what we didn’t know off-site, ex.
• the 3rd needle valve didn’t work due to an initial defect
• the needle valve opening required to stabilize 1200rpm (before sync)
• the delay between needle valve operation and water level response

• What helped us,
• Write code that is easy to change
• mix upload.hotswap to change code on the fly
• LiveDashboard metrics

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mix upload.hotswap

• Can replace the code on a running Nerves�without `mix upload firmware`

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• NOTE: The updated code is not persistent

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• We used this for�RPM hover logic / water level control logic …

20

PGC

LiveDashboard

• Metrics tab
• This idea was introduced by Powell at a Nerves meetup, LiveView and Control Systems

21

Processes tab

Our happy moment

22

Let’s keep building fun things with Nerves.

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Thank you.

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