Pool Systems Automation

Overview

A monitoring and control automation system was installed in the pump house during the 2020-2021 off-season. As of June 2021, continued refinement is still on-going. It was custom designed and installed to aide in the management of the pool, ensuring the pool is safe and comfortable for all occupants while reducing effort for the staff.

Background

The project was officially kicked off in December 2020 with the following goals:

• To alert staff and key maintenance personal of abnormal operating conditions or situations requiring attention.
• To protect aging equipment (i.e. the boiler) from damage due to malfunction.
• To reduce staff workload by automatically filling the pool.
• To record historical data for pressure, flow, temperature and chemical levels.

The following principles drove and will drive all future design efforts:

• The system must not hinder or add complexity to daily pool maintenance.
• In the event of automatic system failure, manual systems (gauges, valves) should be usable to perform all tasks.
• The system must be very well documented and maintainable by moderately technical member-volunteers.

Operation

Monitoring

Main Pump Motor

The main pump motor is monitored via an auxiliary contact on the motor starter. The contacts close upon pressing the Start button, assuming sufficient voltage is present and the overload is not tripped. The contacts open upon pressing the Stop button, or when another motor-stopping event occurs.

The main pump motor auxiliary contact and a snapshot of the flow rate (via the flow meter) determines the operating state.

State	LCD Notification	Condition(s)	Stack Light
Pump Stopped	Pump Stopped	The default state upon initialization. Set when the pump is stopped.	Red (Flashing)
Pump Startup	Pump Startup	The pump is starting up (a 10-second static delay once the motor starter contacts are closed).	Off
Operating	Pump Operating		Green/Yellow (Flashing)
Backwash			Alternating Red-Green

Filter Tank Pressure

Four filter tank pressures are monitored by sensors with internal signal conditioning. When fed by a 24 VDC (each sensor is independently fused), a 1 V to 5 V output corresponds with 0 to 100 psi of pressure.

A value is considered valid if the measured voltage is 1 V or greater (less than 1 V suggests an open circuit or failed sensor). The low and high thresholds are somewhat arbitrary, and based upon observation. A nominal pressure of 10 psi is assumed (add 8 psi to the limit per PHTA guidelines for a single-gauge backwash threshold).

Qualifier	Threshold	Result	LCD Notification	Stack Light
Operating	< 8 psi	Pressure Low	Flt Tank Pres L	Yellow
Operating	≥ 8 psi and < 18 psi	Pressure OK	None	Green (unless overridden)
Operating	≥ 18 psi	Pressure High	Flt Tank Pres H	Yellow

When in the Pump Startup, Operating or Backwash state, each tank pressure is briefly shown on the LCD in psi. If the value is not valid, Fault is shown in place of a pressure value.

Control

Auto-Fill

The pool is automatically filled when a low threshold is met via the surge tank water level sensor.

User Interfaces

Stack Light

The stack light indicates the overall operating state of the system.

State	Description
None	The system is off or the pump is starting up.
Solid Green	The pump and system are operating nominally.
Flashing Green	Same as Green with active pool fill.
Solid Yellow	The pump and system are operating but intervention may be required.
Flashing Yellow	Same as Yellow with active pool fill.
Solid Red	The pump is stopped by the operator.
Flashing Red	The pump is stopped due to the surge tank level.

Design

Programmable Logic Controller

A DirectLogic 06 (D0-06DD2) programmable logic controller (PLC) is used for as the base for monitoring and control. A PLC was selected due to its reliability and relative ease-of-maintenance by volunteer maintenance staff. Within the electrical engineering community, most have had some level of experience with PLCs and/or ladder logic. It is assumed that within the membership, there will always be someone with the knowledge that can volunteer on an as-needed basis.

The controller consists of an internal CPU, 24V inputs/outputs (I/O) and multiple add-on modules for analog measurements. Serial communication carried over WiFi via a Lantronix PremierWave XN, presenting the PLC as if it were connected to a virtual communication port on the host PC.

Digital Inputs

Input	Function	Description
X4	Surge Tank Float	Not presently implemented; reserved for future use.
X5	Motor Starter Status	Active when the main pump is operating.
X10	Boiler Stage A	Active when conditions described in Pool Boiler Stage A: Demand for Heat are met.
X11	Boiler Stage B	Active when conditions described in Pool Boiler Stage B: Flow/Temperature Interlocks are met.
X12	Boiler Stage C
X13	Boiler Stage D
X14	Boiler Stage E

Digital Outputs

Output	Function	Description
Y4	Stack Light Red	Drives the red element of the stack light.
Y5	Stack Light Yellow	Drives the yellow element of the stack light.
Y6	Stack Light Green	Drives the green element of the stack light.
Y10	Fill Valve	Drives the pool fill valve into the open position (spring-return when off).
Y11	Motor Starter Stop	Stops the main pump by mimicking "stop" button closure.
Y12	Boiler Disable	Disables boiler operation when a significant fault condition is observed.

Expansion Modules

Slot	Part Number	Function	Purpose
1	D0-07CDR	4 x 12/24 VDC Input, 3 x Relay	Not used at this time (spare)
2	F0-04AD-2	4 x Analog Input (0-5 VDC or 0-10 VDC)	Sensor inputs (pressure)
3	F0-04AD-1	4 x Analog Input (0-20 mA or 4-20 mA)	Sensor inputs (flow, depth)
4	N/A	Not Populated	Spare

Serial Interfaces

The DL06 PLC has two serial interfaces. On this unit, they are configured as follows:

Serial Interface	Electrical Configuration	Serial Configuration	Purpose
Port 1	RS-232	9600 8O1/None	Programming
Port 2	RS-232	38400 8N1/None	Modbus Polling

Sensors

The following points, as shown in the Process Flow Diagram, are monitored by the PLC:

• 4 x Pressure Transmitter (PT) units, one per filter tank. Each unit is connected in parallel to a non-electrical gauge.
• 1 x Flow Transmitter (FT) unit prior to the pool return. A manual gauge is placed in the vicinity of this paddle-wheel sensor.
• 2 x Depth Transmitter (DT) units, placed near the bottom of the surge tank and in the static level probe well. The latter is optional (may not be implemented).

There are additional sensors, not directly monitored by the PLC, but considered to be part of the system:

• 1 x Temperature Transmitter (TT) prior to the inlet of the boiler. This unit is used as the feedback for the temperature controller, which raises a call for heat to the boiler.
• 1 x Flow Transmitter (FT), pH probe, ORP probe and conductivity sensor. These sensors are fed into the Chemtrol PC2100 and are detailed here.

The following sensors are used:

Parameter	Sensor Part	Range	Output
Pressure	Measurement Specialties MSP-300-100-P-4-N-1	0 to 100 psi	1-5 V analog
Flow (Output)	GF Signet P51530-P1	0.3 to 6 m/s 1 to 20 ft/s	Sine Wave (analog)
Depth	ALS-MPM-2F	0-5 meters	4-20 mA
Temperature	TBD - PT100 RTD	-40 to +400 degC	Resistive

NOTE: The flow meter was previously a GF Signet MK515.3P1. Erratic behavior prompted its replacement in early June 2021.

Drawings

Drawings have been updated as of June 24th, 2021 with multiple corrections and as-built updates. They are still in-process and subject to review and change.

Panel Layout

Pages 5-9 are reserved for enclosure panel layouts.

Sensor Wiring

Pages 10-19 are reserved for sensors (including "flow meters") and their associated wiring.

Programmable Logic Controller

Pages 20-29 are reserved for the PLC and its expansion modules.

Auxiliary

Pages 40-49 are reserved for systems related to the automation system (via interconnection).

Known Issues

• Detection of backwash (versus operating) is occasionally incorrect. This is likely due to the ramp-of of the flow meter.
• Some PLC LCD time display screens are buggy.

Planning

The following features are planned when possible:

• Implement time-of-day auto-fill to reduce the frequency at which the pool is filled during the day. The club is not billed at a time-of-day rate, but as "good citizens" it is ideal to fill during low-demand periods to help reduce water bills for all city residents.
• Add a float switch as a failsafe means for detecting over-fill. In the event that the depth sensor fails.
• Add variable frequency drive (VFD) control for the main pump motor, when and if implemented.

Surge Tank Modulating Valve

If the budget permits, a spring-return modulating valve will be installed on the drain return into the surge tank. This will allow consistent level control via water level feedback. This will restore intended surge tank functionality (maintaining the proper surge tank level regardless of pool level).