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PLC and HMI Programming for a Plant Nursery

In a similar vein to this news post, a while ago we were asked to design an irrigation system for a nursery.
The project needed the following:

  • Humidity Control for 4 hot houses
  • Watering Control for 40 bays of plants
  • Graphical User Interface
  • Control via Internet and smartphone or tablet
  • SMS Alerts for faults
To implement this we supplied and programmed a Delta PLC with add on IO modules and an HMI with in built web access for iOS devices.


A PLC with enough IO for this project was not available so a PLC that could have enough IO modules added on was chosen.
In this case we used a Delta DVP series PLC.

Humidity Control

To control humidity we would normally specify a 4-20mA humidity sensor such as the RHT-003 but because the Humidity was being controlled with a fog mist and could reach over 97% RH (Relative Humidity) capacitive sensors like that used in the RTH-003 are not suitable and will fail within a short period of time.
To measure and control humidity above 95% RH requires the use of the wet/dry sensor method.
This means two temperature sensors are used, one is kept wet with a sponge and one sits in free air. The water from the sponge evapourates and lowers the temperature of the wet sensor. The difference in temperature between the wet sensor and the dry sensor is proportional to the humidity and can be calculated.
In this system we supplied a CMS-121 Wet/Dry Bulb Psychrometer for each humidity zone.
Each of these has two PT100 temperature sensors, therefore 8 PT100 inputs to the PLC were required, this was achieved with two DEC-105 4 Channel PT100 Input Modules.

Watering Control

Because a relatively large number of outputs were required we didn't use a standard stocked relay output module like the DEC-108 8 Relay Output Module but instead used two DVP32SN11TN 32 Transistor Output Modules with the associated cable DVPACAB7B10 and two DVPAETBOR16A 16 Relay Cards for each Output Module as these gave a better price per IO point (64 relays) and were available with a short lead time.
Each bay of plants that needed watering control already had a valve similar to the HER-400 24V Solenoid Valve installed.

Graphical User Interface

For the Graphical User Interface (GUI) we had the option of providing a dedicated HMI like one of these or a computer with GUI Software. In the end we chose an HMI with built in Ethernet and iOS support, where the GUI content and control are all available on an iOS device or PC.
The HMI GUI and PLC were programmed together to give a number of modes of functionality, some of which were:

  • Humidity calculation from wet and dry temperature sensors.
  • Temperature and Humidity display and control.
  • Enabling and Disabling all watering solenoids.
  • A number of watering modes and times for each plant bay.
  • Temperature and Humidity Logging and Trending.

If you have a similar project you need help with or would like a quote for PLC or HMI programming, please Contact Us

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