Integrated Control Systems

Part 2

Getting the Most from an Integrated Control System

The biggest problem with automated greenhouse controls is that they often work so well there is a tendency to take them for granted. Once your system is installed and configured, you can leave the greenhouse pretty much on autopilot as far as climate control is concerned. However, you can get much more out of it if you decide to take the controls yourself from time to time, and develop a system of routine 'in-flight' checks. For this you need to understand how your controller works, the things it depends upon, and a few things you can do to ensure maximum performance of your operation.

Selecting Greenhouse Equipment

If you are building or replacing equipment, and you know that you are going to be using a computer controlled system, try to find equipment that is well suited to automated control. Most manufacturers of greenhouse control systems will be happy to provide advice about equipment and equipment control options. They have a wide range of experience with controlling all types of greenhouse equipment and can often suggest alternatives that will provide the best performance in an automated setting.

Equipment Compromises and Workarounds

Greenhouse equipment and equipment systems are seldom engineered as an integral parts of a whole. Most greenhouses consist of a collection of ‘off-the-shelf’ solutions that represent the owners' and builders' best choices given what was available, affordable, and practical. Quite often compromises have to be made. Individual components are not inherently designed to work in harmony with other greenhouse processes. One of the advantages of using integrated control systems and the challenges for their manufacturers is to find control strategies that can help offset certain shortcomings in equipment.

Some examples include:

	If you have an exhaust fan that is too large for venting on some conditions, you may be able to use your computer system to cycle it on and off to achieve an in-between effect.
	If you have a limited water supply, you can use the system’s capacity management features to limit the number of irrigation tables that can be watered at once.
	Using feed forward calculations, integrated systems can compensate for the thermal lags associated with hot water heating supplies and correlate this with outdoor weather events to achieve the smoothest control possible.

Know Your Equipment Limitations

Although a certain degree of compensation is possible, a control system can only work within the absolute limits of the machinery that it controls. If your heating and cooling setpoints are too close together, you could end up wasting energy and wearing out your equipment prematurely as the system tries to heat and cool simultaneously or in ‘a vicious circle’. Likewise, if your boiler is undersized, no amount of computing power can compensate. However, if there are problems with your equipment design, you might be able to use the flexibility and power of the control system to monitor and measure equipment performance so that you can pinpoint where your weakest points are and address the need for equipment updates in order of priority.

Try to make sure that the equipment you purchase can be controlled with sufficient accuracy to produce the results you need. This has a lot more to do with equipment precision than the method of control. Once you have accurate equipment that can deliver repeatable and predictable results, chances are you can get good control using  a variety of control strategies and options.

Sensor Placement

No matter how accurate your sensors are or how many of them you may have, they won’t do much good if they aren’t representative of the conditions you are trying to measure. For example if you hang your climate sensors near a doorway that tends to get left opened a lot, and you place them several feet above the crop surface, your climate control system will likely do a good job of maintaining the climate setpoints for this spot. However, it probably won’t reflect the actual conditions that most of your crop is experiencing.

You can always get more sensors and scatter them around, but chances are you’ll get a different reading from every one. Which one is correct? Better to find a single spot that is representative of the average conditions you want to monitor. It will usually be somewhere central, and in the case of climate sensors, in or near the crop canopy. You can still use other sensors to check the degree of variation in the climate relative to your selected control point, but you should only have one set of control sensors and they should be monitored frequently and their position adjusted as needed.  

Data Recording

If your system supports it, use the data recording functions to their fullest. By comparing recorded readings over time with your setpoints you can double-check on your settings and spot any problems that might be occurring. You can verify that irrigation programs are functioning properly, and that the system is keeping your climate within your established limits. If there are problems, recorded data can help you find the cause. For example, if the climate temperature exceeds your setpoints you can refer to recorded outdoor values to see if it’s simply due to a hot day, or whether there are problems with your heating and venting equipment.  

Proportional & Integral

Many computerized control systems use sophisticated programs or ‘algorithms’ to maintain setpoints. One of   these is known as proportional control. When a proportional strategy is used the system will increase the degree or response (heating, venting, watering etc.) based on how far away the actual reading is from the desired reading. For example, in a heating program, as the temperature drops below the setpoint the system will proportion progressively more heat to compensate.

Similarly, integral settings are used in a variety of situations to regulate the amount of control based upon how long a current value has deviated from the target. In a cooling situation, the longer the climate temperature remains above a setpoint, the more the cooling response (fans, vents, evaporative cooling etc.) that will be proportioned up to the maximum that is available.

Management by Exception

If your control system supports alarms you should make full use of them by setting up as much equipment and climate monitoring as possible. Although this doesn’t replace the need for preventative maintenance and regular equipment inspections, this ‘no news is good news’ strategy will help free up your time for more productive management concerns. It will also make sure that you are alerted of problems as soon as they occur, rather that as soon as someone notices.

Pulse Width Modulation

Pulse width modulation (PWM) is a control strategy option on some integrated systems that provides increased control accuracy by "pulsing" a piece of equipment on and off to achieve an average effect somewhere between full on and full off. The time period for one cycle (one "on" period plus one "off" period) must be short enough to minimize fluctuations in the condition you are trying to control (i.e. temperature, humidity), yet long enough to minimize premature wear on the equipment. It’s important to point out that pulse width modulation can’t be used in all situations since some types of equipment may be damaged or wear out prematurely if short on/off cycles are used. The quality of control achieved using pulse width modulation can often be as good as when using fully modulating equipment. When set properly, the overall number of equipment operations may not significantly increase, because temperature or humidity cycling (over/under shoot) can often be greatly reduced. Sometimes the energy savings and better control will outweigh the costs of any additional maintenance caused by extra equipment starts.

Sophisticated Irrigation

Most simple irrigation systems use time clocks as the basis for irrigation. With a computerized control system you can integrate time, weather, light, and moisture-based information for much more sophisticated control of crop moisture and nutrients.

Staying in Control

Because your control system will tend to faithfully look after things day after day with little trouble, it's hard not to become a bit complacent. This is the most serious problem with all automated systems. Whether they are opening doors at the grocery store or running nuclear power plants, they all need some supervision. Try to develop a routine checklist for reviewing your system, perhaps something similar to the preflight checks that pilots use. Although integrated control systems allow you the freedom to just  'set it and forget it', truly superior control will always depend upon someone knowledgeable checking in regularly and making adjustments as needed.