good day all,
I am an electrician with 30 years experience, I am in the process of trying to build a controller that will interface a room temp sensor, supply air sensor and modulate a damper and valve motor.
This will be for a class I am wanting to teach and I don't want to use a propitiatory controller from a company. I would like this to be generic so the students can actually see how it components operates with out getting bogged down in a specif company devices.
All the motors will 24 v, the control can either be 0-10 or 0-5 volt or 4-20 mA.
As in all systems the 24 volts will power the devices as a stand alone power source and not be regulated through the board.
I have no idea on how to begin the programming but I have the several devices in mind to use.
Does anyone have any thoughts on how the programming would go or is this even feasible with a Arduino.
a VAV controller is a bit advanced for a beginner.
in order to learn how to pogram an Arduino, you will need to get some switches and some LEDsm ertx
Motors : servos or steppers AND drivers
can respond as an analog postilion.
duct pressure needs to be monitored, and low pressure sensors are very expensive compared to the hobby parts we use.
air velocity is also expensive, probably more expensive than all the other bits combined.
is you were to create a flow-chart diagram of the VAV controller, we can help you replace the diamonds, rectangles and arrows with actual things. and the Arduino is really up to the task.
one of the places that you will find almost no help is in the PID loop for dynamic control. 99.9999% of the help is for things like temperature or water flow, (think glaciers moving or paint drying) duct pressure moves at the speed of sound. hint use only P & I. D is not the correct application, it makes rockets crash and planes disintegrate. 1/D or inverse derivative holds the key there but a bit beyond this stage of your journey.
Another part is the physical motor, linkage, damper. the 'engineering' for air handling is 50 years behind the current capabilities of technology. those in the know call HVAC engineering - casual engineering.
In order to alter volume, you have to create resistance. in HVAC a damper that is 20% open allows about 80% of the volume. that means that your damper actuator is trying to do all of it's control in that 20% range of movement. and there are few people who understand motor hysteresis as applies to final control elements (dampers and valves)
to control a VAV system, you have to first have rock solid control of duct pressure. that means really good control of the fan output. that means a variable speed drive on the motor is a good starting point, with detailed knowledge of the ramp up and down speed.
the VAV Box requires a stable duct pressure so the speed of control of the VAV box should not be any faster than 4x that of the ability of the fan controller to control duct pressure. a second VAV box will cause disturbances when under control so the slow control movements of the VAV box cause system upsets that effect other bits of the system.
Despite all the above, you will be able to get something to work and you will be able to make it yourself.
if you study some of the VAV boxes, the perforated plates that restrict air , the pressure independent control loop or mechanical device that creates a pressure independence of the device, you can replicate the actions 'close enough' to get it to work. And, probably every bit as good as what is on the market.
some engineering.
heat / cool is measured in BTU's
a human body has X BTUs when active and less when sedentary.
lights, computers, other machines have some BTU's
all you are trying to do with the air is to balance the BTU's input and output in the room.
you cannot control the heat, or measure it very well, so all you can do is add more cooling based on the room temperature.
the air changes per hour are part of the equation as is the air temperature of the air you deliver into the space.
when you think about that, when you add more cooling than there is heat, the room will cool.
when you stop adding cooling, the room will heat.
all you are doing it trying to make that balance stay withing a range where 60% of the people do not complain.
20% will always and forever be too cold and 20% will always and forever be too warm. the classic bell curve.
Thank you all for your input, I have purchased vav boxes model Johnson controls TSS-10-HWC.
It has a dx coil, dp air flow, and a damper.
My end goal is to install a supply air temp and a valve actuator in hopes I can raise or lower the supply temp sensor then watch the devices change states.
This will be for a lab that the students and begin to understand the sequence of operation.
All the equipment will be mounted on a board.
As for the air pressure we were going to use shop air and regulate the pressure as to simulate air flow across the coil.