Integrating motor control, heater control, and solenoid valve control into one system

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Joined Mar 2, 2016
I am trying to build a circuit and components to accomplish the following: BLDC motor (with integrated controller electronics) control, temperature control of a heating unit and control of solenoid valves. I have attached a depiction of the basic setup in PDF and will explain it in detail. I have no electrical experience, so although I now have a basic understanding of what I may need, the specifics are eluding me. First, I’ll start with the diagram, starting from the bottom of the page and working up.

There are four boxes near the bottom, but these are basically place settings for what they are noted as. I have no idea about the power supply and what is needed there yet. Valve control is one of the areas I need help with. Heater control is another box, but it is probably the one I have the most understanding of and it is probably the simplest part. Finally, there is a fan control box which notes that there will be two fans (one shown at the top and one at the very bottom). The rest of the drawing is primarily known parts.

Starting from the bottom up, the first part is a BLDC blower (I have the specific part in mind and have all of the stats and ratings). This is the “hot” blower for ease of reference. Once the product is turned on, the fan will need to output a minimum of 1 CFM continuously (or go into standby, which will be discussed later). This blower has integrated control, so setting it up shouldn’t be too difficult. The problem I am trying to solve with this part is that I want the fan rate to be based on the number of valves that are active. There are four output valves (and two exhausts). Basically, when zero to two of the valves are open, remain at 1 CFM. When three are open, increase to 1.5 CFM and when all four are open, increase to 2 CFM. This would be constantly adjusted based on how many of the four valves were open at any given time, but wouldn’t go less than 1 or more than 2 CFM. I don’t know how to convert “number of valves in operation” to changing RPMs of blower, so that is the electrical question here.

The next block is labeled “flow”. This is a basic flow switch placed here to trigger a heater shut down should flow drop below 1 CFM in order to protect the heater. I currently plan on using a flow switch pre-programmed to “fire” at <1 CFM, but it is ~$40 each, so a simpler/cheaper way would be appreciated (maybe a board based flow sensor/switch?).

The heater itself is a 1000 watt, 100 volt industrial process heater. I will need to either develop a controller for it or use an off-the-shelf PID controller. I don’t really need all of the functionality of a PID controller though. All I need is a set temperature (190 Celsius) to be maintained whether running at 1 CFM or 2 CFM. I do need fast response times though. I know the heater is much larger than needed for that temp, but they are fairly slow to ramp up, so I need a large overshoot to compensate. I need the temp to go from room temperature to 190C as quickly as possible and back to 190C as quickly as possible whenever there is a change in flow (valves opening or closing). I am currently looking at a cheap PID controller and a cheap SSR for control (along with the flow “kill” switch and the next “temp” box), but again, I think it might be cheaper if I just made this part of the circuit as I don’t need display outputs or the ability to change the temperature or anything else.

The next block is “temp” and this is just a K thermocouple probe through a compression fitting at the exhaust end of the heater and will be used to adjust the output temp.

The next section is a circle, a half circle and 6 boxes with arrows. This is a basic design of the manifold and valves. The four around the main manifold are normal closed and will provide the work of the system. The other two are the exhaust valves and they are normal open. The half circle is just a depiction of the manifold to get the flow to those valves and can be ignored. When first started (all output valves closed), 0.5 CFM will be exhausted from “A” and 0.5 CFM will be exhausted from “B”.

From here, we will deviate to the left and right. All four output valves each have a corresponding kill switch and momentary tactile switch (on/off switch). The kill switch is a spherically activated tact switch that is located in the main body of the apparatus. When the laboratory slide (not part of this drawing) is inserted into the apparatus, the tactile switch is activated. It needs to be set up so that the valve associated with this switch can’t be activated unless the slide is in place. Additionally, the momentary tactile switch has an LED in it and I would like that to remain dark as long as there is no slide in place. The momentary switch itself should control the associated valve by cycling open every time the button is pressed and remain open as long as it is held down.

Finally, the upper fan. This “cold” fan is similar to the previous, but will be cheaper and won’t have integrated controls. When the heater is on, this fan will be on as well. This one will run at a higher rate though. There will be a total of 5 valves attached to this fan. The first will be normal open and will most likely remain open all of the time. 1 CFM will be blown through this valve and redirected into the body of the apparatus to help cool the SSR and internal electronics. The other four valves will work in tandem with their “hot” counterparts. If hot valve #2 is opened, cold valve #2 will also open. The fan will increase RPMs to output an additional 1 CFM (so if one valve open there would be a total of 2 CFM… 1 CFM to the body and one to output). Maximum flow for the fan would be 5 CFM. I am not describing the non-electrical components here in detail, but basically, after the hot air flows through the slide, it will be combined with cold air at a 2:1 ratio.

I am going to have to pay someone to design this for me, of course. The reason I am posting here is to get a better idea of what I need so I can say exactly what I am looking for. My hope is that I won’t get fleeced by “development” time if I already have a good idea what I need for this to work.

All of that said, here are the questions I know to ask:

1. How difficult will it be to get the fans to adjust RPMs based on the number of valves operating?

2. Can two valves be triggered by one tactile switch (along with the fan RPM increase)?

3. Does the “cold” fan need integrated electronics to accomplish the RPM changes and, if not, can the “hot” fan do the same? The integrated electronic controllers are expensive and since I am not using completely variable flow, I’m not sure if they are even really necessary.

4. Is BLDC really necessary for this application? I am using CPAP blowers due to their long life and reliability, but based on my current design, I know longer need the blower to change speeds based on pressure gradients, so I don’t even know if it is worth the cost.

5. Is it reasonable to expect that if valve 1, 2, 3 or 4 is opened, exhaust "A" can be closed and if two valves are open, exhaust "B" can be closed? I would assume there would need to be some computation there, so I don't know how that would work.

6. Please comment on anything that you see I missed here or questions I don’t know to ask. This is especially important from a safety standpoint. For example, I mentioned standby. If the apparatus hasn’t been used in XX minutes, the heater will shut off and the blowers will remain on for XX minutes to help with cool down. Obviously, I will need a clock/timer integration, but have no idea how that works... pointing things like this out would be helpful.