Design of a Fan controller

Hi all I am new to this group. I am pursuing my masters in electrical engineering. I have opted a control systems course in which I am facing
difficulties. I am doing a small project to design a fan controller like a cooling system present in the cars. I am not good at designing the control systems. I want design it in Matlab/simulink I am good at simulink. I have a block diagram which gives the idea for the project. Can any one help me out in getting started with the cotroller design and what more specifications should I consider for the design. ( i have block diagram but could nto post here as it does not allow) The basic working of the cooling system is, initially the cold water/coolant flows from Radiator through the water pump and gets warmed passing through the oil cooler and flows through the engine where it gets heated. An electric thermostat is used to by pass the water either into radiator where the coolant is cooled by a electric fan (20kw, 2000rpm) and is cycled back into engine through the water pump and oil cooler, the thermostat may also block the flow of coolant to the radiator and directly send into the water pump if the engine is below the operating temperature and when it reaches the operating temperature the thermostat opens and the coolant is passed through the radiator allowing it to cool down. There are temperature sensor that reads the temperature one loacted at the outlet of engine i.e before the thermostat, second one after the water pump and third after the oil cooler and allows us to design the controller for the model. The electric fan is variable speed fan and its speed is controlled by the speed command The position of thermostat and speed command for the fan is PWM signal (0-100% duty cycle)
Given Requirements 1.    Cooling fan mounted between radiator and engine (20 kw, 2000rpm) 2.    The performance of the fan would be same for the both directions 3.    The coolant temperature should be between 120 c to 140 c 4.    Max engine outlet temperature is 110 5.    Max radiator inlet is < 104 c and delta T across rad is 8 c 6.    Oil temperatures around 70 c to 100 c (Max temp allowed is 113 C) 7.    Air temperatures (predicted exiting the radiator) app 80 c 8.    Max heat sink temp <70 c 9.    Max ambient temperature within cool box : 85 c 10.    Min ambient temperature within cool box -40 c 11.    Flow of the coolant can also be controlled by the speed of the water pump
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Hi all I am back please dont feel my problem is lenghty. I do appreciate for your time and help. Regards kumar
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kumarkk wrote:

These specs don't make sense to me.

OK. I assume the typical cross-flow arrangement.

How might it reverse? (A fan whose performance is the same in both directions is necessarily inefficient. Efficient fans have curved blades.)

Measured where? There are three locations.

So how can the coolant temperature be as high as 140C? (BTW, the vapor pressure at 140C is about 38 psi, more than most tires carry.)

Surely, the radiator inlet temperature is substantially equal to that of the engine outlet. Delta T depends on the air flow. If the flow rate doesn't matter, what is the fan for?

OK
OK
What heat sink?

Those are air temperatures? How does this relate to #7?

Do you have a model for how water and air rates affect heat transfer?
Jerry
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Hello Jerry thanks for your reply and happy for the support i would look these questions with specs and my prof and will get back. Hope to continue the same support thanks Regards kumar
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Hello Jerry I will collect the info needed actually i donto have any model for the water and air rates affecting thr heat tranfer is that a compulsion. I have some data which says Fan mechanical power Vs Fan speed (Is this useful). Could you please lemme know what more should i need to collect for the model. Regards Kumar
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kumarkk wrote:

I don't see that you can design a system that can meet the specifications. For one thing, they seem contradictory. For another, you don't know the input/output relations of your system variables. The specifications give radiator's delta T without specifying whether the fan is off (we must guess that it is on) or the speed of the pump. There is no indication of how pump speed affects this. (Slower circulating speed exposes coolant in the radiator to cooling air for a longer time, so it emerges cooler _for the same inlet temperature_ but the coolant remains longer in the engine, and so emerges hotter. The effects offset one another. Usually, faster circulation brings better cooling, but not proportionally. You don't have nearly enough information to design to a specification. Perhaps that's what your professor wants you to say.
In the other hand, you can build something that has a good chance to work. The thermostat that controls how the coolant circulates already does most of the job. When the coolant gets too hot, turn the fan on. When it is cool enough, turn the fan off. That will keep the engine from damage while you make the measurements needed for a more sophisticated design. Real engineering is done that way. You need accurate numbers for _this_ particular engine/pump combination, and _this_ particular radiator, oil cooler, and housing. There are rules of thumb that can provide an approximation, but the actual numbers need to be measured.
Jerry
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Hello Jerry I really appreciate for the time and patience you are taking to educate me and helping me thanx for the support.
I have visited my prof so he explained me the specs so I am showing the changed ones 1. The performance of the fan would be same for the both directions

change when reversed and here he asked me to consider only one direction assuming it pulls air from the radiator. 2. The coolant temperature should be between 120 c to 140 c >>This really did not make sense to me too so we changed that it would be max engine outlet(around110) 3. Max engine outlet temperature is 110 4. Max radiator inlet is < 104 c and ( delta T across rad is 8 c -- he agrees it depends on the flow rate and incr in flow rate affect the heat transfer)

outlet max, giving about 104 cmax radiator outlet, which would then be inlet to fan motor( I did not get in right way could you explain) 5. Air temperatures (predicted exiting the radiator) app 80 c OK 6. Max heat sink temp <70 c

temp at or below 70 c and the fan control electronics to monitor heatsink temp to provide thermal protection for fan power electronics 7. Max ambient temperature within cool box : 85 c Min ambient temperature within cool box -40 c

I am looking at your recent reply for the input/output relations Thanks a lot Regards Kumar
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kumarkk wrote:

change when reversed and here he asked me to consider only one direction assuming it pulls air from the radiator.
What kind of 20 KW motor that needs circulating liquid coolant is reversible? Or is the fan motor 20 KW (about 25 horsepower)?

Does he give any hint about how it changes? There are tables for cross-current heat exchangers, but I don't think you are given enough conditions to use them. Google ans see what you find. What mechanism allows the radiator inlet temperature to be lower than the engine outlet temperature? Another heat exchanger?

outlet max, giving about 104 cmax radiator outlet, which would then be inlet to fan motor( I did not get in right way could you explain)

temp at or below 70 c and the fan control electronics to monitor heatsink temp to provide thermal protection for fan power electronics
What heat sink? Does the motor cooling system also cool the electronics? How?

Well, good luck! Maybe all will be clear in the end.
Jerry
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[snip]
Radiator exit temperature would normally be somewhat lower; around 90 degrees C; maximum. Coolant shouldn't get too low upon exit; unless you have blend control at the thermostat which bypasses some coolant past the radiator to mix at the thermostat to prevent thermal shock to the enging block.

Inlet to fan motor?
The fan causes the amount of airflow through the radiator to change. When the radiator exit temperature becomes too high, then the fan must be switched on or sped up to reduce the exit temperature. If the temperatures keep rising beyond tolerable at the engine, then an alarm should sound to prevent catastrophic engine damage.

That's far too sensitive to temperature. Electronics for automotive use have a minimum requirement to survive 85 degrees C for extended periods. Power transistors tend to have a greater maximum temperature range; not that you should need that with Rds-on in the milli-ohm range on power MOSFETS. Even nicely "rounded" PWM should not be shedding many Watts of electrical power when driving a large fan.
I believe that the whole approach is confused and inherently flawed. It will at best result in a system that only just works; if at all.
It looks like you need to take a step back and review the specifications; figure out what they really mean in the physical world and figure out what's important. Specifications are often "excessive".
In my view; the parameter that actually NEEDS to be controlled is the temperature of the coolant going into the engine.
A vastly-simpler approach is to measure the engine's coolant inlet (post-thermostat) and outlet temperatures. Regulate the thermostat's outlet temperature by radiator fan speed to approximately some intermediate point of its opening range (say 2/3). By regulating to some intermediate point, you can identify certain system "failures", including insufficient cooling capacity.
The engine coolant outlet temperature gives some "predictive" indication of increasing or decreasing cooling requirements but the closing point of the control loop should be the coolant inlet temperature into the engine itself. As such, the engine outlet temperature can be ignored as a first-cut.
The engine's waste heat output (reflected in the temperature difference from inlet to outlet) presumably varies widely; being the reason to have a variable cooling system in the first place. The thermostat determines an engine's coolant inlet temperature; within the cooling capacity of the engine and ambient conditions.
The radiator fan alters the ambient conditions.
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Hello Bernd Felsche Thanks for the suggestions. Now you guys are making me clear of the confused requirements i had. I would once agin sit on that in the mean while if any body can suggest me how to start the design with min requirements as you said only considering the engine inlet and outlet temperatures and regualting the speed of fan to maintain the coolant temperature and preventing the engine from getting over heated. Regards Kumar
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Start with a "classical" PID control loop based on just the thermostat outlet temperature; with the nominal set-point about two-thirds of the way through the thermostat's nominal operating range.
Vary the fan speed to maintain the set-point. Due to the thermal mass of the system, time constants will be quite long. The integral and differential terms of the control system can be ignored initially, giving you a simple proportional fan speed relationship.
Depending on how quickly ambient and load conditions change, that MAY be sufficient to maintain coolant temperature within a few degrees C.
If conditions change rapidly, then the coolant outlet temperature may be a better input to the control loop to obtain "differential" control; i.e. react quickly to fluctuations in waste heat being dumped into the coolant; to be disposed of in the radiator. That temperature measurement doesn't have the same "lag" as the thermostat outlet and fan speed can be changed "in anticipation" of coolant arriving in the radiator after a rapid change in engine outlet temperature.
But for anything more complicated than the PID based on thermostat outlet temperature alone, you MUST have a thermal model. The thermal model must have the heat flow into the coolant; the coolant flow rate; the radiator's operating (convective heat) properties; and ambient air conditions.
On second thoughts; you should have a basic thermal model anyway. Any control system design requires that you model the system being controlled.
As a first APPROXIMATION, you can use the engine's maximum power output at a particular speed (if it's a variable speed application); or the actual load power (for a constant-speed application) as a guide to how much heat will need to be rejected by the radiator. It'll be of the order of the GROSS output power of the engine (depending on technologies); so you have to add loads like oil and coolant pumping power needs to obtain the gross output figure.
The rate of heat rejection by a particular radiator depends on ambient air temperature and speed; as well as the rate of coolant flow through the radiator. If the coolant pump is driven off the engine's crankshaft, then you can use the engine speed as a metric of coolant flow rate in your control algorithm... but the relationship is NOT linear; simply monotonically proportional over the interesting operating range.
I believe that I've mentioned bypass flow before; i.e. not all coolant flows through the radiator so the real system will have to determine what proportion of coolant flow passes through the radiator to be "dead accurate" in controlling temperature. A measurement of radiator outlet temperature, in addition to thermostat and engine outlet temperatures is sufficient to calculate the proportion.
I could go on and on about how one could potentially improve the control of engine coolant temperature; but there are two things to remember:
1. A simple system that's "good enough" is preferable to a complex one that's expensive to implement and difficult to understand. (KISS Principle - "Keep It Simple Stupid")
2. It's not my Master's coursework. :-)
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Hello Bernd Felsche Thanks for the suggestions. Now you guys are making me clear of the confused requirements i had. I would once agin sit on that in the mean while if any body can suggest me how to start the design with min requirements as you said only considering the engine inlet and outlet temperatures and regualting the speed of fan to maintain the coolant temperature and preventing the engine from getting over heated. Regards Kumar
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kumarkk wrote:

First, please excuse my confusing inlet and outlet temperatures at the radiator. Second, I must point out that the availability of two means of control (pump speed and fan speed) without any information about how they interact greatly complicates the problem. Simplify it by fixing the pump speed at its highest practical value. (That minimizes temperature variations within the engine, usually a good thing.) It is wise to tentatively select a scheme before concentrating on details.
One way is turning the fan in at full speed when the coolant reaches a particular temperature, and turn it off again when the temperature drops below some lower temperature. This is how most modern automobiles are cooled. (The fan is rarely on at highway speeds because the cars motion creates enough airflow.)
Another way, perhaps more appropriate to a controls course, is varying the fan's speed so that it increases as the coolant temperature rises. It might start at low speed when the coolant temperature rises above the lowest permissible temperature, and increase with temperature to maximum speed at the highest permissible temperature. (Good design will make that temperature somewhat lower than the specification to allow for calibration error and sensor drift.)
Jerry
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As you've already worked out, the first stage in any control project is to understand the physical behaviour of the 'plant'. It might comfort you to know that as that increases, your understanding of how to control it will also. I wouldn't be too worried about the apparent difficulty of the problem at this stage. Just work through defining the behaviour of the system, plus your control objectives.
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Frankly, as a Master's student, you shouldn't be posting required specs and asking for an approach. You should be posting specs, telling us about what you've tried, the high points and low points of the output behaviors of the approaches you've tried, and what direction you see your next approach taking.
Scott
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Scott Seidman wrote:

Unfortunately, Scott, that depends more on the school than on the student. I knew a Ph.D. in EE who believed that curving one side of the symbol for a capacitor was a matter of artistic license.
Jerry
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That's an American thing - UK capacitors have straight "plates", except for electrolytics, which have the positive plate "thickened" and a + alongside. I was never sure what the meaning of the curved "plate" was - maybe it was just a cultural thing alongside valve/vacuum tube, anode/plate - reflecting parallel development of electronics on both sides of the Atlantic up until about 1970, since when the US has dominated and recent electronic vocabulary and symbology has become unified.
JPG

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JPG wrote:

The curved plate has exactly the opposite significance of the British thickening. It is the negative end of a polarized -- usually electrolytic -- capacitor. Because the US and British conventions differ and each convention is used in several countries, the plus sign is added. My generation rarely uses it, but we're on the way to extinction.
Jerry
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