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Need recommendation for rheostat or potentiometer

(*steve*)

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So, have you considered my suggestion of mechanically restricting the airflow?

Another option is to vector part of the airflow where it won't do any harm and won't provide thrust.

It's probably easier to do the former.
 

hevans1944

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Sorry for the confusion. I didn't really focus on the fans because we are all set on that front. We just need an effective and safe way to control the speed of one of them.
You are NOT all set on that front. Controlling the volume of air produced by the "push fan" is the problem, and your "solution" is to starve the fan for power to slow it down with a variable resistor. That's right up there with controlling the speed of an automobile with the brake pedal while keeping the accelerator pedal depressed at full-throttle. I suppose that is simple and valid for a short duration contest, where wasted energy is irrelevant, but nailing the time to reach the finish line (from 5 to 25 seconds) by trial-and-error is the goal. It's not good engineering in my opinion, but what the heck do 8th grade science students know about engineering?

We expect that most contestants will vary the amount of thrust by mechanically varying to flow of air to/from the push fan.
This is definitely the way to go in my opinion, as was earlier suggested by @(*steve*) in his post #12, and later in his post #21, to which you have yet to reply. All three of his points were spot-on from an engineering design point-of-view. For a basic design, all you need are Venetian-blind louvers at the fan intake. Or, you could try making an iris diaphragm for the same purpose. With either choice, the push fan runs at full-speed, but the volume of air pushed out is limited by how much volume of air is let in. Since these are ducted fans, by the rules of construction, you could also add radial bleed vents for even finer control.

Why do you think a rheostat would provide better control? What are the rules of the contest? Would this be a project for the 2017 Science Olympiad?
 

CDRIVE

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A high wattage Rheostat solution is absolutely archaic! Besides the size of the beast have you considered the weight? Moreover, large high power Rheostats require probably >10 times the torque of a small Pot to turn them. I have some huge ones made in the 1940s that you damn near need a wrench to turn them. I like to think of them as variable heater elements!

I still don't get why a discrete PWM circuit is unacceptable. It's NOT an IC.

Chris
 

hevans1944

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I still don't get why a discrete PWM circuit is unacceptable. It's NOT an IC.

Chris
Chris, @NyeSpy in his post #20 posted a verbatim copy of part of the rules for the competition. With a little help from Google I was able to determine that this came from the rules for the 2017 Science Olympiad. The rules are protected by copyright so I won't post them here. Despite the copyright restrictions, some schools have scanned and posted copies. However, if you go to the Science Olympiad website you will find all the information you need to determine what this is all about. The rules, as well as kits for the various contest categories are available for purchase there. Below is a picture of the hovercraft kit available for purchase from Ward's Science. It includes a copy of the rules with the purchase price.

20296892.jpg


All this is admirable if it gets young people interested in pursuing Science, Technology, Engineering, and Math (STEM), although I suspect that any learning that takes place is incidental to the thrill of participating in a cooperative group against competitive groups. My oldest son in Atlanta supports FIRST Robotics Competition (FRC), which is a step above, engineering-wise, to what I have seen at the Science Olympiad website. I support all these efforts to entice and educate young people towards pursuing a career in science, technology or engineering. There is a reason math is on that short list, although few will make a career in math. That's why it is important to test problem-solving skills as a part of the overall competition, to make sure math is part of the solution.

I hope @NyeSpy succeeds in inspiring a few of his students to become participants in STEM. As we all should know by now, teachers have an almost impossible task of reaching out because of limitations on time and funds available. My high-school chemistry teacher paid out her own pocket the money required to transport a few of her students, me included, from Smyrna TN to Nashville TN to attend and exhibit at the State Science Fair. We need to reward such dedication somehow if we want a nation of doers instead of takers.

After thinking more about it, I would like to applaud the idea of using a rheostat to control the pusher fan speed. This is thinking "outside the box" but very apropos to the intent of the Science Olympiad competition. The tap positions, as a function of time-to-finish, should be marked on a paper strip affixed parallel to the linear resistor with its sliding contact. Place time-to-finish calibration marks on the paper strip after direct experimentation before the competition and then update (correct) them during trials at the competition. Use two different colored markers so as not to mix them up. The experimental calibration before the competition is just to provide benchmark points that will be adjusted during the competition. If allowed, always use a fresh battery at the beginning of each trail run. If allowed to use the 7.2 V NiCd battery pack, you should consider purchasing a 5 V fan for the pusher and make sure the 12 V fan will lift the vehicle.

You might want to consider using two (or more) resistors in series, or parallel, or series-parallel combinations to obtain a finer control of the time to finish. Experimentation is important! Since the competition is limited to 9 V "transistor radio" type batteries, and the total active time is less than a minute from power-on to power-off, heat dissipation is not a significant factor. But do the math. And best wishes for Good Luck in the competition!

Hop
 
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NyeSpy

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This is indeed for Science Olympiad. We are not sure how well the rheostat idea is going to work. If we are unable to achieve consistent, reproducible results with the rheostat we will scrap that idea and go with the mechanical restriction of airflow. It is all about experimentation and learning!

Discrete PWM is allowable, but the risk is that an untrained volunteer judge might disqualify us if they believe it is an IC... and that has happened in the past. I don't need crying students who invested blood, sweat, and tears into a project only to be told they cannot compete. :)
 

hevans1944

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I don't need crying students who invested blood, sweat, and tears into a project only to be told they cannot compete. :)
Amen to that! Gotta stay with the rules AND have fun!

Please measure the current applied to the pusher motor at your full battery pack voltage. The product of the voltage and current will tell you what the maximum power consumed by the motor will be. Any resistors you place in series with the motor will not only reduce the voltage to the motor but will also dissipate some power. The good news is this power dissipation will never exceed the maximum power the motor requires without the resistor(s).

If you have access to a nearby Radio Shack, you can purchase a package of 500 resistors containing a variety of 1/4 watt resistors (multiple values, multiple samples of each value) for less than fifteen bucks. They also offer a package of 100 resistors rated for 1/2 watt dissipation for less than ten bucks, but I don't know what values are included. You can always connect like-valued resistors in series or in parallel to increase the power dissipation. For example, two 100 Ω resistors rated 1/4 watt each connected in series will be able to continuously dissipate 1/2 watt and exhibit 200 Ω resistance. Connect them in parallel for 50 Ω resistance and the combination will still be able to dissipate 1/2 watt continuously. Only difference is, the 200 Ω combination will require 10 V across the two series-connected resistors whereas the 50 Ω parallel-connected combination requires only 5 V to dissipate the same power.

The brushless DC computer fan motors may not "like" to operate at reduced voltage. They may run, but not start, when too high a value of resistance is placed in series with the motor in attempting to run it at low speed. I would check for this undesirable operation right away since you are not allowed to touch the vehicle once it is placed on the track and turned on.

Perhaps, if this occurs, you could wire a magnetic reed switch in parallel with the resistor, to bypass it temporarily, until the motor gets up to a higher speed. A rare-earth (samarium cobalt) magnet worn on a finger-ring could actuate the normally-open reed switch while you turn the fan on. Remove your hand and the reed switch opens to allow the resistor to take effect, slowing the motor down to the desired speed. If allowed, you could instead install a manually operated push-button switch on the vehicle to do the same thing: press the "start" button, turn on the battery power switch, release the "start" button. On the well-known principle that it is often better to ask forgiveness than to ask for approval, I would opt for the magnetic reed switch. But please experiment!

You can find powerful rare-earth magnets inside of "bricked" hard disk drives and at least one of your students surely has one! They were used in the mechanism that operated the pivot arm that carried the read/write heads across the disk platters. Usually only a small piece, broken off of the brittle original magnet, is required. Be careful in breaking off a small piece. The shards are of course powerful magnets too!

Hop
 
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NyeSpy

Feb 7, 2017
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We unboxed the fans today and started planning the build. The push fan is working marvelously, but the lift fan is definitely going to need more power than my 7.2v NiCd battery can provide. We even tried wiring both batteries together in parallel and the fan still will not spool up. It shudders for a few seconds and then stops. If we manually start the fan (with fingers), it will spin up for a few seconds but then stops again.

Here is the fan - http://www.digikey.com/product-detail/en/delta-electronics/PFM0812HE-01BFY/603-2045-ND/6580735

Why this fan? From what I've been reading, we need a high static pressure fan to inflate the skirt where it will hold the full 2kg of mass that gets us the best possible score. This fan generates an insane amount of static pressure compared to other computer fans.

Will switching from 1500-1800 mAh NiCd batteries to Lipos make any difference? I had the NiCd batteries laying around the house and I love FREE, but if investing in different batteries gets this fan spinning I will do it.

I am going to bring in some 9v batteries tomorrow to see if I get any different results.

Definitely open to suggestions.... and hoping I am not going to have to send it back for refund. But that is an option if I have to go that route.
 

NyeSpy

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Just discovered the 8.4v NiMh on Amazon. :) I'm thinking that could make a huge difference. Worth a shot???
 

davenn

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We even tried wiring both batteries together in parallel and the fan still will not spool up. It shudders for a few seconds and then stops.

yes, because it isn't getting the correct voltage, you said the "lift" fan was 12V .... 7.2V does not equal 12V

Will switching from 1500-1800 mAh NiCd batteries to Lipos make any difference? I had the NiCd batteries laying around the house and I love FREE, but if investing in different batteries gets this fan spinning I will do it.

no it wont make any difference ... a wasted investment .... you need a 12V battery
 
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NyeSpy

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yes, because it isn't getting the co9rrect voltage, you said the "lift" fan was 12V .... 7.2V does not equal 12V



no it wont make any difference ... a wasted investment .... you need a 12V battery

So it is impossible to operate a 12v fan with less voltage? Wouldn't it just run slower? Other teams seem to be successfully using 12v cooling fans. Confused.

Fortunately it can go back if necessary.
 

davenn

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So it is impossible to operate a 12v fan with less voltage?

well you discovered that 7.2V isn't enough ;)

Wouldn't it just run slower?

brushless motors operate a little differently to brushed motors

Other teams seem to be successfully using 12v cooling fans.

but what voltage ?? have you definitely checked what they are doing ?

and it may be a different fan type to the one you are using ??
 

(*steve*)

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Having read the rules and the FAQ,it seems that more weight is a good thing, so a rheostat may be reasonably acceptable.

However the limitation on max voltage means you really need to find a fan which will operate at 9V (or lower).

The rules clearly allow discrete electronics. I would assume that as long as you make the circuit diagram available in your documentation and have the part numbers clearly visible, and possibly include datasheets for these components, you wouldn't have any problem.

The use of brushless DC motor driven fans is problematic if you want to use a resistor to limit speed. The controller might not like it.

If you use a brushed motor driven fan and a resistor to limit speed, a problem you may have is that the fan may run with the resistor, bit not start with it. Placing a capacitor across the resistor will briefly provide sufficient voltage and current to start the fan.

An alternative to a variable resistor would be a series of fixed resistors each half the value of the next. A series of switches across each resistor allow you to select a value using a binary combination of positions. Teach your students binary!

This may be cheaper, lighter, and more easily reproduceable than a variable resistor. It also gives the students an exercise in determining the maximum power dissipation in each resistor, and perhaps how to create this value and power rating from available resistors.
 
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NyeSpy

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Just realized the issue with the lift fan and it will indeed need to be exchanged. 9v is below the minimum range for that particular fan. Found another that generates 1019 pascals static pressure that will operate with voltage as low as 5.5v.

The students aren't the only ones learning! :)
 

(*steve*)

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The rules have an interesting wrinkle in the scoring. They reward heavier craft. In a sense it rewards over-engineering and waste (i.e. inefficiency)

Why don't they award points for lighter vehicles, or the "cargo" they can carry, or even better the ratio of the cargo weight to the weight of the craft?

As far as I can tell, increased mass allows you to take advantage of the energy required to accelerate it being larger than the enegy required to overcome friction from the skirt touching the ground, or the craft sliding along the edge of the track.

Incidentally, what are you doing to reduce friction from the latter of these two issues?
 

NyeSpy

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A ton of great suggestions and questions here. As to the skirt, we are hoping to be able to fabricate one that has an open bottom which will minimize friction. If we cannot pull that off, we will try a closed skirt with lots of tiny holes (think air hockey table top). The push fan we purchased should theoretically be able to overcome the friction of either design, but right now we just need to get both fans operational before we can proceed.
 

NyeSpy

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As to target mass, we want to get as close to 2 kg as possible without going over. A 2 kg craft that hits the targeted time will score maximum points.
 

Audioguru

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When you power up the motor then you should measure the voltage from the battery at the same time. Your battery might be too old to provide the extremely high starting current without its voltage dropping very low. The wiring must also pass the extremely high current without dropping the voltage.
 

NyeSpy

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I wired the two 7.2v NiCd in series and tried the lift fan again. This is an illegal configuration (over 9v), but that fan is an impressive bit of technology. Its too bad we cannot use it. :)
 

hevans1944

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I wired the two 7.2v NiCd in series and tried the lift fan again. This is an illegal configuration (over 9v), but that fan is an impressive bit of technology. Its too bad we cannot use it. :)
This is an example of applied engineering. It is important to specify components of a design completely and then select particular components that meet those specifications. In this case you originally chose a motor and fan combination that met the air flow and pressure specifications but failed to meet the voltage specification. That particular motor is also a brushless DC motor with an operating voltage specification of 10.8 V to 12.6 V, clearly not withing the 9 V (maximum) voltage allowed.

There is a huge difference between a brushed motor and a brushless motor. Every DC motor requires some means to reverse the armature polarity within the external stator magnetic field. With brushes and a segmented commutator this is accomplished mechanically and not very efficiently. With brushless commutation, external circuitry senses armature position and "automagically" reverses the current applied to the armature at the correct time to effect continuous rotation.

The brushless commutation circuitry for small computer cooling fans is now contained within the motor housing and has virtually completely replaced brushed commutation for small permanent magnet (PM) motors. It's all about economics of scale: it is cheaper to produce, in large quantities, brushless DC motors than brushed DC motors of equal performance. Unfortunately, adding electronics to the mix means complication and a limited range of operating voltages. Varying the input voltage to a brushless motor has NO direct effect on the armature current, which controls the torque produced by the armature. What a variable input voltage does do is require the switching electronics to accommodate the variable input voltage while still producing commutation at the designed rotation rate. If the motor slows down as a result of reduced input voltage, that is a sign that the electronics is failing to do its job of maintaining a constant rotation speed, which is why a limited voltage input range is specified for proper operation.

In general, brushed DC motors are simpler to design, and in sufficiently large quantities less expensive to produce. Their performance will almost always be less than a brushless motor of comparable size and torque at comparable rotation rates. Another down-side is they generally run a LOT faster than brushless motors and usually require a geared speed reducer for relatively low-speed fan operation. Again, it's an engineering decision and you have to know all the factors that affect it. It appears from the original rule, before it was amended, that the competition envisioned the use of small brushed PM motors to which contestants would add their own shrouded fans. It is difficult to see (without purchasing one) whether the Ward product follows this paradigm, but brushed PM DC motors are inexpensive and readily available. I would recommend using one for the pusher fan.

Just realized the issue with the lift fan and it will indeed need to be exchanged. 9v is below the minimum range for that particular fan. Found another that generates 1019 pascals static pressure that will operate with voltage as low as 5.5v.

The students aren't the only ones learning! :)
We should always be learning, whether in a formal educational environment or not. It is difficult to teach young people that their education is a process, not a goal to be discarded once a certain level is achieved. Learning should be a life-long process.

Congratulations on finding a lift-fan that will operate from your 7.2 V NiCd battery pack. Make sure it will accept this voltage without significant overheating and possible failure. If it's rated to operate at only 5 V it may run hot with 7.2 V (44% more voltage) applied.

You should purchase a small, brushed, PM motor and fit a propeller or fan to it. It will be much easier to control the speed of such a motor by inserting a variable resistance in series with the motor leads. Try a model airplane plastic propeller and operate the motor at the full voltage of your NiCd battery pack to see how much thrust it produces. Most electric airplanes operate at very high propeller rotation speeds, probably much faster than you will need to propel your hovercraft, so you may need to shorten the length of the propeller blades, or use a shrouded turbine propeller design.

Are you certain you need a skirt to contain pressurized air for lift? The inverted air-hockey table approach could be viable and much simpler. Maybe a Styrofoam tray with lots of holes you could tape over from the inside to control lift would be simpler. You can make clean holes in Styrofoam by heating a copper tube attached to a wooden handle, remembering to remove the Styrofoam plugs every once in awhile. For smaller holes, a heated copper wire of solid 10 AWG will also work. Since the hovercraft will constrained by the rails, it would appear that a skirt is not necessary to constrain air escaping from the sides. A couple of light and flexible fore and aft flaps might be all you need!

Hop
 

Hover Coach

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Can you suggest where to purchase a suitable rheostat to control the thrust of the propulsion fan? I plan to use a brushed dc motor and 8.4 volt NiMh batteries. Also a suggestion: Rather than a bag skirt would it be better to use a ring of round foam window insulation around the bottom of the hovercraft? The insulation I have has a cross section of about 5/8 in. I have tried it, hot glued to the bottom of a Styrofoam platform. Seems to work well.
 
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