Electric Circuit With An Electric Motor - Satellite Project

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1. Sep 13, 2015

korkotyan

I am a software engineering major, and I am having some trouble with an electric circuit in my satellite project.

Basically I am trying to generate 1G on the inner surface of a cylinder with a radius of 0.03m, connected to an electric motor which has to rotate at about 173 rpm (to generate 1G on a 0.03m cylinder) according to my calculations.

In the circuit I need to connect obviously a battery to the motor, and regulate the rpm with a microcontroller (like an arduino). So I need a part or a circuit that will allow me to control the voltage via the microcontroller, to increase or decrease the rpm of the motor. Since all batteries are DC I guess it is preferred that the motor will be DC as well.

This is an approximate of the general circuit I imagine (http://s10.postimg.org/pibf3x2fd/satellite_circuit.jpg).

I just need the part or a circuit that will allow me to control the voltage via the the microcontroller.

Thanks for any help

2. Sep 13, 2015

Staff: Mentor

Will it be a feedback controller? Where the computer senses the speed, compares it with desired speed, and generates a control signal based on that difference?

3. Sep 13, 2015

korkotyan

Inside the cylinder will be an accelerometer, I did not yet figure this one out, how to send the data from the accelerometer to the microcontroller. But any way it is not to hard to make a mechanism, to determine the rpm of the cylinder.

4. Sep 13, 2015

Staff: Mentor

What are your requirements for accuracy and stability? If accuracy is loose enough, you may not need the feedback.

What are your equirements for power consumption? temperature? EMI? The better your question, the higher quality answer PF can provide.

5. Sep 13, 2015

korkotyan

Technically it should be a (0.1m)^3 box satellite. So accuracy and stability are very important. I do not think power is going to be a concern, since reaching 173 rpm requires low voltage. I am not sure about temperatures, but again low voltage means low temperatures, the only concern here is to keep the battery warm enough when the sun is not visible. I forgot to mention that the satellite should be at a 300km orbit thus the period is about 90 min (I think). As I said I am a software engineering major, so I am not to familiar with the subject, does EMI mean noise?

6. Sep 13, 2015

Staff: Mentor

Electromagnetic interference. Noise, right.

Designing a satellite is a lot of work, and moving parts are tricky in particular. The satellite will have significant temperature variations between day and night. You get internal mechanical stress, all materials (like oil) have to handle the temperatures without freezing or evaporating, and so on. The battery has to work with the large temperature variations.
If power is not an issue, you can keep the battery warm just by using it more. I would expect that it is an issue, however. Lower power consumption probably means longer lifetime.

The microcontroller can regulate its output voltage depending on the readings of the accelerometer, but you'll have to see if it can drive the motor directly or if you need an additional transistor to get the required power.

7. Sep 14, 2015

korkotyan

I did not even think about the option of powering the motor with the microcontroller, I thought it will burn the chips and the buses, but it may actually work.
I found this (http://www.ebay.com/itm/12V-DC-300-...c-Motor-Hot-/150530611989?hash=item230c52db15) DC 300rpm motor on ebay, it says in the description "Horse Power Cont.: 5W" and "Voltage/current: 12V DC", it means the max current is 0.4166A? And if the max rpm is 300, and I want it to run at about 173rpm, I simply reduce the voltage to 12 * 173/300 ≅ 6.92V? And the current is now ~0.2403A?

The microcontroller limitations (link 1, link 2) state that the recommended current per pin is 0.02A, but the max is 0.04A (from link 1), for the ATMEL ATMEGA2560 from all the pin SINK and SOURCE the current limitation is 0.8A (link 1, almost at the end), which is by far more that the motor needs.
Does this all add up? And how is the current affected by a mass connected to the motor?

Thanks for the help mfb and anorlunda. I am sorry for asking really basic stuff, I am just not to familiar with the subject.

8. Sep 14, 2015

Baluncore

How much power will it take to rotate the tube?
Maybe consider a small brushless motor with a permanent magnet armature and a three phase field winding. The frequency of the drive voltages will determine the rotation speed. Those three phases can be generated directly by a microcontroller. By monitoring the phase lag of the cylinder, with a hall effect sensor, you can determine how much power is needed to keep it rotating at the correct speed. You might use phase modulation to reduce power consumption once it is running at the required speed with the optimum phase.

9. Sep 14, 2015

korkotyan

The brushless motor is a nice concept, unfortunately the low rpm motors are very expensive. And from what I understand brushless motors have a minimal voltage (most popular from 6V), so I simply will not be able to get 173 rpm from a few thousand rpm motor.
Regarding my calculations in the previous post, are they good?

10. Sep 14, 2015

Baluncore

11. Sep 14, 2015

korkotyan

Well, I do not know. I am asking theoretically, how any mass (lets assume 0.1-0.2kg) will affect the voltage and current.

12. Sep 14, 2015

Staff: Mentor

The mass determines how fast you can accelerate the tube. At constant speed, the power is determined by friction in the bearings. You will probably have to experiment to learn how much friction you have.

Edit: you are getting a micro course on engineering. Design can not begin until you know the requirements.

13. Sep 14, 2015

korkotyan

I do not think friction affects to much the values, I am asking if the mass affects only the voltage or the current as well?

14. Sep 14, 2015

Staff: Mentor

A spinning mass will continue spinning forever with no power if there is no friction. Think of the sattelite itself. It would tumble forever with zero power if we let it.

15. Sep 14, 2015

korkotyan

Emm... I am confused, you just said friction is a factor and now you say friction is not, and by the way it is incorrect that a mass will tumble forever with zero power, every satellite falls eventually, without sustaining its velocity.
But please, are my calculations correct (post #7)?

16. Sep 14, 2015

Staff: Mentor

You need the frisction to answer Baluncore's question from #8. Your calculations in #7 say nothing about the tube you are spinning. Before you design, before you select a motor, you must determine the requirements.

Sattelite use energy to prevent spinning. When they run out of fuel, spin can not be controlled and they fail.

Where is the motor that keeps Earth spinning once per day? There is none. Your thinking about power and friction is backward.

I suggest that you review Newton's Laws of motion before attempting this project.

17. Sep 14, 2015

korkotyan

I am sorry, I did not try to offend you in any way, if I did I am truly sorry.
Lets just assume it has the starting speed to rotate around Earth, and after a few weeks it falls.
I am trying to build the inner part of the satellite and it only.

18. Sep 14, 2015

Baluncore

Any dynamic load in the tube will cost energy.
Why do you rotate the tube ? What is the experiment ?
What is in the tube. Is there something that moves or rolls?

19. Sep 14, 2015

korkotyan

I though it was clear, by generating 1G (centrifugal force) I am creating artificial gravity. And yes, I am aware of the fact that if the whole satellite spins it makes the forces inside the satellite go haywire, lets just assume the satellite is always perpendicular to Earth and only one side of the satellite faces Earth.
In my first post there is a link to an image, the circuit is not really relevant anymore if my calculations are correct (I can not get a confirmation on them), but the motor and the cylinder are still as planed.

20. Sep 14, 2015

Staff: Mentor

Don't worry, nobody is getting offended. But you are repeatedly resisting thinking about requirements before selecting a motor, or how to control the motor. How will you know if the motor is right sized?

You also ignored MFB's cautions in #6. If the lubricant becomes stiff when cold, friction increases. If the lubricant freezes, the tube will get stuck and your project will fail. Baluncore added more critical questions in #18. These things must be addressed before selecting or controlling the motor.

It is the same in software engineering. What would you say to a programmer who rushes to select a language and platform before asking what the software is required to do?

You also seem to not understand Newton's First Law. It is critical. Later, when we address the problem of starting up this system from stop, Newton's Second Law will become critical.

21. Sep 14, 2015

Staff: Mentor

... which is again an effect of friction, here in the atmosphere of Earth.

Concerning the spinning satellite: this is an important point. Spinning up your centrifuge means the satellite will start spinning in the opposite direction. You either need some flywheel to cancel it (and a separate motor for the flywheel), or you have to live with a spinning satellite (which also means your motor rpm has to go up to get the same net spin).
You'll still get some rotation. If your microgravity environment has to be good or if you rely on the correct orientation of the satellite for other reasons, you might need an active 3-axis stabilization.

Motor currents and voltages don't scale in that way. It will depend on the motor.

Compared to the launch costs?

22. Sep 14, 2015

korkotyan

I understand all of the cautions, I resist them because they are not relevant now, especially the most detailed facts as oil freeze. You can not start a project like this, by looking into the minor problems, because you will never start the project at this rate, and there will always be more and more problems like this! You need to start big and move in deeper, and deal with these kind of problems. I obviously am going to test it on Earth with normal conditions countless times, before I even consider sending it to space.

I can not understand how Newton's Laws are even relevant at this stage, at a non moving environment, except for the cylinder.

23. Sep 14, 2015

Staff: Mentor

Freezing oil is not a detail. It limits the choice of components for the whole mechanical system. It will influence the friction and therefore the necessary power of the system, which then influences the choice of electronics. To make it worse, the power consumption of the electronics will then influence the temperature of the satellite which feeds back to the viscosity of the oil.

Yes, designing space systems is not easy.

24. Sep 14, 2015

Baluncore

The consumption of energy by the spinning tube, once it is spinning, is determined by the dynamic contents of the tube.
It takes almost no energy to spin an empty tube in space.
Your post #7 is computing over one watt for a motor doing almost no work. Your computations in #7 are irrelevant.
There is something you are not telling us. I believe that must be something about the tube contents. Why generate artificial gravity ?

25. Sep 14, 2015

Staff: Mentor

That is where more experienced engineers disagree sharply with you. You begin a project by carefully determining the requirements. With requirements in hand, you begin the design to meet those requirements. You are trying to jump to the last step first.