Electric Circuit With An Electric Motor - Satellite Project

[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

 

[anorlunda]

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?

 

[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.

 

[anorlunda]

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.

 

[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?

 

[mfb]

does EMI mean noise?

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.

 

[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-RPM-High-Torque-Gear-Box-Electric-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.

 

[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.

 

[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?

 

[Baluncore]

How much power will it take to rotate the tube?

 

[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.

 

[anorlunda]

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

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.

 

[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?

 

[anorlunda]

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?

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.

 

[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)?

 

[anorlunda]

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.

 

[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.

 

[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?

 

[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, let's 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.

 

[anorlunda]

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.

 

[mfb]

every satellite falls eventually, without sustaining its velocity.

... 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.

I found this (http://www.ebay.com/itm/12V-DC-300-RPM-High-Torque-Gear-Box-Electric-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?

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

unfortunately the low rpm motors are very expensive

Compared to the launch costs?

 

[korkotyan]

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.

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.

 

[mfb]

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.

 

[Baluncore]

I though it was clear, by generating 1G (centrifugal force) I am creating artificial gravity.

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 ?

 

[anorlunda]

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!

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.

 

[korkotyan]

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.

I can use a magnetorquer - 0 moving parts

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

Can I use a shunt motor to be able to scale that way? But if I buy this series motor (the ebay one I mentioned), how to calculate the voltage for specific rpm? And how to calculate the voltage for specific rpm with a mass? Are there any formulas on this matter?

Compared to the launch costs?

Hahaha you are absolutely right, but for some reason all of you think that this project may actually end up in the Thermosphere.

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.

Ok, what about thin materials that can melt (and there are a lot of those), or hitting space debris, it is an endless loop of problems. If you (not you specifically) start projects from things like that, you should not even bother starting them. From the simple reason, things will not go as planed, therefore you need to be able to overcome problems like that.

Yes, designing space systems is not easy.

It is not correct, I am not designing a rocket, nor I put the satellite box in orbit and give it a starting speed, I do not even need it to work for years or even months, only a few weeks. And again, the chance it will actually be in space.

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.

That is good new, it means I can put inside the cylinder some sort of Bluetooth transceiver, to send the data from the accelerometer to the microcontroller to regulate the voltage on the motor.

Your post #7 is computing over one watt for a motor doing almost no work. Your computations in #7 are irrelevant.

Could you please let me know how you calculated this, and maybe include some formulas as well, that is exactly what I was asking since the beginning.

There is something you are not telling us. I believe that must be something about the tube contents. Why generate artificial gravity ?

Hahaha I am not hiding anything, I just want to use the microcontroller, and want some unique project with it, so artificial gravity always fascinated me. I am not saying it is going to lead to new discoveries, this has been done already, it is just a unique project.

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.

I answered on this in this post. I am not doing it to actually send a satellite to space. I am setting the requirements low so I will be able to start programming the microcontroller.

 

[Baluncore]

Could you please let me know how you calculated this, and maybe include some formulas as well, that is exactly what I was asking since the beginning.

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 current drawn by a simple DC motor is proportional to the torque it is generating, while the unloaded speed is proportional to voltage. Without knowing the dynamic tube contents we cannot calculate the load and therefore the current that will be required by the motor. I = W / V only works for a fixed power. You have not yet identified what that power is doing, or where that energy is going. You got 5W from ebay.

That is good new, it means I can put inside the cylinder some sort of Bluetooth transceiver, to send the data from the accelerometer to the microcontroller to regulate the voltage on the motor.

That just transfers the connection problem from communications to power supply. KISS.

Hahaha I am not hiding anything, I just want to use the microcontroller, and want some unique project with it, so artificial gravity always fascinated me. I am not saying it is going to lead to new discoveries, this has been done already, it is just a unique project.

So exactly what is in the tube?

 

[korkotyan]

The current drawn by a simple DC motor is proportional to the torque it is generating, while the unloaded speed is proportional to voltage. Without knowing the dynamic tube contents we cannot calculate the load and therefore the current that will be required by the motor. I = W / V only works for a fixed power. You have not yet identified what that power is doing, or where that energy is going. You got 5W from ebay.

What do you meant by fix power for I = W / V?
Lets say the cylinder mass is 0.2kg. So what will happen if I try to stop the shaft from spinning, are the voltage and the current both going to increase?

That just transfers the connection problem from communications to power supply. KISS.

What do you mean by that? I will need to put a small battery inside the cylinder to power the Bluetooth transceiver (3.3V, 30mA).

 

[Baluncore]

What do you meant by fix power for I = W / V?
Lets say the cylinder mass is 0.2kg. So what will happen if I try to stop the shaft from spinning, are the voltage and the current both going to increase?

The motor voltage will be that generated by the drivers from the battery voltage. The motor current will be due to torque on the motor.
If cylinder and contents are solid and balanced, with no windage, then only motor brush and bearing friction will be important, so there will be very low torque or motor current. Starting or stopping will need more current, but that can be done slowly under software control. So it could use a brushless 3ph motor driven from a microcontroller with PWM to minimise current once running.
Your application of the ebay motor wattage specifications assumed constant maximum power. That is not the case. You were computing maximum current or torque before damaging the motor by overheating in a cooling atmosphere, yet you do not say where all that power was going.

What do you mean by that? I will need to put a small battery inside the cylinder to power the Bluetooth transceiver (3.3V, 30mA).

A cylinder can be powered from the motor or the end bearing brushes. If all you want is to measure acceleration on inner surface of a cylinder then just mount a magnet on a spring and sense that as it passes; “Wot! no power?”
https://en.wikipedia.org/wiki/Centrifugal_governor

So exactly what is in the tube?

As you have persistently ignored my question, good luck, I have unwatched this thread.
Repeatedly writing “What do you mean by … ” is childish and shows that you cannot actually reason about technology. You have no understanding and just want an equation to plug your numbers into.

Your ignorance is wasting our time.

 

[korkotyan]

As you have persistently ignored my question, good luck, I have unwatched this thread.
Repeatedly writing “What do you mean by … ” is childish and shows that you cannot actually reason about technology. You have no understanding and just want an equation to plug your numbers into.

Your ignorance is wasting our time.

You took it out of any proportion, I asked only twice "what do you mean by that", and it was not out of disrespect or something, I truly am not familiar with those topics, that is why I mentioned in the beginning, that I am a software engineering major.
And Yes, I do want a mathematical form of that, so I could start dealing with the software part of this project, the thing I am familiar with.
And god, How am I ignoring you question, about the contents of the cylinder?! What do you want me to say? My goal is to use an arduino (or some other brand) microcontroller (to program it), and I do not want to do the common projects.

 

[mfb]

I can use a magnetorquer - 0 moving parts

Fine, just make sure it is fast enough for your still unspecified requirements.

Can I use a shunt motor to be able to scale that way? But if I buy this series motor (the ebay one I mentioned), how to calculate the voltage for specific rpm? And how to calculate the voltage for specific rpm with a mass? Are there any formulas on this matter?

Every motor will have different curves, you need the datasheet and experiments to check the voltage and current at given rpm and torque (you have to consider both).

Ok, what about thin materials that can melt (and there are a lot of those), or hitting space debris, it is an endless loop of problems. If you (not you specifically) start projects from things like that, you should not even bother starting them. From the simple reason, things will not go as planed, therefore you need to be able to overcome problems like that.

Don't use materials that melt. Satellite design needs several iterations because some dependencies go in circles, but you can focus on some points first and then care about minor issues. If those issues turn out to be larger, you might have to change something in the design. The very first things are always the requirements as you don't want to change those.

 

[korkotyan]

unspecified requirements

You are right, I realize now that some of the problems are critical, thus should be addressed before designing the satellite.

Every motor will have different curves, you need the datasheet and experiments to check the voltage and current at given rpm and torque (you have to consider both).

That is probably the most suitable way for me to start designing the satellite after specifying more requirements.

Thank you very much mfb, anorlunda and Baluncore for your help, you enlightened me on very important parts of this project. I will start now from the real beginning.