Circuit design help?

Discussion in 'After Hours Lounge (Off Topic)' started by Jeff Perry, Aug 28, 2003.

1. Jeff Perry Stunt Coordinator

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Hello! I have been lurking here for awhile. I have a circuit design question I am hoping someone can help me with.

The scoop: I am building a device to allow me to remotely steer a trolling motor on my boat either manually or via autopilot from computer input.

My progress: I have a programmable device to sense motor travel from a pot and apply a DC voltage (configurable from 1-12 volts) to drive the motor. To reverse motor direction, the device simply swaps the voltage. So I can control the exact position of a shaft on a motor, like a servo, only cheaper.

My problem: My controller only outputs a max of 50mA. My steering motor - a power window motor for a car - draws from 3 to 10 amps depending on load. I need to build a small circuit that will take a +/- 1-12 volt dc input, draw under 50mA and switch the higher amperage load driven directly by my 12 volt marine battery. It also needs to reverse the polarity on the output voltage when the input voltage does the same.

I considered using a couple of relays, taking care to make sure only one was on at a time, but I'm not sure that solution would be ideal. They would have to switch several times every second while making course corrections, and I'm thinking they would wear out fast. I plan to use this system for hours at a time, so I doubt the relays would last long. They may also take a bit longer than expected to switch - I need to switch this quickly, so the steering appears fluid.

Is there a better solution I can use, other than relays?

If nobody knows the answer to my question, could you point me in the right direction? Maybe a web site or something that will help me? I understand really basic electrical theory but I know little about circuit design. I can, however, follow a basic schematic and solder...

Thank you!!

2. Holadem Lead Actor

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3. Jeff Perry Stunt Coordinator

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Hi Holadem! Thank you for the reply! Can you elaborate? I don't know what an IGBT is or how to put them in an H-Bridge configuration. Do you have a link to any material I might be able to read on the topic?

Thank you for your help! Jeff.

4. Justin Lane Cinematographer

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Here's a brief description of how an H-bridge operates:

http://mechatronics.mech.nwu.edu/mec...circuitry.html

As a controls guy, I am interested in what micro-controller you will be using, and how exactly you will be using a power window motor to accurately steer your trolling motor. If you have a design schematic of some sort I may be able to give you pointers as to how you should implement this system.

J

5. Jeff Perry Stunt Coordinator

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Justin, thank you for the link. Very informative, and easy enough to implement.

So, what's an IGBT, then? I assume I would use them as the switches in the H-Bridge configuration? Where can I learn more about how they work?

To answer your question: I don't know a lot about microcontrollers, but I do know about industrial automation. I am using AMX control equipment to drive the system. Specifically, I purchased a pan/tilt interface designed for controlling video cameras. The interface has motor outputs and pot inputs. Utilizing an AMX master controller and Axcess programming language, I can monitor the pot status in real time and issue positioning commands. The system then applies the appropriate voltage until the resistance on the pot - coupled to the shaft of the motor - is where it needs to be. Of course, since it's made for controlling tiny camera motors, it isn't quite up to the task at hand without some intermediate circuit to take the load!

This gets really fun when you realize that the AMX system has serial inputs, so I can hook a GPS enabled computer up to it and communicate that way. Touchscreens are also standard AMX fare, which would allow me to input positioning commands. The end result should be a system that will hold or change my position on the river via GPS. I can stay in one spot without dropping anchor, or even input a path I wish to follow and the speed I wish to follow it at. Some quick calculations and my boat will do my bidding while I fish. I may even attempt to tie in a depth finder and try some "AI" to automatically navigate around too-shallow waters.

At least, that's the idea. For now, I simply want to get it working with an industrial joystick coupled to another input on the AMX system. That way I can control the trolling motor from the other side of the boat. Once that works, I can focus on the more advanced features of the system.

I chose the power window motor because of the high torque, cheap price, and ability to hold it's position when powered down even under reasonably strong influence from external forces. The AMX system I chose because I already know how to program them. Typically this equipment would be very expensive, but I've bought most of it used on eBay. Other than the GPS enabled laptop (that I already own), I think I can come in under \$500 for this whole project.

Can I ask another question? Where did you learn circuit design? Can you recommend any good, hands-on books or kits I could buy? I would love to be able to tackle problems like this on my own. I think I could figure it out, if I only knew where to start.

6. Holadem Lead Actor

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Is the output voltage of your AMX device constant? Is the motor displacement time-controlled then? It doesn't sound like a very accurate method to me, but Justin might be better qualified to tell you.

In any case, an IGBT stands for Insulated Gate Bipolar Transistor. It is basically a device that combines the best of both BJTs (Power) and MOSFETs (Fast Switching). It was designed specifically for this type of power switching, such as motor drives.

You need an H-Bridge because you need to control direction. Otherwise, a simple current amplifier built around one big fat BJT or FET would have sufficed.

The following link does a decent job of explaining what an H-Bridge is. Any type of electronically controlled switch can be used in an H-Bridge, including relays as they show on the page. However, IGBTs are the best suited for this application.

http://www.dprg.org/tutorials/1998-04a/

As far as books go, I have heard good things about the Art of Electronics, which I plan on acquiring myself very soon.

--
H

7. Justin Lane Cinematographer

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Looks like a design with potential. I am not too familiar with AMX products though, so without further research, I will avoid commenting further.

8. Justin Lane Cinematographer

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Whoops---Holadem and my own post crossed. :b

J

9. Jeff Perry Stunt Coordinator

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10. Jeff Perry Stunt Coordinator

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OK, I think I'm getting the hang of this. I have a few more questions, if you don't mind!

Can you tell me if this IGBT would be appropriate?

!! Something is wrong with this forum. It won't let me post a link! It says "In an effort to cut down on the number of members that just post a thread to post a URL to another site you are not allowed to post URLs to other sites until you're here somewhat longer." That is annoying.

The IGBT I tried to link to is at:
www dot digikey dot com /scripts/us/dksus.dll?Detail?Ref=175763&Row=213141

It switches up to 600V @ 14A. I'm only switching 12V, but it's possible my motor could draw up to ~10 amps if resistance to it's movement becomes too great, so I chose this due to it's higher amp rating. Also, it says "w/diode". Does this mean I won't have to provide diodes across the "H" legs?

Few more questions:

1. The IGBT will only turn on with positive voltage applied, right?

2. I assume it will not be harmed if given a negative voltage?

3. How do I wire them? I understand the H configuration, and I understand the power for the motor goes into the collector and out the emitter. That's all very simple. What I don't understand is the wiring to the gate contact to switch the voltage. I have a +12v and a ground wire from my controller that swap depending on which way the motor should run. If the circuit is not complete, the voltage will not flow, and there is only one pin for the gate input. So how do I work that out?

Thank you again!! This place is an amazing wealth of information.

11. Jeff Perry Stunt Coordinator

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*bump* Can anyone help answer my last couple of questions?? Pretty please?

12. Holadem Lead Actor

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Please provide a part #, the link doesn't work.

1 - Yep. The IGBT will turn on at the thresold gate voltage listed on the datasheet.

2 - +/- 20V seems to be the typical absolute maximum Vge. But they should never have to see negative potential (relative to the emitter). See below.

3 - There are logic tables next to the drawings in the link I provided. You could:

a) Build those functions yourself with a couple of logic gates ICs (NOT RECOMMENDED). The main thing to remember is NEVER to turn on 2 IGBTs on the same side at the same time. This would cause a direct short that would blow your circuit fast. You will have to use some sequential logic to implement sufficient delay between direction reversals. That and a boatload of other considerations.

OR

b) Buy a goddamn H-Bridge Driver (RECOMMENDED ). By far the safest solution, I wouldn't touch option (a) with a 10 foot pole. As you can guess, an H-Bridge driver basically takes your differential input, the 4 outputs are connected to the gates and voila! Check International Rectifier for these devices.

A few random thougths:

- You should isolate the control section of your circuit from the drive section. It is standard practice. While with only 12V, your safety is not really a concern, your components' is. Motors are inductive devices, and can easily produce voltage spikes that would fry any IC in sight. Use optocouplers as interface between the logic and the drive.

- You can use an H-Bridge chip rather than discrete components for the bridge itself.

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H