Motor power...
Pezi_Pink!
Posts: 32
Hello all,
I was hoping someone could assist me with a problem I have with one of my current projects. The basic idea was to take a cheap RC monster truck I have, remove the circuitry, and then construct my own motor drive circuit, add a mounted Ping))) with servo onto the front of it, powered by a SX28. I am also adding the Parallax RF units so commands can be issued from the PC or set to auto roam mode.
The project has come quite far, I have mounted the servo and Ping))) successfully on the bonnet and have a basic motor control H-Bridge circuit to drive the two motors. This is all connected up to the SX and it works quite well. My problem is that when I put everything together, the combined extra weight of the servo, bracket and Ping))), the top shell, and the extra circuitry is too much for the motors to move it all. It works OK without the top on and no Ping))) but this isn’t really what I wanted.
I have done lots of research into motor control circuits, however these are only very small motors and most the “decent” circuits use MOSFETS which I don’t have enough voltage to switch on.
Basically the SX is powered through a 9v battery and regulator, and the H-Bridge is powered by the rechargeable 4.8v NiCad battery pack that came with the unit. I seem to remember the truck moving with a fair bit more power before I hacked it up even with the minimum weight/components for powering the motors on board. I could use another 9v battery in place of the 4.8v although this doesn’t seem to make much difference, and I know these motors can go faster than this..
I have tried several different circuits to no avail. I am hoping one of you electronic masterminds can help me here. Given that the unit used to go quite fast using the 4.8v battery pack there must be a technique I can use to increase the power to the motors? I thought this would revolve around amplifying the current, perhaps using darlington pairs or something similar? I have tried a few things to no avail. The other problem I have is space. I can’t use much in the way of additional circuitry, although there is still some room.
The old board had a whole load of transistors on it, hence my thinking that aside from the H-Bridge they must be used to amplify the current somewhat. I don’t mind if the batteries don’t last that long as long as I can get it to work..
Here’s a picture of the shell with the Ping))) and servo:
This is the space inside the shell where the H-Bridge is:
And you can see here the CPU and RF etc will go in the “pickup” section of the truck quite nicely:
And finally this is the simple h-bridge I am using at the moment, formulated from several different circuits I have looked at / tried using!: (note the gnd is both to the battery and sx like it should be!)
If someone could point in the right direction here that would be great, it’s annoying to get so far and be stumped by such an frustrating problem! I don’t really want to be wasting lots of time searching the wrong avenues if there is something silly I have missed!
Thanks for any help..
And on another note, check out my little sonic car!
Pezi
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Beware the pink squirrel!
I was hoping someone could assist me with a problem I have with one of my current projects. The basic idea was to take a cheap RC monster truck I have, remove the circuitry, and then construct my own motor drive circuit, add a mounted Ping))) with servo onto the front of it, powered by a SX28. I am also adding the Parallax RF units so commands can be issued from the PC or set to auto roam mode.
The project has come quite far, I have mounted the servo and Ping))) successfully on the bonnet and have a basic motor control H-Bridge circuit to drive the two motors. This is all connected up to the SX and it works quite well. My problem is that when I put everything together, the combined extra weight of the servo, bracket and Ping))), the top shell, and the extra circuitry is too much for the motors to move it all. It works OK without the top on and no Ping))) but this isn’t really what I wanted.
I have done lots of research into motor control circuits, however these are only very small motors and most the “decent” circuits use MOSFETS which I don’t have enough voltage to switch on.
Basically the SX is powered through a 9v battery and regulator, and the H-Bridge is powered by the rechargeable 4.8v NiCad battery pack that came with the unit. I seem to remember the truck moving with a fair bit more power before I hacked it up even with the minimum weight/components for powering the motors on board. I could use another 9v battery in place of the 4.8v although this doesn’t seem to make much difference, and I know these motors can go faster than this..
I have tried several different circuits to no avail. I am hoping one of you electronic masterminds can help me here. Given that the unit used to go quite fast using the 4.8v battery pack there must be a technique I can use to increase the power to the motors? I thought this would revolve around amplifying the current, perhaps using darlington pairs or something similar? I have tried a few things to no avail. The other problem I have is space. I can’t use much in the way of additional circuitry, although there is still some room.
The old board had a whole load of transistors on it, hence my thinking that aside from the H-Bridge they must be used to amplify the current somewhat. I don’t mind if the batteries don’t last that long as long as I can get it to work..
Here’s a picture of the shell with the Ping))) and servo:
This is the space inside the shell where the H-Bridge is:
And you can see here the CPU and RF etc will go in the “pickup” section of the truck quite nicely:
And finally this is the simple h-bridge I am using at the moment, formulated from several different circuits I have looked at / tried using!: (note the gnd is both to the battery and sx like it should be!)
If someone could point in the right direction here that would be great, it’s annoying to get so far and be stumped by such an frustrating problem! I don’t really want to be wasting lots of time searching the wrong avenues if there is something silly I have missed!
Thanks for any help..
And on another note, check out my little sonic car!
Pezi
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Beware the pink squirrel!
Comments
I'm a software guy, not a circuity guy, but even I see some REAL problems with your H-Bridge and how it's constructed. Here is an H-Bridge that should work for you which also uses Darlington transistors:
http://www.bobblick.com/techref/projects/hbridge/hbridge.html
Note the criss-crossing of the two wires near the center of that schematic. That does not appear in yours, and is part of the H-Bridge theory or format. There should be no problem in using your BC-nnn transistors in lieu of the TIP-nnn transistors shown. Both are general purpose Darlingtons as far as I'm aware.
The other matter I find a bit odd, is the existance of 4 logic inputs. Generally H-Bridges operate with only 2 logic inputs. At least that's been true of those that I've seen that seem to work okay. I suspect the problem there is that 4 additional transistors are missing from your "hacked" schematic, but I stand to be corrected there.
Regards,
Bruce Bates
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re the 4 inputs, you are correct and even this circuit could have just two inputs, it's just thats how I built it first for some reason and it's stayed like that whilst I am trying to get it to work [noparse]:)[/noparse]
That circuit I am using I have seen lots of similar ones on the net, such as here althoug this uses mosfets, www.4qdtec.com/bridge.html and here www.mcmanis.com/chuck/Robotics/tutorial/h-bridge/bjt-circuit.html
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Beware the pink squirrel!
That H-bridge circuit you have there looks a little odd,at least to me.
I believe some changes are in order,but it's just my humble opinion.
The combination of NPN and PNP transistors seems wrong for the purpose.I would use a set of 4 matching NPN units.
You might change the base bias resistors to a lower resistance to ensure full saturation.
Then connect inputs 1 and 4 together,also inputs 2 and 3.
Be sure to breadboard the circuit and work the inputs manually before connecting it to the controller,things happen sometimes.
Good luck, Jim Richey
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Thanks, Parallax!
your motor will only "see" 3.6V ...I would be willing to bet that there are/were MOSFET transistors on the original board.
The 1K resistors seem a little on the light side to deliver an adequate amount of current, you might get better results with
smaller value resistors assuming the transistors can thermally deal with the demand.
MOSFETS with a low RDs "ON" would be a better choice for delivering the most power considering the supply voltage level.
See a previous reply I had to a similar thread....
http://forums.parallax.com/showthread.php?p=563780
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
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Beau, I understand the drop of voltage across the transistors, and the original board certainly doesn't look like it uses any MOSFETS, and the little cars I have don't seem to either.·· However that's based on the assumption that all MOSFETS I have seen are a lot bigger than the BJTs as all the transistors are pretty much the same size on the boards.··I'm most probably wrong there then!· I have a device that analyses transistors, maybe I will take some off and see what they are.
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If it's the voltage that's the main problem, if I used a 9v battery wouldn't this then be supplying 7.6v (ish) across the transistors·which should be easily enough voltage (maybe too much, I have no idea what the motors are rated at)..· This doesn't work too well either, although the transistors do handle the extra power/heat.· So I conclude that it's still a combination of both voltage and current...?
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I'm getting a good grounding with transistors now, it's just there are so many types and values it's sometimes difficult to know what you are looking for to solve a given problem.· In some respects there seems to be a lot of conflicting ideas with the articles I have read on the net.
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Thanks for your help guys
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Beware the pink squirrel!
Post Edited (Pezi_Pink!) : 5/21/2006 9:24:50 AM GMT