Let's Make Robots!

Ok this is a project I have wanted to do for a while now and have finally started since I needed a good topic for my Senior Exit Project.

So far I have attached the two wheel chair motors that I stole (only paid $25 for the entire chair) to a sturdy platform, and added a caster in front. Doesn’t sound like much but, it is what it is for the time bean. My intent is to basically build an oversized Boe-Bot (Parallax) you know just a nice maneuverable platform that I can add sensors to and just play around with different things.

Stuff that I want to do with it but just need to find time for/ read up on:

1.       IR – To since obstacles and stuff, also I love Chris’s IR Beacon idea (have the stuff)

2.       Sonar – General obstacle detection and distance measuring (would need to purchase)

3.       GPS – Would be awesome to have this thing drive around campus next year all on its own (have the GPS chip, but its 3.5v and my BS2P is 5v logic, any ideas)

4.       Encoders – On the wheels for PID control (have two just need to make an adapter)

5.       Vision – Played around with MSRS a little (have webcam)

6.       Wi-FI – Have an old laptop and desktop so the only thing I need is a (piece of software)

7.       Power – I want to get some better batteries (two 12v 18Ah would be nice)

Like I said so far I have just mounted the motors. Right now I’m working on speed controllers; I’m trying to design them around the HIP4082IP MOSFET driver. So if you have any thoughts on that they’d be appreciated.

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Justin Hulbert's picture

So looking at what I have so far, do you think I would be fine ordering the parts we have talked about? Any extra parts I should buy in case something fails or needs a different value?  

Justin Hulbert's picture

I don’t know if you were joking or what but I am enjoying the project quite a bit. We have to do all sorts of things like writing a research paper, interning with someone for at least ten hours, conducting at least two professional interviews, and a whole list of other things. As you could imagine a lot of the kids think it’s really dumb and a waste of time, but I think its very rewarding and an extremely good use of our time. Its worth our entire fourth quarter grade in English class so you don’t graduate if you don’t do it. Though I don’t think I’ll be done by May sixth, that is when I present what I do have along with my research, to a panel of judges so whish me luck.

Oh and what about the batteries? I know there big and heavy but they were cheap so…

Mike's picture

I love the vision of a "senior exit project" - should be easy to achieve with those batteries ...

 

... * no seniors were harmed in the making of this robot * ...

Justin Hulbert's picture

I sort of want to do that but it’s really not that simple. Like the control board that runs the original speed controllers is like all integrated and very complex, hosting multiple unmarked micro processors.  And since my project is on designing speed controllers, it would sort of be nice to actually build one. Besides it’s something I could use multiple of, so if I can get a design that works I’ll make a few.I have a few drawings, let me see if i can clean them up and post them...

Justin Hulbert's picture

Thanks for taking a look at the data sheet, but from my understanding (though I could be wrong )  the HIP 4082IP is just a FET driver. Which means it converts the 5v logic signals to voltages high enough to switch on and off the FETS. So I won’t be driving the motors directly from that chip. The motor controller that came with the wheel chair has two of these on the board next to the large FETs so I am certain that I can make them work it’s just a matter of selecting a few FETs and some diodes. Take a look at this article that I’m trying to sort of follow: http://www.parallax.com/dl/docs/cols/nv/vol2/col/nv52.pdf     

Thanks,  I really need to get going on this should be a fun project.

robologist's picture

but the HIP4081 in the N&V article has an internal charge pump in addition to the bootstrap circuit.

 

 

 

Had to re-edit that, been a while since looking at these.

Justin Hulbert's picture

Robologist,

Sorry for the mistake.  As you mentioned the N&V article dose make use of the HIP4081 not the HIP4082 as I said. Though I have a set of both just in case… don’t you just love free samples!!!

robologist's picture
Not really a mistake, both can be used, but the 4081 should be able to go fully on or fully off since there is a charge pump working on the gate voltage. Bootstrap circuits generally only allow operation if 5% to 95% PWM is being used so that there is something pulsing to "bootstrap" a gate driving voltage from. 
Justin Hulbert's picture

Question, when you solder two MOSFETs of equal value in parallel you lower the RDS (on) and more power can be dissipated right? Dose this also increase the overall ID? Like say I had two MOSFETs rated at 50A, could I control a motor that draws close to 100A? I don’t know, I’m just having a hard time picking MOSFETs and I really need to get them ordered soon! This list is intimidating: http://futurlec.com/TransMosIRF.shtml  

I’m thinking about the IRL3803 but the dada sheet makes it sound like it can only handle the high current at 10v even though it can handle 30v overall. http://futurlec.com/Transistors/IRL3803.shtml

It’s always a trade off when it comes to the all the different values isn’t it.

robologist's picture

Technically, you are doubling the current capability by soldering another FET in parallel. Practically, the case limit of a TO220 case comes into play long before the current limit does. A TO220 packaged only can dissipate 75 watts or less typcially, before a thermal failure of the internal joints occurs. Watts being I^2R losses that come directly from Rds-on. Remember to adjust the gate drive resistors if beginning to parallel FETs.

I think you can use a number of different FETs, since I seem to recall wheelchair controllers being rated at 80 watts or so. It might be advisable to look at other suppliers though, since I've heard a few folks mentioning long delviery times from Futurlec.The IRL3803 would probably be workable with a 12 volt supply, inductive spikes from a motor might take it over that 30volt Vds-br if used with a 24 volt supply. THe IRFZ44 oddbot mentioned is a good standard duty FET, with a 55 volt Vdss for a higher voltage supply.