I am currently in the designing phase of a Segway that I am planning on building during the summer. I am an electrical engineering student, thus have a fairly good grasp on the electrical components of a segway. However, there are a few different things on the mechanical side of things that I am not sure about. My goal is to build this project under $400-$450, so the items purchased will have to work along with that. I have been using your book as a reference, as well as a few other project logs on the internet, one of those being http://p1r.se/robot/segway/. I have also been referencing the LawnBot guides behind some of the mechanical components that are similar between projects. I appreciate any tips or advice you have for me concerning this project, and hope this thread and these questions will help others who have a similar project.
Known Components and Product to be Used:
IMU & Sensors: https://www.sparkfun.com/products/10121 6 Degrees of Freedom ITG3200/ADXL345 $65
Mircocontroller:https://www.sparkfun.com/products/11021 Arduino Uno $30
One of the biggest questions I have what is the approximate amount of current the motor would theoretically pull under maximum strain. One motor controller I am considering, which is the one you used for your own, is the Sabertooth 2x25. However, is one that can withstand that amount of current completely necessary for this project? Is there any cheaper ones that are manufactured that may withhold a smaller amount, but still be a viable option?
I have also considered that option of making the Triple 8, or using other H-Bridge schematics available, using the 47A P- channel and 50A N-Channel Mosfets make this build around $50-$60, including some of the other materials needed to make it.
Ultimately, what is the most bang for the buck when it comes to motor controllers, I know the 2x25 would certainly do, but is it overkill? When considering the current draw from the motors in this application, what is the probability that a triple-8 would burn up? And could even a simplified/cheaper version than it suffice?
I would first like to mention that I have been keeping an eye on ebay for reasonably priced wheel-chair motors, but would like to ask if there if anything specific I should look for when doing so. I'm not quite sure if all wheel chair motors are "all created equal" or not. I have been particularly been trying to keep my eye on ones with wheels attached. As I assume finding wheels that meet specifications and having the right/convenient hubs would be both difficult and expensive. Not to mention I lack the proper mechanical skills to do it properly.
What kind of tires would work best for this segway (excluding the wheelchair tires).
I was also wondering what your opinion was on using scooter motors instead of wheelchair motors, and the advantages/disadvantages of doing so.
I believe I squashed enough questions into one post. I appreciate any info you have, Thank You.
for the motor controller, the Sabertooth has one main advantage and that is reliability. Since it has a built in over-current protection circuit, it is nearly impossible to overload and break it. It will shut itself off for a second if it hits its maximum limit, it is also extremely powerful for its size and has a pretty nice control interface built in that you can use without worrying about sending the wrong signals and messing something up.
Having said that, the sabertooth 2x25 is no match for the Triple8 when it comes to raw power output. Of course, if you try to go cheap on building the triple8 and don't use the 4oz copper-clad board, it is possible to burn a pcb trace or even blow up a mosfet if the voltage is too high. When I had the triple8 running the Lawnbot400, it would pop a wheelie with little effort, but also could blow the thing up if you weren't keeping an eye on it during extended use to make sure it didn't overheat. The sabertooth would simply trip it's current limit if you tried to pop a wheelie with it, but it would also keep on running for hours and you never have to check on it.
So, in short - I used the sabertooth on the Segbot to keep from having to worry about the motor-controller. When riding something 5-10mph, you would like to be sure you aren't going to blow up your motor-controller and go flying off the front of it. It was a safety issue for me. But as to your question of whether it is overkill? I have never hit the overcurrent limit on my Segbot, so maybe it is. I can tell you from experience that the sabertooth 2x12 is NOT sufficient to carry 150-200lbs, so don't try that one.
You can probably make a motor-controller that would handle the Segbot for less than $120.. for instance, I have a newer design called the 2x4 which uses 2 mosfets on each leg of the H-bridge and it would be plenty powerful and only cost around $70 to build two (each one only controlls one motor). It is more reliable than the triple8 - mainly because the pull-up resistors for the high-side mosfets use a voltage divider setup that allows you to select how far down you need to pull the P-channel mosfets in order to turn them off, without pulling them all the way to GND - which when using a 24v battery supply, gets awefully close to the typical 25v Vgs (gate-to-source voltage) limit that can blow up your mosfets if exceeded. With the voltage divider you can select the 2 resistor values that keep the mosfet pulled down to around 12v below the battery source, keeping the Vgs far below its limit. This was the only downfall of the triple8. Either the triple8, 2x4, or sabertooth would be plenty of power for this application. You shouldn't be drawing more than 20 amps with a geared motor setup, and all of the mentioned motor-controllers will handle more than 20amps (continuous).
The wheelchair motors are all very similar and just about any from ebay should work - you will likely have to remove the electric brakes though to get them moving.
You are right to look for wheels with your motors, as the hubs to mount other wheels to them will be expensive and hard to find, though it can be done with chain and sprockets. Also, a good size for wheels is generally 12"-15" diameter... don't try anything smaller than 10" or you will probably wish you had gone larger. If you are buying wheels separately, I would go with a 13" tire from Harbor Freight for $15, or the 12" wheel/tire/sprocket combo from Northern Tool, though those are a bit pricey at $65 each.
Scooter motors would be plenty powerful enough and I have seen them used before, but you would then have to use chain/sprockets to connect them to the wheels which is more work and less reliable than wheels mounted directly to the motors. But it would work just fine and they are way cheaper.
Feel free to ask away... though I have been working a lot lately and am slow to respond sometimes.
forgot to mention that the IMU situation is bleak.. I have only written code for several analog version and those are all discontinued (to my knowledge). I have promised to decode the I2c versions of the IMUs available but have been too busy to complete this as of yet. It is in the works though and I will be posting an update as soon as I get that working.
also, any arduino will do just fine... even the mini versions.