Skid-Steer: The Movie
Bill Chennault
Posts: 1,198
All--
Here is a picture of my skid-steer.
Pay no attention to the "blocks" I am using to keep the 'bot off the carpet for testing purposes! (I have multiple hobbies.)
And here is a link to a very poor movie. I wish I knew how to make them better. I wonder if there is a "Do It Better" button on my Sony Mavica? I suppose the only advantage is that I am in this movie! (I am the one with the socks on.) In the movie, although all five processors are up and running, the BS2p40 is blinking an LED, one of the OEM BS2's is counting encoder pulses from both gearmotors but doing nothing with them, and another OEM BS2 is doing all the work without sensors. (It is using my best guesses as to how much to turn, etc.) The other two OEM BS2s are powered up and just kinda hangin' out.
http://www.billnkc.com/images/Robotics/Crawler/SS-10-29-07.mpg
You might have to play it twice . . . once to cache it and once to watch it. (I'll make another one outside on the flagstone patio. There is a lot more light there.)
--Bill
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You are what you write.
Here is a picture of my skid-steer.
Pay no attention to the "blocks" I am using to keep the 'bot off the carpet for testing purposes! (I have multiple hobbies.)
And here is a link to a very poor movie. I wish I knew how to make them better. I wonder if there is a "Do It Better" button on my Sony Mavica? I suppose the only advantage is that I am in this movie! (I am the one with the socks on.) In the movie, although all five processors are up and running, the BS2p40 is blinking an LED, one of the OEM BS2's is counting encoder pulses from both gearmotors but doing nothing with them, and another OEM BS2 is doing all the work without sensors. (It is using my best guesses as to how much to turn, etc.) The other two OEM BS2s are powered up and just kinda hangin' out.
http://www.billnkc.com/images/Robotics/Crawler/SS-10-29-07.mpg
You might have to play it twice . . . once to cache it and once to watch it. (I'll make another one outside on the flagstone patio. There is a lot more light there.)
--Bill
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You are what you write.
Comments
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EPIC FAILURE -·When everyday run-of-the-mill failure just isn't good enough.
Yes. You are correct. It is noisy. However, it cannot be heard from the upper floor.
--Bill
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You are what you write.
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--DFaust
I calculate it at 50 feet per minute. For an inside machine, whatever it is REALLY moving is fast enough!
I will try to measure it, though.
--Bill
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You are what you write.
Great job so far, do you have any plans ofr sensors?
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--DFaust
Sensors? You bet! The first ones I will implement will be the gearmotor encoders. Then probably some IR sensing and some whiskers and some ultrasonic and . . . you name it.
--Bill
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You are what you write.
I don't think there is any way to silence those gearmotors. I just took it outside and ran it a little bit and it was not so loud . . . I think! I will have a new movie so you can judge for yourself.
--Bill
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You are what you write.
P.S. nice house!
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A complex design is the sign of an inferior designer. - Jamie Hyneman, Myth Buster
I am ramping the speed up and down in both the forward and reverse directions, as necessary. (This machine will definitely not coast!)
All ramping the speed (up or down) means is that the motor is turning at X rpm and you want it to turn at Y rpm, but you do not want it to go from X to Y in one step, instantly. That could shorten gear train life or cause other undesireable results.
Remember that either X or Y (current rpm or desired rpm) could be 0 rpm. So, assuming the current rpm is 0 and I want to make it say, half of the maximum rpm available using this motor environment, then I might simply increase the rpm in the smallest increments possible until I determine it has reached half of its maximum rpm. When the motor has reached that point, then we can say that it was ramped up from 0 rpm to 50% of its total rpm capability.
That was an example of ramping a motor up. It is just as important to ramp one down from its current rpm to your desired rpm. And, it works the same way. Using the same idea of changing the rpm the smallest amount possible, we simply decrease the rpm the smallest amount possible over time until the desired rpm (which may be 0 rpm) is reached.
In the robotics world, motor rpm is usually changed via Pulse Width Modulation (PWM). This is vastly different than changing motor speed by simply increasing or decreasing the voltage. With PWM, the duty cycle is changed. The duty cycle is the amount of time the motor is on. During this time it receives full current and full voltage, therefore it operates at full power. If the duty cycle is less than 100%, then the motor's rpm is decreased from its maximum potential, but the torque of the motor remains constant. (It produces just as much power, but the rpm goes down.)
Implementing PWM is simple by using one of the many, many DC motor controllers available. Parallax offers several varieties, I believe. The one I am using on my current machine is their newest, I think: The HB-25. The HB-25 is extremely simple to use (it would have to be or I couldn't use it).·Like most motor controllers, you simply send it a command and it takes care of handling everything else. In other words, the motor controller accepts your command and creates the proper PWM signal to send to the motor.
There are DC motor controllers and there are DC servo motor controllers and they are different. Some share characteristics of the other, like the HB-25 DC motor controller. Some do not. But, that is not the question you asked me.
I am new to this world of digital electronics. There are many here that are not new by any means and have FAR, FAR more expertise than I will ever possess. I hope they chime in, correct my mistakes above and continue to guide you along this exciting path!
--Bill
ps The house sits on a cliff overlooking a golf course. There are large ponds and small lakes at the bottom between the cliff and the course. I was glad my programming did not drive little Skid Steer off the cliff! (The actual Stamp code that controls the HB-25 can be downloaded free from the Parallax site. I use its setup routines as the "front end" to all my programs that use the HB-25 motor controller. They are simple, short and sweet.)
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You are what you write.