zedin
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Re: Robotics platform; build or buy? and motors or servos?

Thu Apr 12, 2012 3:16 pm

So I have been tinkering with an arduino (thanks for those suggestions in my other thread) and learning a bit.  I want to try and move onto starting to work with a platform so my robot can move about.  So I had some questions in case folks have any suggestions.

First.. For a mobile platform would you recommend buying something like the DFRobot 4wd or build my own out of plexiglass?

The other question is somewhat related.. for moving the robot.. dc motors or continuous servo?  I am not real clear on what the specific differences between them would be and the pros and cons of each.

To give folks an idea of my ambitious first robot I want to make a mobile robot that is your typical wandering robot with sensors to not bump into walls.  The fun aspect is I also want to attach a sensor up front at say a 45 degree angle that can detect movement in front of the robot (and of course take into account the fact the robot is moving).  If it detects something moving in front if the robot that is not just due to the robot's speed I want to fire a nerf gun.  So basically a automatic wandering nerf turret =p

Ravenous
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Re: Robotics platform; build or buy? and motors or servos?

Thu Apr 12, 2012 4:21 pm

That one's got four driven wheels - I would check first if it's able to do a complete turn on carpet without friction stopping it.  (Do the wheels have enough torque to turn with weight on the back, basically.)

A differential drive one (only two driven wheels, plus an extra castor wheel at the front and back to stop it tipping) wouldn't have this problem, but they tend to be more wobbly...

Having said that it seems a good price for what you get, so my comment shouldn't be taken as a criticism.

The DC motors look like they have encoders (only two, not all four?) so you could keep count of distance travelled and angle rotated, depending on the accuracy.  Though tyre slippage when rotating will probably affect the encoder accuracy - but that's probably the case for all four wheelers, my guess.

You might first want to search for the "Where am I" PDF book (file name is pos96rep.pdf) which is an old literature research survey which contains a lot on this.  It's huge and contains more stuff than I could possibly do, the bits which talk about odometry (using the encoders) are worth reading.

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exartemarte
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Re: Robotics platform; build or buy? and motors or servos?

Thu Apr 12, 2012 4:34 pm

Continuous servos have their adherents - they're easy to use and don't need a separate motor driver - but they're limited to quite small robots. My preference is for geared dc motors.

The chassis you've linked to looks nice, but for a first mobile robot I would pick something cheaper, such as this chassis from Sparkfun. Whatever you start with, you'll probably want to move on before too long.

Both of these chassis have dc motors, so you will need a dual h-bridge motor driver. I'm not an Arduinist, but I imagine there will be motor driver shields that plug into your Arduino.

With all of this set up correctly you will have independent pwm control of both motors - make sure the motor driver can handle the motor stall current.

zedin
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Re: Robotics platform; build or buy? and motors or servos?

Thu Apr 12, 2012 4:46 pm

exartemarte said:



With all of this set up correctly you will have independent pwm control of both motors - make sure the motor driver can handle the motor stall current.


Without sounding dumb.. (trying to read up on all this before I get started).  What is the motor stall current?  I have seen it listed (along with other current/voltage specs) on motors but not really sure what it means.

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exartemarte
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Re: Robotics platform; build or buy? and motors or servos?

Thu Apr 12, 2012 4:55 pm

zedin said:


exartemarte said:



With all of this set up correctly you will have independent pwm control of both motors - make sure the motor driver can handle the motor stall current.


Without sounding dumb.. (trying to read up on all this before I get started).  What is the motor stall current?  I have seen it listed (along with other current/voltage specs) on motors but not really sure what it means.


Counter-intuitively, a dc motor draws maximum current when it's not moving. This may be because it has stalled under load, or straight after switch-on when it hasn't got started yet, and it is known as the stall current. In the case of the Magician chassis, they claim it's about 1 Amp. A motor driver using the common L298N controller would be adequate.

Michael
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Re: Robotics platform; build or buy? and motors or servos?

Thu Apr 12, 2012 8:18 pm

I guess it depends on your skills.

My woodworking skills are good, but my precision metalworking skills are poor and I enjoy the electronics and software side of things so I would be tempted by a kit or completed platform rather than build one myself.  If I were to build something myself, I would probably opt for plastic and wood rather than metal.  But you or someone with differing skillsets might make a different choice.

Regarding motors, I would probably opt for stepper motors although they are more expensive.

Hopefully we'll see some Raspberry Pi specific platforms and kits emerge over the next couple of years.

MG2R
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Re: Robotics platform; build or buy? and motors or servos?

Thu Apr 12, 2012 10:08 pm

zedin said:

Without sounding dumb.. (trying to read up on all this before I get started).  What is the motor stall current?  I have seen it listed (along with other current/voltage specs) on motors but not really sure what it means.

The technical explanation of the stall current being higher than working is that the solenoid inside a DC motor generates a counter-EMF (EMF stands for electromotive force) when it is turning, which works against the applied voltage. That means that the total voltage over the solenoid will be lower than the source voltage, thus lowering the current flowing through the motor. The faster the motor is spinning, the higher the counter-EMF will be.

As a rule of thumb you can say that StallCurrent = 7*NominalCurrent

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SN
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Re: Robotics platform; build or buy? and motors or servos?

Thu Apr 12, 2012 10:19 pm

this is why you can blow the windscreen washer fuse on your car if the pipes/nozzles are frozen when you try to use them (I think) - been there, done that, swapped a 10A for 20A fuse and all is OK now
Steve N – binatone mk4->intellivision->zx81->spectrum->cbm64->cpc6128->520stfm->pc->raspi ?

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exartemarte
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Re: Robotics platform; build or buy? and motors or servos?

Thu Apr 12, 2012 10:31 pm

SN said:


this is why you can blow the windscreen washer fuse on your car if the pipes/nozzles are frozen when you try to use them (I think) - been there, done that, swapped a 10A for 20A fuse and all is OK now



Hmmm...

While serving Her Majesty, I once heard the No 1 on a radar detachment telling his operators, in all seriousness, that they must "never replace a fuse with one of the wrong value - if you haven't got the correct fuse then wrap the silver paper out of a cigarette packet round the old one and use that".

zedin
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Re: Robotics platform; build or buy? and motors or servos?

Fri Apr 13, 2012 6:23 pm

So I think I am going to go ahead with motors with encoders.  The question I have is what sort of motor driver to get.  I have seen most can handle 2 motors but I also saw http://www.robotshop.com/dagu-.....ler-5.html which can do 4 motors with higher end amps.  For future scaleability is this better then say the arduino motor shield that can only do 2 motors at 2A?

MG2R
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Re: Robotics platform; build or buy? and motors or servos?

Sat Apr 14, 2012 2:21 pm

zedin said:


So I think I am going to go ahead with motors with encoders.  The question I have is what sort of motor driver to get.  I have seen most can handle 2 motors but I also saw http://www.robotshop.com/dagu-.....ler-5.html which can do 4 motors with higher end amps.  For future scaleability is this better then say the arduino motor shield that can only do 2 motors at 2A?


That motor controller looks quite fancy However, it has also been designed to power motors up to 2A, shutting down when it detects a stalling motor. It's your choice, really.

SeanD
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Re: Robotics platform; build or buy? and motors or servos?

Sat Apr 14, 2012 3:44 pm

I have a couple of the DFRobot 2WD chassis that I use with my kids for basic robotic development and play platforms.  For the money they are excellent. The only issue we have had with them is that the supplied wheels are a bit of a pain to get onto the shafts of the motors but all it takes is some perseverance.

Motors and a motor controller are the way to go.  Assuming that you are going to be doing this whilst waiting for your RPI then I can thoroughly recommend the DFRobot Romeo board. It is an Arduino plus 2 channel motor controller all in one for sensible money.  Even has some really nice features like additional power connectors by each pin so you can use very neat cabling with sensors and servos.

One of the sensors that you will pretty quickly find you need are encoders on the motor shafts. Trying to do anything useful without them will end up with great frustrations as motors will run at different speeds, and these differences will be accentuated as the batteries fade.  The encoders will let you know how far each wheel has turned so you can make them turn very precisely.  I am pretty sure DFRobot sell encoder kits, I used some I had sourced from SparkFun for the 2WD chasis but have also fabed my own in the past.

Power supply is also something to think about.  The DFRobot kit as I remember somes with an AA batter holder.  After a while you will probably find you want to move up to LiPo as it is amazing how fast you can go through a set of batteries and the more sensors, servos and actuators you add to your platform the higher charge to weight ratio you want from your power source.

One last tip, because it is fun. As I recall DFRobot sell a tiny PCB that enables you to easily (i.e. without getting some wire cutter out) interface with a Wii Nunchuck. I think it is a couple of bucks. Anyway the Nunchuck makes and awesome remote control input.

n31l
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Re: Robotics platform; build or buy? and motors or servos?

Sat Apr 14, 2012 7:30 pm

exartemarte said: 


Hmmm...

While serving Her Majesty, I once heard the No 1 on a radar detachment telling his operators, in all seriousness, that they must "never replace a fuse with one of the wrong value - if you haven't got the correct fuse then wrap the silver paper out of a cigarette packet round the old one and use that".



I once tested the fusing point of a few house hold fuses using a fuse tester, then we wrapped the fag packet silver paper round the fuse and we tested it. It popped the fuse in the tester after going very black. must have held more than 50A!

LinuxCircle
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Re: Robotics platform; build or buy? and motors or servos?

Sat May 04, 2013 2:11 am

A few affordable robot board are readily available, I have tried two options:
1. Gertboard with Dagu 4 channel motor expansion
2. DFRobot Romeo V2, which has 2 channel motors.

I find that DFRobot Romeo V2 is much easier to configure. You just basically need to plug it in into your Raspbery Pi and run Arduino IDE to program it, whereas the Gerboard needs several customisation on your Raspberry, and several jumper connections to program its ATMega board to make it Arduino compatible. :ugeek:

The Romeo comes with several Analog ports and Digital ports, as well as 2 motors, and takes power from Raspberry Pi's USB port.

A lot of superb articles written about the two: http://www.linuxcircle.com
www.linuxcircle.com

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