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Author Topic: Building a hexapod - i need help .  (Read 1136 times)

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Offline bilalsTopic starter

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Building a hexapod - i need help .
« on: February 26, 2010, 01:04:15 PM »
Hello everyone, im new to this forum. I just got into robotics (2 days ago). I have been reading about hexapods since then. Its a project for the university and im really interested in building it.

I have 18 servo motors (type DS8911) in hand. Below are the specifications:

Torque at 4.8V: 25.0kg.cm
Speed: 0.19sec/60 degrees
Dimension: 35 x 21 x 40mm
Weight: 69g

Obviously, there will be 3 servos per leg hence 3 degrees of freedom.

I was looking for ready made microcontrollers and i came across Axon II which i suppose can easily handle 18 servo motors (correct me if im wrong).

1st question:
The servo motors are to be connected to the pins on Axon II that provide unregulated battery voltage. So, what does unregulated here mean? Does it affect the servos? What battery voltage is needed to operate my servo motors? Will the battery be able to operate all 18 of them?


I want to operate the robot wirelessly using an Xbee adapter.
2nd question:
Can i connect the Xbee adapter to the Axon II MCU? If not is there any other type that i can use?

Another thing im not sure about is the programming and controlling the robot. Let me explain:

Lets say i want to move the robot forward 1 step. Im not sure how, but lets forget about that bit for now. This will require a couple of programming lines.
I have to upload the program to the mcu. Can i then for example execute the program using the PC?

Im not sure if that was clear. let me explain more.

I can write a program that can move the robot forward. I can also write a program that moves the robot backward.I do have to upload them, right ?Will they run automatically (I dont want them to) ?
Can i then send a command from pc to execute either the move forward program or the move backward program or any program i have uploaded ?
What if i executed the program twice. I mean can I send two execution commands, for example by executing the move forward program twice, the robot will move two steps forward.

One more thing is that i live in the United Arab Emirates - Abu Dhabi (Middle East). I want to order Axon II and i think it can be delivered via UPS? Any idea on how long will it might take to be delivered ?

Can i finish the project in one month time if i get all components needed ?


« Last Edit: February 27, 2010, 12:09:37 PM by bilals »

Offline blackbeard

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Re: Building a hexapod - need help .
« Reply #1 on: February 26, 2010, 03:32:51 PM »
"The servo motors are to be connected to the pins on Axon II that provide unregulated battery voltage. So, what does unregulated here mean? Does it affect the servos? What battery voltage is needed to operate my servo motors? Will the battery be able to operate all 18 of them?"

regulating voltage means limiting the voltage supplied. most things run at 3.3-5v and at a low current. since servos are motors they require more current and if powered from an insufficient regulator they can burn out said regulator. you should see how much power your battery can supply and how much your servos draw. if you are conscious of battery weight you can note that they will not all be running simultaneously and plan/program accordingly.

"Can i connect the Xbee adapter to the Axon II MCU? If not is there any other type that i can use?"

this i can't confirm but i'm pretty sure that it can. unless it's an Xbee arduino shield.

"Another thing im not sure about is the programming and controlling the robot. Let me explain:

"Lets say i want to move the robot forward 1 step. Im not sure how, but lets forget about that bit for now. This will require a couple of programming lines.
I have to upload the program to the mcu. Can i then for example execute the program using the PC?"

if it's like the arduino and i'm assuming the axon can do everything and more then the arduino then you should be able to send signals via the serial port to the mcu. you could use a parallel port if you REALLY wanted but why? you would loop your forward, turning and backward movements and then just go to them under certain circumstances that you decide. if i understand correctly then what you want to do is you want to tell it where to go via pc right? that's entirely possible but i'm not sure how to do it. i know a combination of visual basic and mcu has been used in the past.

"Can i finish the project in one month time if i get all components needed ?"

dunno. how good are you with programming? assembling the robot is easy if it comes with a prebuilt chassis. you probably wont even need to solder anything. to be honest though if it's your first project then go with wheels. even NASA uses wheels because it's simpler and is the most effective.
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Offline waltr

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Re: Building a hexapod - need help .
« Reply #2 on: February 26, 2010, 06:58:15 PM »
Blackbeard answered your questions well. To add:

Most of the XBee modules can pass serial data from your PC with the proper level translation hardware, MAX232 chip or a XBee to RS232 or USB adapters, to a microcontroller's  (Axon) serial port (UART). The Series 1 XBees running the 802.15.4 protocol are the easiest to use but first check on your country's radio regulations to be sure these are legal to operate. For testing your code it would be better to run a cable from the PC to the robot. Once that is working then add the radio link.

Yes, you will need to write and load a program into the Axon processor. The code would 'watch' for data on the serial port then check what command it is and execute the appropriate code to act on that command. Once you learn programming this is not hard to do.
There is a large user base for the Axon processor with lots of code examples to get your started.

Good luck and have fun.

Offline bilalsTopic starter

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Re: Building a hexapod - need help .
« Reply #3 on: February 27, 2010, 02:30:51 AM »
Blackbeard:
Thanks for your help, but i have more questions regarding the battery. If for example all 18 servos are running with 20ms delay between them.
servo1 moves-20ms later-servo 2 moves-20ms later-servo 3 moves.....etc

Can this be considered as one servo running at a time? 5v or 6v battery would do the job ? Are the servos connected in parallel, such that they'll have the same voltage?

waltr:
Thank you also for the help, i would like you to give me a link for the Xbee module that would work on Axon II.

So guys, do you advise me going ahead and ordering the Axon II and Xbee.

Again, thanks for your time and effort helping me guys.

Offline HyperNerd

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Re: Building a hexapod - i need help .
« Reply #4 on: February 27, 2010, 12:34:54 PM »
The 20ms between each servo signal does not mean that the servo isn't drawing power afterwards.
When you send the signal, which is 1 - 2ms long, that tells the servo what position to move to.
Depending on your servo, the motor may be running for over a whole second (1000ms) after this pulse to reach the position you told it to go to.
This means that you would need to time how long it takes one of your servos to turn fully from one extreme to the other (1ms pulse to a 2ms pulse), then leave at least this amount of time between each servo moving if you only wanted one to move at a time.

And yes, the servos would be connected in parallel.

 -HyperNerd
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Offline bilalsTopic starter

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Re: Building a hexapod - i need help .
« Reply #5 on: February 27, 2010, 03:00:23 PM »
This means that you would need to time how long it takes one of your servos to turn fully from one extreme to the other (1ms pulse to a 2ms pulse), then leave at least this amount of time between each servo moving if you only wanted one to move at a time.
 -HyperNerd

Thanks. I got it for the case where one moves at a time. Now if i want to move as many servos as i want at the same time, will that affect the battery? and How? Will all servos move?

Offline HyperNerd

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Re: Building a hexapod - i need help .
« Reply #6 on: February 28, 2010, 04:14:55 AM »
If you want to move more servos at once, you will need a powerful battery.
If each servo draws say, 500 mA, then if 5 are going at once, that's 2.5A draw on the regulator / battery.

Check your battery's specs to see how much power it can supply at any one time. Call this number in Amps, A.
Connect an ammeter, or multimeter set to Amps, to your servo and see how much power it draws when moving. Call this number in Amps, B.

The total number of servos which can move at once will be (A/B)-1
You could get away with A/B, but the -1 is just to leave some breathing room for your battery, so to speak.

Example:
Battery specs show 3A max output.
Servo draws 0.75A (750mA) when moving.
This means that I can use (3/0.75)-1 = (4)-1 = 3 servos at once.

 -HyperNerd
There are 10 types of people in the world - those who understand binary, and those who don't.

Offline bilalsTopic starter

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Re: Building a hexapod - i need help .
« Reply #7 on: February 28, 2010, 07:32:31 AM »
Thanks HyperNerd for the thorough explanation , i got it now.

Offline chelmi

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Re: Building a hexapod - i need help .
« Reply #8 on: February 28, 2010, 10:36:09 AM »
Connect an ammeter, or multimeter set to Amps, to your servo and see how much power it draws when moving. Call this number in Amps, B.

Does it make a difference whether if the servo is under load?

 


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