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Anyone know much about servos?

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#1 SushiKitten

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Posted 27 March 2013 - 06:24 AM

Hey guys!

 

So I found this beautiful thing last week: http://www.robotis.com/xe/bioloid_en

 

And I couldn't stop thinking about it, so I rooted around for a base price and it's $1200: http://www.robotshop...obot-kit-2.html

 

Since I am in university and have access to many machines and supplies, I thought I may be able to make this thing for cheaper. So I started looking at the things it comes with. The biggest thing is it comes with 18 $40 servos, which is way too expensive, I might as well buy the kit at that price.

 

I don't know a whole lot about servos, that's going into mechanical engineering and I'm studying computer. I was wondering if anyone with a bit more knowledge could tell me if this servo I found on Sparkfun has the same functionality as the $40 one.

 

The $40 servo: http://www.robotshop...o-serial-6.html

 

The servo from Sparkfun: https://www.sparkfun.com/products/9347

 

What is it that's driving up the price? Just all the extra features? Could I still make a joint move with the Sparkfun servo?



#2 SpleenBeGone

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Posted 27 March 2013 - 07:42 AM

First of all, do want.

 

Now, the mechanics side of things is more my style, lets see if I can help.

 

The expensive server has very little information, so it's hard to compare. It lists the gear ratio, but not the motor speed, so you don't know the turn speed. The cheaper one has half the operating voltage, so you'd have to use a different power source to them. They spin fast enough to make the joins work however, and seem to have plenty of torque. They're plastic geared, but that shouldn't be a problem unless you're trying to lift heavy things. 

To use the cheap ones in a join system, you're going to need to make some mounting plates. You'd need a laser cutter for plastic, or a cnc for metal, unless you have really, really steady hands and drillpress skills. 

 

Overall, it'd be some work, but the cheaper ones can be made to do the same thing. I'd start by first contacting the company though, to see if there's a student discount. 


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#3 SushiKitten

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Posted 27 March 2013 - 07:56 AM

Thanks so much Spleen! They seem to be on backorder, so I might find another place that sells them, but I'm glad that with some work the cheaper ones would be fine. 

 

I've seen a small servo from Sparkfun work off an Arduino at the university's rocket society, so that was my plan to power it. If I try to follow the bioloid design, I'll need a large Arduino or possibly something else to control them all. The Uno I have now wouldn't hold all 18 servos, it only has 9 or 10 PWM outputs.



#4 SpleenBeGone

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Posted 27 March 2013 - 09:21 AM

Yeah, you'd need a bigger board. Those appear to be 360* servers as well, so make sure when you program things, don't let it just spin all the way around. 


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#5 SIlhouette

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Posted 27 March 2013 - 10:06 AM

$40 Servo-

unidirectional

two modes Actuator and continuous 360

15kgf/cm torque

59rpm at 12 volts

 

cheap servo-

monodirectional

single mode continuous 360

3-5kg holding torque

60-70rpm at 6 volts

 

basically you are paying for a multidirectional and stronger servo, it doesnt go as fast but will hold that speed under more stress then the cheapo.

up to your judgement for which you want for what job. Ill have a closer look when I wake up tomorrow since I didn't look at what you needed it for yet.



#6 SpleenBeGone

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Posted 27 March 2013 - 10:18 AM

Where did you see the cheap one being monodirectional? 


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#7 SIlhouette

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Posted 27 March 2013 - 10:31 AM

Where did you see the cheap one being monodirectional? 

 

That is the strangest bloody thing... I could have sworn it said that!

 

I might have gotten the stats on the other one wrong also so for simplicity Ill just post the entire stats and you can help her or I will in the morning when I wake up.

 

This is for the $40 dollar servo.

 

INCLUDES
• AX-12A : 1 set
• B01-HORN : 1 set (pre-equipped)
• FP04-F2 : 1 set
• FP04-F3 : 1 set
• BPF-WA/BU Set : 1 set
• 3P Cable 200 : 1 set
• Bolt PHS M2*6 : 16 set
• Bolt BHS M3*10 : 1 set
• N1 Nut M2 : 16 set
 
 
SPECIFICATIONS
• Weight : 54.6g
• Dimension : 32mm x 50.1mm x 40mm
• Minimum Control Angle : 0.29¡Æ X 1,024
• Gear Ratio : 254 :1
• Holding Torque : 15kgf.cm (at 12V, 1.5A)
• No Road Speed : 59 RPM 12V
• Motor : Cored Motor
• Operating Mode :
1. Actuator Mode (0¡Æ ~ 300¡Æ)
2. Wheel Mode (Endless Turn)
• Operating Temperature : -5C ~ +70C
• Operating Voltage : 9V~12V (Recommended Voltage 11.1V)
• Maximum Currents : 900mA
• Standby Currents : 50mA
• Command Signal : Digital Packet
• Protocol Type : Half duplex Asynchronous Serial Communication
(8bit, 1stop, No Parity)
• Link (Physical) : TTL Level Multi Drop (Daisy Chain Type Connector)
• ID : 254 ID (0~253)
• Communication Speed : 7843bps ~ 1 Mbps
• Feedback : Position, Temperature, Load, Input Voltage, etc
• Material : Engineering Plastic
• Position Sensor : Potentiometer
 
 
INITIAL SETTING (default)
• ID : 1
• Baud rate : 1 Mbps
• Please change the setting of ID and baud rate depending on your operating environment.
 
 
COMPATIBLE PRODUCTS
• Controller : CM-5, CM-510, CM-2+, CM-700
• Interface(I/F) : USB2Dynamixel
• Frame : Frames for Bioloids


#8 SpleenBeGone

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Posted 27 March 2013 - 10:45 AM

Yeah, the expensive one is going to be much higher quality, but I think for her purposes, she can make the cheap one work. 


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#9 SushiKitten

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Posted 27 March 2013 - 12:05 PM

I think I will go for the cheaper one. The more expensive one is better quality, but I just want to see if I can construct this thing cheaply first. I can replace them later down the road when I can afford it. I'll just be careful to keep the frame light so I don't overload the servos.



#10 SIlhouette

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Posted 27 March 2013 - 10:03 PM

Yeah its mainly the torque that is key.

 

Likely wont be able to make the dinosaur or human model with such low power, but the spider looks like it should work well.



#11 Dr. Strangelove

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Posted 03 April 2013 - 01:45 AM

This is probably way off what you guys are talking about, but may as well give it a shot.

I've looked into various 'hexapod' designs before and these I found are quite attractive. I would say if you used light materials (balsa wood for example) with el cheapo servos first as a test (as to not strain the petit servos too much, and see their limits), persay, then you could progress to heavy duty expensive stuff as the funds increased.

http://www.instructa...es-coming-soon/

http://www.instructa...-based-on-FPGA/

(Instructables for those who dont already know about it, is a magnificent website)

https://www.youtube....h?v=2IAZpuwhqew

 

These are a few, as far as I know they're not as expensive as the $1200 beast of a critter, but they do a good job.


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