TinyRC.com - XMODS, XMOD, Micro Flight, ZipZaps, ZipZaps SE, Bit Char-G, MicroSizers, TTTT, Plantraco Desktop Rover, SuperSlicks, Digi Q, Forums, News, Pictures, Parts, and Shop
Forums, XMODS, XMOD, Micro Flight, ZipZaps, ZipZaps SE, Bit Char-G, MicroSizers, TTTT, Plantraco Desktop Rover, SuperSlicks, Digi Q
XMODS Hop-Ups, XMODS Parts, XMOD Hop-Ups, XMOD Parts, Bit Char-G Hop-Ups, Bit Char-G Parts, MicroSizers Hop-Ups, MicroSizers Parts
  #1  
Old 09-18-2007, 02:22 PM
Edblad
Guest
 
Posts: n/a
Linear servos?

Hello gang,

New member here. Have been admiring TTTT and other 1:87 scale RC/IR models for a few years now. Finally made a decision to make a start. For the first project I want to make my own micro servo. I know I can manage to construct the mechanical parts, but the electrics are worse.

My question is; how does the servo selfcenter and how is this system constructed?

Thanks for helping a newbe out!
Reply With Quote
  #2  
Old 09-19-2007, 05:14 AM
Fred's Avatar
Fred Fred is offline
Registered User
 
Join Date: Nov 2006
Location: 3rd planet, tiny solar system
Posts: 24
Quote:
...My question is; how does the servo selfcenter and how is this system constructed?
...
Hi Edblad,

servos use a feedback mechanism, comparing the desired position (usually derived from a PWM signal [1-2ms pulses every 20ms]) with the actual position. In most standard RC servos, a potentiometer connected to the drive shaft senses the actual position. Some of the basics are explained here:

http://www.geocities.com/bourbonstre...rvobasics.html

Instead of a pot for position sensing, many alternatives are possible: Hall sensors (linear sensors or digital ones [together with a magnet on one of the shafts driving the gear; the microcontroller calculates the actual position by taking the gear ratio into account]), optical sensors, etc.

I have built my own (digital) servos (using optical and Hall sensors) for special applications. In many cases, you can trim down the case of a commercial microservo so that it fits into a 1/87 scale model.

Another very helpful source of information is the open servo project:

http://www.openservo.com/


All the best,

Fred

Last edited by Fred; 09-19-2007 at 08:25 AM.
Reply With Quote
  #3  
Old 09-19-2007, 12:04 PM
Edblad
Guest
 
Posts: n/a
Hello Fred,

If I understand it correctly, the microcontroller counts how far (number of rotations of the axle) the servo horn travels when the transmitter stick is set back to the center position and then run the motor in the other direction for equal number of rotations, so that the servo horn ends up in the center again.

I've no experience what so ever about PIC-programming so I got some reading to do. What kind of info does the microcontroller need and how to put it into a code?


Regards and many thanks,
Alexander
Reply With Quote
  #4  
Old 09-19-2007, 01:14 PM
Fred's Avatar
Fred Fred is offline
Registered User
 
Join Date: Nov 2006
Location: 3rd planet, tiny solar system
Posts: 24
Hi Edblad,

servos with switch type Hall sensors (the ones that can be used to count each shaft rotation) are tricky in terms of centering, unless you store the absolute position (as it relates to number of rotations). An alternative is to provide stops (at each end of the servos's travel). When the servo is powered up and the microcontroller goes through its initialization, it lets the servo run to both stops, keeping track of the position of each stop and defining the center as a certain position (e.g. 50%) between both stops.

There are many microcontroller families. I personally use the Atmel AVR family, many people prefer other tyes, e.g. PIC controllers. Google will help you find everything you need to know about programming these devices (use search terms such as "introduction microcontrollers" or "microcontroller tutorial", optionally adding the name of the controller family). Free development software and easy programmability (DIY interface to a parallel port or similar) are advantages, but not essential.

Writing servo software is probably quite challenging as a first microcontroller project and may end up in frustration. To get started, a more straightforward projects may be preferable (such as writing software to control the lights of a micromodel; adding flashing lights etc. makes for some interesting programming; you can later use the routines that analyze the RC signal for a servo project). Learning from the open servo project may be the next logical step.


All the best,

Fred
Reply With Quote
  #5  
Old 09-19-2007, 02:27 PM
kalkurap kalkurap is offline
Registered User
 
Join Date: Feb 2005
Posts: 40
If you are building a linear servo, can you not use a linear potentiometer such as the one listed in the following link directly at the output to measure the position?

http://www.panasonic.com/industrial/...OL0000CE13.pdf

For example, the 9mm stroke type has an overall dimension of 9mm X 16mm, not really tiny, but you might be able to reduce it even further by trimming it. What might be good is that you may not have to build any circuit, instead you might be able to use the circuit from any existing servo provided the resistance the linear potentiometer matches that of the pot in the servo.

Following along what Fred mentioned, if you chose to use switch type of sensors, can you use a sensor at the center as well? That way you can take the guess work out of the equation, and use the sensor to determine the center.

By the way Fred, did you use any sensor at all for you wheel loader steering mechanism? How does the model drive if the motor drives an open loop system with the operator acting as the feedback? I can understand why closed loop systems are better for larger models, but I am interested to know how such a system works at these scales.
Reply With Quote
  #6  
Old 09-20-2007, 03:36 AM
Fred's Avatar
Fred Fred is offline
Registered User
 
Join Date: Nov 2006
Location: 3rd planet, tiny solar system
Posts: 24
Quote:
Originally Posted by kalkurap View Post
...use a linear potentiometer such as the one listed in the following link directly at the output to measure the position?
Hello Prasanna,

you are certainly right. Here are some commercial servos (top 3 on this page) where linear pots were used in micro servos:

http://www.wes-technik.de/English/light_ng.htm

Quote:
... can you use a sensor at the center as well? That way you can take the guess work out of the equation, and use the sensor to determine the center.
That is certainly right, sometimes you may just not have sufficient room available when you try to build a real tiny servo.

Quote:
By the way Fred, did you use any sensor at all for you wheel loader steering mechanism? How does the model drive if the motor drives an open loop system with the operator acting as the feedback?...
It turned out that the stepper motor I am using for the steering mechanism is strong enough. It is not missing any steps. Therefore, the microcontroller can simply count steps to identify the center position.

Warm regards,

Fred
Reply With Quote
  #7  
Old 09-23-2007, 06:12 PM
Edblad
Guest
 
Posts: n/a
Thanks for your help!

Now I realise how much work it is getting the servo working. In the meantime I've found another option; the muscle-wire servo. Is there anyone who have any experience with these?

Regards,
Edblad
Reply With Quote
  #8  
Old 09-25-2007, 10:08 PM
kalkurap kalkurap is offline
Registered User
 
Join Date: Feb 2005
Posts: 40
Not a muscle wire based servo, but is more like a linear motor, by Mr. Koichi. You might find it interesting:

http://www.oyajin.jp/~toko/pic/0062/index.html

Google translation:

http://72.14.203.104/translate_c?hl=...language_tools
Reply With Quote
  #9  
Old 09-26-2007, 12:28 AM
Edblad
Guest
 
Posts: n/a
Thanks a lot! This servo/actuator would be perfect. 33 positions sounds pretty acurate, at least for steering. Just have to learn PIC- programming now... The code is available at the page but have no clue how to put it in the chip.
Reply With Quote
  #10  
Old 09-26-2007, 12:50 PM
kalkurap kalkurap is offline
Registered User
 
Join Date: Feb 2005
Posts: 40
You are right, the code is available on that site, but is in .hex format. That is fine if you want to program it in to your chip, but if you want to make any changes or use a different chip you might need to do more work on that such as converting it back to assembly.

It will be interesting to know how much torque an actuator like this one has.

As far as programming the chip goes, the fastest way probably is to contact someone who has a PIC programmer and ask them to program it for you. I could have done it for you, but I am in the states and I do not know if the trouble of shipping it from here is worth it. Let me know if you are interested, I will be glad to help.
Reply With Quote
  #11  
Old 09-26-2007, 01:45 PM
Edblad
Guest
 
Posts: n/a
Great thanks for your offer! Is there much trouble learning programming? Maybe I could solder up the interface to start with and then learn about the actual programming. I guess it's much more work invold than just "copy and paste" the code?

Yes, it would be interesting to know how much torque it will produce, especially before the work is started. But, the aerodynamic forces acting on the rudder, which the actuator must overcome, can't be less than needed for the steering of a 1:87 car, I think.
Reply With Quote
  #12  
Old 09-26-2007, 04:23 PM
kalkurap kalkurap is offline
Registered User
 
Join Date: Feb 2005
Posts: 40
Programming isn't all that difficult, especially the PICs. Any prior experience with programming such as microcontrollers, microprocessors or even 'C' etc makes it even easier. It is logical and fun, hence anyone with enough interest should be able to tackle it.

Provided you have the programmer set up and are using the exact same chip as mentioned in the original article, then programming the chip is as easy as "copy and paste". But that isn't often the case, and that is why you might be required to know how to write the program yourself.

For example, Mr. Koichi's circuit uses the chip PIC12C509A which is an EPROM based device meaning it can be programmed only once. I prefer devices such as 12F625 which are EEPROM based, and can be re-programmed. Even though these chips are very much similar to each other, the hex file generated for 12C509 may not work on 12F625 without some minor modifications. In an ideal situation, you will need to have the source code of the program in assembly language (.ASM file as opposed to .HEX file), make a few changes as required and generate your own .hex file.

If you have the slighest interest in programming these chips, there is absolutely no reason for not doing it.
Reply With Quote
  #13  
Old 09-27-2007, 12:27 AM
Edblad
Guest
 
Posts: n/a
When you put it this way it really sounds possible. I've always wanted to learn PIC-programming and build my own IR TX and RX, maybe I'm getting closer to my goal.

I made a search for the chip that you mentioned but it seems that it can't be found here in Sweden. The other one is available though. And cheap too, so if one or another fails I can always try a new one.

Do you know where I can find a layout for an interface?

Thanks for helping me out!
Reply With Quote
  #14  
Old 09-27-2007, 10:27 AM
kalkurap kalkurap is offline
Registered User
 
Join Date: Feb 2005
Posts: 40
Alexander,

I am not familier with the current generation of DIY PIC programmers. I did build one long time ago, but have been using programmers sold by Microchip for many years.

Having said that, there are a couple of things you need to keep in mind before you start. First thing is you need is a software which allows you to write the code, assemble it and simulate it on the PC. MPLAB IDE is probabaly the best one out there and you can download it from here:

http://www.microchip.com/stellent/id...&part=SW007002

Next, you will need a programmer. I have found several sites that show circuit diagrams of easy to build programmers. In my opinion it is better to go with a serial (RS-232) programmer as RS-232 can be still found on laptops etc. When you build your pgorammer you will have to choose the right type of socket for the PIC that you are going to program. For DIP packages ZIF sockets work well (although a bit expensive). For SMD devices you might need to make your own socket as some people have done (for example, look at Mr. Koichi's site). If you are doing in circuit programming, then you might need to make a pig tail connection. Here are some of the programmers:

http://www.jdm.homepage.dk/newpic.htm

http://www.myke.com/elcheapo.htm

And finally you need a program that can download the code you have written into the programmer. Assuming that you are using Windows, the following might be a good choice:

http://freenet-homepage.de/dl4yhf/winpicpr.html

It is mentioned in that site that is software works with the above mentioned programmers and doesn't need any additional drivers.

Hope this helps,
Prasanna.
Reply With Quote
  #15  
Old 09-29-2007, 03:28 PM
Edblad
Guest
 
Posts: n/a
Thanks for the links. Before starting with the servo I'll start with a simpler project. Instead of creating a new thread I ask here. What I had in mind is a circuit connected to a radio receiver that allows you to control a couple of LEDs (only one channel, on and off function). Anyone who knows how it can be done?
Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Aftermarket servos nchogberg XMODS 1 08-11-2004 02:14 AM
XMODS NOT Linear! xmodSpeed XMODS 11 05-02-2004 12:56 PM
i think i have a new mod for the servos Silent_NightR34 XMODS 6 10-26-2003 12:33 AM
Cheap micro servo's? Tazzy TTTT 8 10-05-2003 11:16 AM
faller servos dtmracer TTTT 1 03-09-2003 10:13 PM


All times are GMT -5. The time now is 05:46 AM.


Powered by vBulletin® Version 3.7.3
Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Mini Inferno Sale - Up to $85 Instant Savings!
Micro-T Hop-Ups
RC18R, M18, Micro RS4, Mini-LST, TamTech-Gear, Minizilla, RC18T, RC18B, RC18MT
shop.tinyrc.com Products
Tiny RC Community News
[03/17/10] Kyosho Mini-Z F1 MF-010 SP2... : The Kyosho Mini-Z F1 MF-010 SP2 Carbon Limited ASF 2.4GHz Tx-Less Chassis Set - more»
[10/23/09] Kyosho Mini-Z MR-03 In Stock! : The Mini-Z MR-03 (http://mr-03.com) is now In Stock and Pre-Orders are shipping! - more»
[09/06/09] Labor Day Savings - $5 Off... : Labor Day traditionally marks the unofficial end of summer - but we're not ready to let go! So, we - more»
Mini-Z, Mini-Z Racer, MR-02, MA-010
M18, M18T, RC18T, Mini-LST, Mini-T, Micro RS4, XRay, 1/18, 18th scale
XMODS, XMOD, Micro Flight, ZipZaps, ZipZaps SE, Bit Char-G, MicroSizers, TTTT, Plantraco Desktop Rover, SuperSlicks, Digi Q
Mini Inferno, Mini Inferno ST, half EIGHT, 1/16, 16th scale
Epoch, Indoor Racer, 1/43, 43rd scale
E-Savage, eSavage, eZilla, e-Zilla, HPI
Robots, Bots, Bipeds, Wheeled, Manoi, Roomba, NXT, Lego, Hacking
Crawling, Crawlers, Micro, RC, Losi Mini-Rock Crawler, Duratrax Cliff Climber
Kyosho Minium, Caliber 120, Minium Forums
Mini-Z Hop-Ups, Mini-Z Parts, Mini Inferno Hop-Ups, Mini Inferno Parts, M18 Hop-Ups, M18 Parts