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  #1  
Old 09-18-2007, 01:22 PM
Edblad
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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!
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  #2  
Old 09-19-2007, 04:14 AM
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Fred Fred is offline
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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 07:25 AM.
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  #3  
Old 09-19-2007, 11:04 AM
Edblad
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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
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  #4  
Old 09-19-2007, 12:14 PM
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Fred Fred is offline
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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
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  #5  
Old 09-19-2007, 01:27 PM
kalkurap kalkurap is offline
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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.
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  #6  
Old 09-20-2007, 02:36 AM
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Fred Fred is offline
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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
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