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Author Topic: Doing a Sentry Gun Project, Need Some Advice on Servos  (Read 2137 times)
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Hey there guys,

I'm in the process of designing an airsoft sentry gun that will automatically track and shoot flying targets in the air. This is my 4th year project for mechatronics engineering. To be honest I don't think this is going to be an arduino project since I will most likely be using a computer to control the gun. However, I will be using an arduino at first to test out the high torque servo motors I will be using and to control the gun with a joystick. I have a couple questions about the best method to control and rotate the gun.

I need the gun to be very accurate, so the servos which will be rotating the gun will need to have very good positional accuracy. So with that said:
- What do you guys think will work best to rotate the gun? Are servo motors the ideal tool in this case? Or will using other types of actuators be better?
- What determines the positional accuracy of a servo?
- Are some microcontrollers better at controlling servos?
- Will using a servo motor controller provide better accuracy? Is there even a point in using one?

Those are all the questions I have for now. I'm fairly new to arduinos and servos so I hope you guys can help me, thanks in advance for the replies!
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Hey there guys,

I'm in the process of designing an airsoft sentry gun that will automatically track and shoot flying targets in the air. This is my 4th year project for mechatronics engineering. To be honest I don't think this is going to be an arduino project since I will most likely be using a computer to control the gun. However, I will be using an arduino at first to test out the high torque servo motors I will be using and to control the gun with a joystick. I have a couple questions about the best method to control and rotate the gun.

I need the gun to be very accurate, so the servos which will be rotating the gun will need to have very good positional accuracy. So with that said:
- What do you guys think will work best to rotate the gun? Are servo motors the ideal tool in this case? Or will using other types of actuators be better?
- What determines the positional accuracy of a servo?
- Are some microcontrollers better at controlling servos?
- Will using a servo motor controller provide better accuracy? Is there even a point in using one?

Those are all the questions I have for now. I'm fairly new to arduinos and servos so I hope you guys can help me, thanks in advance for the replies!
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Maine
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There are a few different sentry gun projects out there, most of them use an Arduino or some form of micro-controller to act as an easy way to interface the motors with a PC. The purpose of a motor controller is to provide an easier way to interface motors with a micro-controller. You can try building your own h-bridge, but chances are there is already an all-in-one package that is cheaper and easier to use.
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Electronic props for Airsoft, paintball, and laser tag -> www.nightscapetech.com

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Hmm ok cool. Is easier interfacing the only advantage of a servo motor controller? Does using one improve the performance (ie positional accuracy) of the servos in anyway? I'm only going to be using two servos, one for each axis of rotation. I'm pretty new to this stuff so I would appreciate it if people here can answer some of the questions I've posted up top.
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If you use a servo controlled by a conventional pwm signal as used by most hobby R/C systems, the Arduino can control them directly. However, you would need a fairly hefty servo to give fast, accurate positioning of something as heavy as a gun and I don't think an ordionary small hobby servo would be up to the job. Bigger servos are available, however. I expect you already know this if you're studying mechatronics.

If you choose to use a conventional DC motor/gearbox and have the Arduino do position sensing and feedback control, that's when you would need an H-bridge drive circuit to control the motor.
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Thanks for the advice. I would use some higher end servos but I unfortunately don't have the budget. The gun I'll be using is a light-weight airsoft gun, I'll also be spending lots of front end time minimizing the inertia of the system. I think at least for my prototype some high-torque hobby servo motors should work, I just need to know how accurate they will be and how I can maximize their accuracy. These guys have been building something similar to what I'm trying to do and they seem to be doing well with just hobby servos - http://paintballsentry.com.
« Last Edit: November 08, 2012, 06:22:49 pm by FloatingArk » Logged

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I just need to know how accurate they will be and how I can maximize their accuracy.

Rotational speeds (unloaded and at probably at the higher end of the applied voltage range) for standard servos are usually listed for for the servo. In some simple test I found a standard servo could position at ~425 positions in ~190 deg of rotation. Servos have some built in dead band (~5us) to reduce position hunting. Best to get a servo and start experimenting.
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If it's anything like the turrets I've seen on Youtube it'll be jumping all over the place when it fires, so I wouldn't worry too much about positional accuracy in the first version - I suggest you get something working any way you can, and then see what needs fixing.
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The http://RealSentryGun.com/ works so well that it kinda scares me o_0

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Most of the people I know that have built paintball ones have used the following:-

http://projectsentrygun.rudolphlabs.com/

We have the parts to build three of these, which we'll then make some enhancements too for Bluetooth remote control from a smart phone, and automatic position and status reporting to a live game map. As with anything, just need the time to get back on to it.
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I've tried both. The one from http://www.realsentrygun.com/ is a lot better IMO. It's more popular and has been around a lot longer. It tracks better. The accuracy is much better. The target filtering is much better. It's a lot less buggy too. Their system has been used by Valve for their portal turret, by The Discovery Channel, and by channel 5 in the UK. This isn't an arduino vs pololu thing, it's just that the sentry project's software is a lot more developed. It's likely because they've been at it a lot longer.
« Last Edit: February 11, 2013, 09:23:07 am by David82 » Logged

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Where you planning on using ready-made software or create it yourself?
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