Power Functions Servo at 12v

[BrianZ]

Hey all,

I'm working on the second phase of my 2.4GHz radio controlled Power Functions, which is to run them off of a 11.1v 3S LiPo battery. I have read posts on this indicating that it may not be so bad for the motors, but the IR receiver is probably not up for it. That's no big deal to me because my radio receiver can handle up to 30v. So I'm all full steam ahead, and I just now remembered that the servos have electronics in them that may be sensitive to those voltages.

Has anybody run a Power Function servo at 12v yet? Did it work?

Brian Z

[Tamas Juhasz]

Sorry for off, but if you use non Lego bettaries and receiver, etc., then why don't you use a custom servo too? Is the Lego PF servo motor cheaper for this application, than RC ones? If you use an RC system, you could use non Lego motors (also servo) with much bigger performance. --> more fun.

Edited January 19, 2014 by Mbmc

[MrNumbskull13]

I have no idea why lego servos are so expensive when you can buy an rc one for about £5?!

[BrianZ]

I agree about the non-Lego servos. I'm trying to strike a balance between using only Lego parts and parts that are not Lego. The way I'm striking that balance is to pretend that I work for Lego (because who wouldn't like to work for Lego?) and I'm in charge of developing a system for Power Functions that makes building radio controlled cars fun. I don't think my bosses at Lego would be cool with me mounting a Futaba servo in my car when we already have a servo developed. On the other hand, we don't build batteries, and I think customers would like a high-powered rechargable LiPo battery pack option, so I'm working on that.

Before I started on this project I considered that my imaginary employer was Futaba instead, and what kind of Lego compatible housings could I build for our servos, receivers, battery packs, and motors? Basically a Futaba Power Functions system (which would be really cool, hint hint Futaba) that worked seamlessly with Lego. In real life I design food, dairy, and pharmaceutical processing equipment, so I have a lot of 3D modeling experience, and I thought about going that way. Research shows that it is difficult to produce 3D printed parts that work well with Lego though, due to the tight tolerances required, so I'm taking the Lego employment route.

You'll know I've gone officially crazy though, when I start listing my imaginary employers on my resume.

[Boxerlego]

9 volts is the most I put on the servo.

I been thinking about the servo motor, and wanted to share my re approach on a servo design. The route I'm taking is back to the basics and divide the servo motor up back in to its basic parts. The servo is basically made up of gears motors and electronics. Lego has 2/3 of this already made, all they need is the right electronics to be flexible enough to accommodate a fluid construction for servo function.

[BrianZ]

Boxer, are you suggesting a module that clicks on to the front of a PF motor, which sends position feedback to a controller? You could add servo functionality to any of the motors like that. The more I work with the Lego PF servo through my radio control circuit, the more I like it for steering though. It's the Lego IR transmitters that hold it back.

[whale2]

I share BrianZ's point, there is no sexy way to connect RC servo to the rest of Lego construction. If size and weight of Lego servo is acceptable for given application, I'd go with it.

[Boxerlego]

Yes! Check this out. It all start with an idea a LEGO IDEA. What you see down below is the 5206 speed computer and two servo motors one of them is dismantle to have a better view of the main subject. What If the speed computer was made out to be a servo driver. This can connect up with any motor up and establish a basic servo function control over a motor.

Edited January 19, 2014 by Boxerlego

[BrianZ]

I've got to think about that. A do-it-yourself solution would involve a microcontroller that would listen to the signal the receiver is sending, and also to the value of a potentiometer that was mounted around the shaft, and it would output a signal to a motor driver that would turn the motor in the direction needed so that the difference between the motor position and the receiver signal position is 0 degrees. Which is, of course, how servos work, but this microcontroller would need to interpret the pulse width modulation signal of the IR receiver or my radio controlled receiver. I think the parts on my circuit board (the Picaxe 14M2 microcontroller, the SN754410 motor driver, and a 5v regulator) are all you would need to make this happen.

[whale2]

This is exactly what I did.

[Boxerlego]

I've been thinking about using the 555 timer on the controller end to produce a pwm signal for the servo driver. However, I do like the thought of using a microcontroller. Does the microcontroller need to have a spring in the controller for the return to center steering work on the servo. With the Lego speed controller it uses a red button to center the servo motor. Which allows the servo to hold any of the possible positions with no input on the controller end but when you try to power your wheels with a drive motor that is connected up to the same receiver that is holding the position of the servo motor at the same time the servo motor instantly returns it shaft to center.

[BrianZ]

Using a 555 is hardcore old school. There is no spring for return-to-center. The Lego servo uses a pulse width modulation signal on lines C1 and C2 (which is a high power signal, used to actually power the drive motors) to tell the servo where to go, and it uses the 0v and 9v lines to power the motor in the servo, via a control circuit. It's a wierd signal protocol because 0V on both C1 and C2 will not power a motor, but that's the signal to go to center for the servo. The output of your device must emulate that PWM protocol in order to control the motor. The device that hooks to the motor must have a vairable resistor that the microcontroller can read to tell it where it is, and use that information to calculate the difference between where the motor is and where it's supposed to be, and send an output signal accordingly.

If you want to get into microcontrollers and you have no previous programming experience, let me suggest the Picaxe. It would make this project so, so easy. It is only a dollar or two more expensive than a 555 anyway. I will admit that the Picaxe has very little street cred though, certianly less than the 555.

[Boxerlego]

Very interesting. I understand your electrical response for how the Lego servo returns to center. My thoughts were steered towards the mechanical process going on the RC controller for the servo motor to return to center. I just recently watched you test video

and 30 seconds into the video you described how when you release the steering wheel on the radio controller is able to center the servo but the radio controller is not in the picture. I watched your first video and the only time I see the servo was able to hold its position is when you use the Trim dial on the controller. My main focus on the microcontroller is the controller part behind all this. When I'm thinking about using a 555 timer to control my servo motor the only way I see how to have a return to center capability for the servo is to have a spring on the controller part. However, using a spring on the steering control removes the servo ability to hold its position when I take my hand off the wheel. I want to have both options on the servo one that can hold a position anywhere I set it and also a option for a return to center function for the servo. So that means if I wanted both options here with a 555 timer I would need a method for switching between the two possible control options for the servo and also have a button that is able to make the servo return to center. Now this is wheres my thinking goes in to realm of microcontrollers and that is being able to program a servo driver with programmable buttons. These buttons could be programmed to make a model walk or a transmission gear shift pattern and alot more with just one press of a button.

[BrianZ]

Oh, Ok. I get it now. I was thinking controller=microcontroller in the housing of the thing that snaps to the motor, you were thinking controller=transmitter. Yeah, my transmitter has a return-to-center spring, but sometimes people will put a foam doughnut behind the wheel on the transmitter so that it does not return to center. What is your ideal wireless communication method between the transmitter and your model?

[Boxerlego]

Right now I have a 433mhz transmitter. But I've been looking at the 2.4ghz transceivers.