Bot Builder's Info

Guides => Tom Servo => Topic started by: Packratt on February 04, 2018, 06:39:02 PM

Title: MechaServo
Post by: Packratt on February 04, 2018, 06:39:02 PM
While I'm building MechaCrow, I thought I'd share the details around the MechaServo build I did last year.

First, the MechaServo BOM:

This build can play audio of Tom Servo's voice recorded from video and converted to WAV file format. The Arduino, paired with the motors, allows you to also swivel the head and move the jaw in time with the audio of Tom's voice at the press of a button or any other assorted trigger. Since it's battery driven, it's portable and essentially replaces the puppeteer's hand with electronics.

Most of the steps in construction follow along with any other Servo build, except at certain steps where we shorten the control pipe, install some custom designed 3d printed parts, and install the electronics into the barrel and hover skirt. All of which I'll detail in subsequent posts if anyone is interested in trying their hand at this modified build.
Title: Re: MechaServo
Post by: Packratt on February 04, 2018, 06:56:15 PM
Possibly the most important parts in the MechaServo build are the custom-printed parts that allow the servo motor that drives the mouth to sit on the neck pipe itself, which helps avoid torsion problems in the mouth control rod that would occur when the head would turn. The first image shows the basic design model I made, which is a combination gear/band drive where the stepper motor turns the head and the servo motor mounts at the end of the pipe to open and close the mouth via a control bar that links the servo arm to the bottom jaw where the string would have went for a standard build.

Here you can see how this works for the mouth control: (
Title: Re: MechaServo
Post by: Packratt on February 04, 2018, 07:28:02 PM
As you can see in the previous post, the control pipe that usually runs all the way down into the hover skirt in a traditional Servo build is cut shot, about 2 inches past the point where it enters the barrel (chest) and the gear drive mechanism is attached at the pipe's end. While you could print a mouth control arm that runs down the pipe to the lower jaw longer, the longer that control arm is, the greater the risk of it breaking under load. The mouth control arm I designed is about 130mm long (first pic). In this pic, you can see how you can attach the control arm to the bottom jaw with an M3 bolt and nut, and the same with the servo motor arm at the other end.

The stepper motor that turns the head is mounted upside down at the top of the chest barrel and fitted with the printed stepper gear and stepper pully, you can see this a little more clearly in the second pic. I use the combination of gears and a banded pully so that the rubber band pully helps keep the gears closely meshed while also stabilizing the neck, and the gears allow for sufficient force to be used to turn the head. Either, by themselves, just doesn't work well. Mounting the stepper motor upside down also hides the screws used to mount it under the turntable, an added bonus.

The only other part mounted in the chest barrel is the small speaker, which I've placed behind the engine mount on the inside of the barrel so, again, the screws can be hidden. I also drilled some holes through the barrel for the sound to get out better, but those are also hidden behind the chest engine. While mounting it somewhere in the head positioned towards the mouth would have been ideal, it was just too large to fit even in Servo's large mellon.
Title: Re: MechaServo
Post by: Packratt on February 04, 2018, 07:31:49 PM
Here's a better pic of that mouth control arm so you can see how it runs from the servo motor, up inside the neck pipe, and attaches to the bottom jaw.
Title: Re: MechaServo
Post by: Packratt on February 04, 2018, 07:53:21 PM
Tom's Brains:

So, remember the BOM earlier... The first pic shows a 'fritzing' diagram of how it's all wired together on a breadboard. As you see, this was the first gen model where I just used buttons as a trigger, where the buttons were soldered to a board I attached to the bottom of the hoverskirt so they could be pressed by the person holding Tom. This works ok, but can be persnickety when the batteries start to drain a bit and the signal range that distinquishes each button from another lowers beyond the normal range.

The second pic shows what all this translates too once it's all put together... Tom's brains, so to speak.

These were the first gen designs, which worked sufficiently, but I'm working on some revisions and will post them later. The main problems with this design were the inconsistent buttons I mention above and excessive battery drain (9v batteries would drain in 30mins). So in the new design I'll move the servo motor to it's own 4x AA power supply and change the trigger to a 4-button RC fob.
Title: Re: MechaServo
Post by: Ron on February 22, 2018, 03:31:45 PM
Very cool!

I've always thought I wanted a semi-autonomous Servo
Title: Re: MechaServo
Post by: fastbilly1 on March 01, 2018, 01:24:08 PM
If I ever build a second servo for myself, I am going your route Packratt.  That is phenomenal.