Servo - September 2016

12 SERVO 09.2016

unit is able to recharge two batteries simultaneously, greatly easing the burden of keeping them operational. It will certainly make my life easier (Figure 4)!

The Banshee three-axis controller is powered by its own 9V NiMH battery. I already had a couple of these, so I should always have one powered up and in reserve.

All of the other components operate on their own power supplies. The light-up brain uses three watch batteries; the LEDs and the wireless microphone use AAs; and the speaker, GoPro camera, and phone are recharged via USB. I think the biggest challenge in operating Jarvis will be making sure all his batteries are kept charged!

Servos — My servos of choice come from Hitec. When I first started building my own animatronics, they are the ones I used and I have become very comfortable with them. They have been extremely reliable and have done everything I have reasonably asked of them. The variety of options they offer means I can always find just the right combination of features and price to fit my build budget.

The output shafts, connectors, and direction of rotation can vary between brands, so it is less complicated if you pick one brand and stick with it.

I selected the Hitec HS-5485HB servos for this project (Figure 5). I would be powering these at 6V; at that voltage, they provide 89 oz-in of torque. This is compounded when combined with the SPG400A-CM gearboxes from ServoCity, and they provided all the power I required. By switching the gear ratios, you can achieve a variety of torque ranges and speeds. It is a tradeoff, so pick the combination that best fits your needs.

I had started with the 2:1 gear sets since my requirements did not call for additional torque. However, I found that even though these are digital servos, I could not slow them down enough to get the smooth motion I wanted. I switched to the 5:1 ratio gears for the elbows and the 7:1 sets for the two shoulder joints. The slower speed gave me the fluid movement I sought.

Linear Actuator — The addition of a RC micro linear servo from Actuonix Motion Devices (formerly Firgelli Technologies) was instrumental in solving the issue I was having with the head nod (Resources).

Moving the head assembly ended up to be an unexpected challenge. I designed the mechanism so that the attachment point for the head would be in the back. This allowed all the components to be visible and complemented the Steampunk look I was after. After making all the decorative and electronic additions to the head, it ended up being much heavier than I had planned.

Even with the addition of a spring to take a bit of the load, the servo I had planned on using could not handle it. The solution I came up with was to use an RC linear servo (Actuonix model #L12-30-50-6-R). I had plenty of room to mount it and the added torque was up to the task (Figure 6). These actuators use the same wiring harness as a standard servo, and function in the same manner. Since they provide a linear motion instead of a rotary motion, they can offer another alternative when you are designing your mechanisms. I have found them to be very easy to work with and they have solved a number of design challenges for me.