The AlexMos Gimbal controller is the most capable DIY control board currently on the market. Intended for brushless gimbal motors only, the new 32-bit processor and dual IMU (inertia measurement unit), allow it to attain a level of stabilization up to 30 times more precise than the 8-bit board it replaced. What makes this different than the more common Martinez , is the software GUI used to tune its performance characteristics. While the Martinez board is open source code for those that like to tinker, you will need to scour the internet and read hundreds of user posts in order to get it up and running. The AlexMos has a software license on its boot loader, and will recognize and work with the firmware that was specially developed for our needs. Its also capable of running on up to 6 cell lipo, and will handle even the heaviest cameras and gimbal motors.
AerialPixels in California, is the largest supplier of Alexmos control boards in the USA. Prior to Multrotor Pilot magazine, I purchased several of the older style 8-bit boards, along with the parts to build a 3-axis gimbal for a DSLR. The customer service is stellar, and the support is second to none. We will be giving you an intro to this new control board, to help you get over the fear of building your own pro quality gimbal.
After receiving the control board, go to SimpleBGC.com and download the latest software, driver, instruction manual, and firmware. Its PC and Mac compatible, and will be needed to get up and running. Connections are pretty simple. All three brushless motors connect to the board without any add on components. The latest driver chips control the motors, and all most connections use servo style connectors. In addition to the gimbal motors, you will need to connect a power source, a receiver/ and or thumb joystick if you plan on controlling the gimbal, the IMU module(s), and finally the optional, but recommended Flight controller servo leads. By connecting your flight controllers gimbal output controls to the AlexMos FC Pitch and FC Roll ports, stabilization is further improved while used in a multirotor. The servo outputs provide attitude information about the copters angle, and the AlexMos board can use it to understand more effectively maintain accurate positioning.
While tuning a gimbal can often be a time consuming a frustrating procedure, I will give you some tips to help you avoid some of the mistakes to make the job a bit easier. The manual covers everything you will need to know in order to get the board up and running and this overview is not intended to replace that or even cover the steps which are very clearly defined.
Here are some tips to help you through the rough stuff:
- Your gimbal frame is the weakest link. Make sure its good quality, rigid, and friction free. Any binding or flexing is going to cause you a lot of grief.
- Since most DIY gimbals for DSLR cameras are fully adjustable, you absolutely must make sure that every axis is perfectly square. For example, if the pitch axis is slightly longer on one side, as the camera lens tilts down, the IMU will sense that there is a combination of both pitch and roll movements taking place, and this will confuse it, and you. Gimbals that use round carbon fiber tubes are the most adjustable, but the tube can rotate in the clamps when you loosen them for adjustment, and this can cause one side of the camera to be farther forward than the other. This will also mix up the gimbal controller.
- The IMU must be positioned square to the frame as well. Soft, thick double side tape can allow the IMU to flex or tilt and this will also lead to some head scratching behavior.
- Two IMU’s are better than one. Extreme angles with a single IMU will cause the gimbal to twitch. With two, much steeper angles are possible.
- Be careful how you run your IMU cables. These wires should be very flexible, thin, and routed away from power and motor leads if possible. If you can run them in such a way that the wire can’t get jerked if the pitch axis flips over, this will reduce the chance of damaging the cable, or the micro connectors they have at each end.
- Run all motor leads, receiver connection and IMU cables through Ferrite magnet choke rings by making at least 3-5 winds around the ring. This will limit magnetic interference, which can cause IMU errors.
- Consider a yaw motor with a hole in the middle that can accommodate a slip ring. Slip rings allow you to make connections to power and flight controller above the gimbal, but prevents the wires from twisting when you pan the 3rd axis more than 360 degrees. Slip rings are available from Adafruit.com. They look like little black cylinders with wires exiting each end. For power and both FC connections, you will need at least 6 wires.
- Mount the Alexmos board in a safe place and position. Aerialpixels sells custom enclosures for the board and IMU, and its cheap insurance against accidental damage, and make for a neat and tidy install.
- If you choose to not enclose the board, I recommend not mounting the board with the USB port pointing down, if the gimbal should flip out while tuning and fall on the cable, you will break the USB connector off the board.
- Have a stand. Trying to calibrate, adjust or work on a gimbal without some type of rigid way of holding it is almost a waste of time. Often just placing your finger on a stationary IMU can mess with proper calibration. Trying to hold the whole gimbal is like trying to hold down a child with A.D.D.
- Once you begin tuning the P.I.D. settings, work with one motor at a time. The battery pack will need to be connected during this step, otherwise your usb connection will get a bit upset. After you get a motor tuned disable or unplug that motor and move to the next one.
- If you have connected a receiver to the board, make sure you have the correct type of receiver selected in the software. Sbus, PPM, Spectum, PWM.
- The transmitter should also be on during tuning if you have a receiver connected.
- Vibration from unbalanced motors or props on your multirotor will make the gimbal IMU freak out. Make sure you minimize these vibrations, and isolate the gimbal with some type of damper or compression mount.
By Joe Papa