[NotInControl]

A lot of good information here in this thread, and compensating for gravity to hold a static position by having a proportional gain relative to the angle will work, but I did want to mention a caveat to be aware of when developing your control system using that assumption, and also provide a bit more background on gravity compensation.

Background:
Gravity compensation is very popular in industrial manipulator control development (like a 6DOF arm), because the load of the arm and its motion is something which needs to be accounted for in the control. However, the development of these control systems account for 3 things which allow it to be more accurately controlled:

1. Uses Motion profiles so it can control the acceleration, jerk, and velocity of each joint through out the motion
2. Calculates the inverse dynamics (Toque required per time considering the dynamics of the motion)
3. Does not need to account for induced dynamics due to a moving base

Here is why I bring this point up.

Using the proportional method above is a good linear approximation if your drive train is stationary, and your manipulator is not subjected to other outside dynamic forces (defense by another robot, on uneven terrain, etc, induced acceleration while the driving). As you can imagine, if you are trying to hold position while driving, or while being hit, or going over an uneven terrain, there will be additional dynamic acceleration acting on the manipulator other than gravity, and the position of your manipulator will move off its set point during those acceleration, because those forces will not be counteracted in the gravity compensation.

Now your gravity compensated control loop, will act to counteract those forces, and if tuned well enough, , with enough finite control, it will be able to regain its position, once those forces stop acting on the manipulator, or become constant. However the point I am trying to make is to understand the limitations of the approach to know when it will work, when it may not, and to have more educated conversations based on the requirements of your strategy.

If you desire to only maintain position with the drive train is in a stationary position. The above approach is good.*
If you desire to maintain position accurately while in motion, or in the presents of other dynamics forces which you may or may not be aware of**, you may need to consider the above approach will not hold position during the motion, and either more detailed assessment of the scenario dynamics is needed (i.e understanding the dynamics of motion for all cases, or considering adding a mechanical brake to keep position, in which case the control system is not responsible for holding the load, and can be off).

*Also, you also want to be mindful of the current draw required to hold position, and the length of time you need to do so. This requires attention to gearbox design to ensure that the efficiency of the gearbox is as high as possible in the operating window where the load is being held. While you are holding position, you will be using power, and if you require to hold that position for long durations of time, or have an inefficient gearbox a power assessment would need to be done to ensure you are not depleting your battery unnecessarily. Obviously, adding another mechanical break adds complexity to the system, and that is why control development is a marriage between mechanical, electrical, and software. Each discipline needs to be involved, and understand the limitations and compromises so the end system can behave as expected.

** Many times in control development the final system doesn't behave as expected because there were forces acting on the actual system not understood or captured in the development model, and then the system needs to be redesigned, or modified live on the hardware in the presents of the unmodeled dynamics.

Just wanted to throw these points out there so that this discussion can be more complete for future use, and highlight some of the pros and cons of different approaches to maintaining position control

Good luck and have fun. Off season is the best time to build your control toolbox