Custom motor mixing is a technique used to adjust motor output power, optimizing the performance of your FPV drone.

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Understanding Custom Motor Mixing

Motor mixing, or the output mixer, is essentially a table that dictates the force magnitude applied to each motor of a multirotor. This setup is crucial, particularly for X4 quadcopters, which typically exhibit 100% output force across all three axes due to their symmetrical design.

Flight controller firmware like MultiWii, BaseFlight, CleanFlight and Betaflight offer several default mixes for various configurations such as tricopters, hexacopters, and octocopters. However, if your multirotor frame is asymmetrical or uniquely designed, it might still fly adequately with a default output mixer, but there’s a chance the motors could be providing inappropriate thrust levels. Ideally, with custom motor mixing, we aim to minimize the need for corrections and thus enhance your drone’s performance.

Note: Altering your motor mixer settings can disrupt motor outputs and cause significant issues if done incorrectly. Do not directly copy and paste the cmix table from this post into your CLI; instead, input it manually. Use custom mixer settings at your own risk.

Is Custom Motor Mixing Necessary?

The necessity of custom motor mixing depends on several factors. Motors positioned further from the center of gravity require more thrust to achieve the same movement as those nearer to the center, due to their longer travel distance.

Additionally, the layout of the motors and mass distribution significantly impacts quadcopter performance. For instance, an FPV drone often has a more centralized mass along the roll axis than the pitch axis, influenced by the placement of cameras and batteries. This setup allows for higher P and I gains on the pitch than on the roll, simplifying the tuning process.

For frames with standard rectangular motor layouts, adjusting your P and I gains appropriately may suffice without altering your motor mix. However, for non-rectangular layouts—such as dead-cat or T-copter configurations—and more complex designs like A-tail or V-tail, custom motor mixing becomes more essential.

Custom motor mixing also facilitates the remapping of motor output pins, enhancing configuration flexibility. Here’s a tutorial on how to accomplish this: http://intofpv.com/t-remap-motor-outputs-use-spare-motor-output-pins

Implementing Custom Motor Mix in Cleanflight

For this guide, I’m using a Naze32 flight controller with Cleanflight, and I’ll explain how to set up custom motor mixing specifically for this system. Each motor’s output is defined using a custom mixing table, formatted as follows:

mmix [motor number] [throttle] [roll] [pitch] [yaw]

For a standard X configuration quadcopter, the layout typically looks like this:

  4CW      2CCW
         /
     Naze32
     /    
  3CCW     1CW

Here’s the predefined motor mix for this motor layout:

mmix 1 1.000 -1.000 1.000 -1.000 (Rear Right motor)
mmix 2 1.000 -1.000 -1.000  1.000 (Front Right motor)
mmix 3 1.000 1.000 1.000 1.000 (Rear Left motor)
mmix 4 1.000 1.000 -1.000 -1.000 (Front Left motor)

Note: In Baseflight, the command is cmix, while in Cleanflight, it’s mmix.

The values are mostly set to 1, indicating that 100% force should be applied. This standard setup works well due to the symmetrical geometry of an X configuration quadcopter. For controllers like KK2, which use percentage-based values, they would use 100 instead of 1 in their custom mixer table.

However, many popular quadcopter frames, such as the Deadcat450, Blackout 250 mini quad with extended 6′ arms, and the QAV250, deviate from the perfect X configuration. This deviation is where custom motor mixing proves beneficial in optimizing flight performance.

To illustrate, I’ll use my ZMR250 mini quad frame as an example. After measuring, I found that the frame’s width is longer than its length, prompting adjustments in the motor mixing to better suit its unique dimensions. While this guide focuses on simpler quadcopter configurations, the principles apply to more complex layouts like hexacopters and octocopters, although those are beyond the scope of this discussion.

Remember, while it’s advisable to start with calculated values for custom motor mixing, feel free to tweak these settings based on your personal preferences and flight style.

Calculating Custom Motor Mix: The ZMR250 Example

When customizing motor mix settings, we’re essentially looking at four key values: throttle, pitch, roll, and yaw. Throttle is typically set to 1 for all active motors in this configuration, with unused set to 0. The yaw value, which indicates the direction of motor rotation, is straightforward—CCW motors are set to 1 and CW motors to -1. Since all four motors on a symmetrical frame like the ZMR250 are oriented similarly, their force magnitude defaults to 100%.

The real customization, however, comes into play with the pitch and roll values, which range from 1.0 to -1.0. To find these values, we calculate the ratio of each motor’s distance to the pitch and roll axes compared to the longest distance across these axes on the frame.

Confused yet? Let’s break it down with a hands-on example.

First, get a 3D CAD drawing of your frame or sketch it out on paper to measure the distances from each motor to the roll and pitch axes.

For my ZMR250, which is longer in width than in length, the measurements were:

• Motor to Pitch axis = 7.75 cm
• Motor to Roll axis = 10.125 cm

Using the longest distance (10.125 cm) as the denominator, we can calculate the custom motor mix:

Motor1:

• Roll: 10.125/10.125 = 1
• Pitch: (-) 7.75/10.125 = -0.765

Motor2:

• Roll: (-) 10.125/10.125 = -1
• Pitch: (-) 7.75/10.125 = -0.765

Motor3:

• Roll: 10.125/10.125 = 1
• Pitch: 7.75/10.125 = 0.765

Motor4:

• Roll: (-) 10.125/10.125 = -1
• Pitch: 7.75/10.125 = 0.765

Here’s how you’d input these into Cleanflight for a ZMR250:

mmix 1 1.000 -1.000 0.765 -1.000
mmix 2 1.000 -1.000 -0.765 1.000
mmix 3 1.000 1.000  0.765 1.000
mmix 4 1.000 1.000 -0.765 -1.000

This method, while seemingly simple, is crucial for optimizing the performance of asymmetrical frames by accurately distributing motor force according to physical layout.

A Trigonometric Approach to Motor Mixing

An alternative method inspired by the KK2 board uses trigonometry to determine custom motor mix settings, eliminating the need to measure distances directly. This method relies solely on the angles between each motor and the flight controller (FC) relative to the roll and pitch axes.

Here’s how to calculate it for motor 2 of the ZMR250 frame:

1. Measure all angles between the motor-FC to the roll axis and the pitch axis using a protractor. Assume the maximum angle as your denominator.
2. For motor 2:
   • Angle A = 51.6°
   • Angle B = 38.2°

Calculating pitch and roll values:

• Roll: sin(A) / sin(A) = 1
• Pitch: sin(B) / sin(A) = 0.6184 / 0.7837 = 0.789

These values closely resemble those calculated through direct measurement, showcasing the consistency and reliability of trigonometric calculations in custom motor mixing.

So how do we know if this actually works? If we look deeper, you will realize this second method can be transformed into the first method.

sin(A) = a / c
sin(B) = b / c

so when calculating Pitch, we have sin(B) / sin(A) = (b / c) / (a / c) = b / a.

With Roll, we have sin(A) / sin(A) = (a / c) / (a / c) = a / a.

In essence, whether you prefer direct measurement or trigonometric calculations, both methods ultimately serve to enhance your drone’s responsiveness and flight stability, particularly in custom or non-standard frame designs.

How to Set Custom Motor Mix in Cleanflight

Steps to configure custom mixer in the CLI:

1. Enter mixer custom to enable the custom mixing.
2. Type mmix reset to erase the any existing custom mixing.
3. Use mmix statement for each motor in this syntax: mmix NUMBER THROTTLE ROLL PITCH YAW

Other use of Custom Motor Mixing

While the primary function of custom motor mixing is to enhance your drone’s flight performance, it’s versatile enough for other practical applications. For instance:

• reverse motor rotations
• if you have damaged motor output pin/solder pad, you can use a spare motor output pin
• change motor order

Edit History

• Mar 2015 – article created.
• Apr 2024 – fixed grammar.