Custom Motor mixer is used to adjust the motor output power, in order to optimize your multicopter performance.

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What is Custom Motor Mix

Motor Mixing is basically a table which specifies the magnitude of forces that should be applied to each motor of your multirotor, therefore it’s sometimes called output mixer. For a X4 quadcopter, each motor has 100% output force in all 3 axis, that’s because it’s symmetrical in all 3 axis.

Multiwii, BaseFlight and CleanFlight etc they all have many default mixes for different configurations, tricopter, hexacopter, octocopter etc. But when you have a multirotor frame that is not symmetrical (unique design), it might still fly fine with default output mixer, the motors might be giving too much or too little thrust, but the FC can sense it and correct it. With custom motor mixing hopefully we can reduce the need for correcting, and therefore optimise the performance of your drone.

Warning: Changing motor mixer could mess up your motor outputs and causes problems, if you have done it the wrong way. Don’t copy and paste the cmix table in this post directly in CLI. Make sure you type it in manually. Use custom mixer in your own risk.

Is Custom Motor Mix Necessary?

The way it works is, motors that are placed further away from the centre of gravity will require a greater amount of thrust to travel the same distance (or arc length) as a motor that has been placed near to the centre of gravity.

Layout of the motor is one thing, another thing is mass distribution, which generally has equally important impact on the quadcopter performance.

A mini quad typically has more centralised mass on the roll axis compared to its pitch axis (due to how the Gopro/mobius and Lipo are mounted). Therefore, it can have higher P and I gains on pitch than roll. When people put in a custom motor mix, P and I gains are now closer to roll and as a result it makes tuning easier.

Therefore if you mini quad frame is already a rectangular motor layout, by simply tuning your P and I gains appropriately, you should be fine and no need to change your motor mix. If you have a non-rectangular motor layout (like a dead-cat or T copter), or especially if you have an A-tail or V-tail, or something more complicated like that, then a custom motor mix is more likely to be needed.

There are other uses of motor mix, such as remapping motor output pins. Here is a tutorial on how to.

How Custom Motor Mix works in Cleanflight

Since I am using the Naze32 with Cleanflight, I will be using this system as an example. Each motor must be defined with a custom mixing table for that motor in this format:

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

For example for a quadcopter of standard X configuration we have this layout:

  4CW      2CCW
         /
     Naze32
     /    
  3CCW     1CW

And 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, in Cleanflight it’s mmix.

You can see there are lots of 1’s there, 1 means 100% force should be applied. This is due to the fact that the X Quadcopter has a symmetrical geometry. KK2 boards use percentage bases, so they use 100 instead of 1 in their custom mixer table.

Many popular quadcopter frames, such as the Deadcat450, Blackout 250 mini quad (with 6′ extended arms) and the QAV250 are not perfect X configurations. This is where we can apply custom motor mixing to optimize flight performance.

I took measurement of my ZMR250 mini quad frame, and I realize the frame width is longer than the length. So I will use this as my example to calculate the custom motor mixing for this frame. It will get a bit more complicated in layouts with more motors, such as Hexacopter and Octocopter, but we will not cover it in this short post.

Note: it’s advised to use the result from calculations, but you should be able to adjust the values according to your personal liking.

Calculate Custom Motor Mix: Example of ZMR250

There are four values we need to get, throttle, pitch, roll and yaw.

Throttle is 1 for all motors that are used in this configuration, unused set to 0, so we don’t need to worry about it.

Yaw indicates the direction of the motor rotation, CCW = 1 and CW = -1. I think if all four motors are on the same dimensional plane and facing the same direction, we can safely assume their force magnitude is 100%. (But If it’s not, it will get tricky)

So that leaves us only pitch and roll values to find, both accepts values nominally from 1.0 to -1.0.

The simplest way to calculate these values, is to use the longest distance between the motor to either the pitch axis or roll axis, as the denominator, to divide the distance between each motor to the pitch axis and roll axis. And the ratio is what we are after, it’s the force of the motor that should contribute to that particular axis.

I guess you are confused? Don’t worry, take a look at this example.

• Get a full size 3D CAD drawing of your frame and print it out (make sure there is no shrinking or enlarging). If you can’t find one, just outline your frame on a piece of paper, that’s what I did.

• Measure all distances from motors to roll and pitch axes. Since the four arms are mirros of each other, the distances to the roll and pitch axes are the same.
  • Motor to Pitch axis = 7.75cm
  • Motor to Roll axis = 10.125cm

• Take the longest distance to be your denominator, so the result is within the range of -1 and 1. In my case the denominator is 10.125 cm.
• Here we can work out the CMIX for all four motors
  • 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

So we have this custom mixing table for my 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 is kind of boring, as this frame is symmetrical, but it should give you the idea how I calculate cmix. You might notice the only difference from the default X quadcopter mixer is the pitch value. It actually makes sense if you think of it this way, the quad’s length is shorter than the width, so it’s quicker to rotate around roll axis (pitch forward or backward), therefore we need to reduce the force on that axis.

Another way of Calcuating – Using angle and Trigonometry

Inspired by how KK2 calculate their custom mixer, it’s also possible to calculate this with trigonometry. With this method all we need to know is the angles between the motor-FC to roll axis, and the between the motor-FC to pitch axis. There is no need to measure any distance, therefore it’s more accurate if you are not confident with your distance values.

Here is the ZMR250 frame. I will calculate the roll and pitch values for motor 2 as an example. First you need to measure all the angles with a protractor, and take the maximum angle as your denominator.

A = 51.6
B = 38.2

In this case, it’s angle A.

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

The values are very close to the above results!

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.

And there we have it! It’s exactly calculating the same thing. The reason for the result difference is because of the imperfect frame drawing, and measurement.

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

Apart from adjusting motor authority over flight controller to get better performance, there are also some other creative uses of Custom mixer. From a quick search I found the following:

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