Yesterday, I found that the Arnold of Maya 2018 has a directional error for the motion vector blur method made in the AOV channel. The motion blur method I wrote earlier applies to Maya 2016 and earlier versions, so I spent a little time studying it today. The problem has been solved now, and cloud rendering will use a small case to explain the production steps. 
Step 1: Creating the ball as a simple animated scene.
Step 2: Turn on motion blur: Render a reference to motion blur.
Step 3: Follow the method given in the official Arnold documentation:
Change the camera's Shutter Start and Shutter End to 0.5 (why changed to 0.5 because my motion blur type is Center On Frame).
Step 4: Create a channel for AOV motion blur:
Here I will create two motion blur channels:
The first is the motionvector channel that comes with Arnold;
The second is to use the custom motion vector AOV of motion vector shader (giving the aiMotionVector material in the AOV channel).
Cloud rendering note: MBlur is named according to your preference. The setting of aiMotionVector was wrote in the previous article.
Step 5: Render this image to see two motion blur conditions.
It can be seen that both the motionvector and MBlur motion blur channels in our AOV channel are in the wrong direction.
Step 6: Next I will officially explain how to solve this problem: From the beginning.
A: Turn on the Motion Blur button in the render settings: I chose the Center On Frame mode here.
B: Change the Shutter Start and Shutter End in the camera Arnold attribute to 0.5, respectively, corresponding to the Center On Frame mode.
(Note: When rendering the motion vector AOV, the camera Shutter Start and Shutter End should be the same value.)
C: Create a new MBlur_2 channel in AOV (to compare with the previous MBlur.)
D: Open the material editor and find the two nodes aiMotionVector and aiVectorMap under Arnold's material panel.
1) Connect the outColor of the aiMotionVector node to the input of the aiVectorMap node.
 2) Then connect the outValue in the aiVectorMap node to the DefaultValue in the Shader property of the MBlur channel in AOV.
3) Change the XYZ mode of the Order in the aiVectorMap node to YZX mode and remove the hook of the tangentSpace.
4) In the aiMotionVector node, enable Encoding Raw Vector.
E. After the above steps are completed, start rendering the image and view it in Nuke.
As shown in the figure: Maya's default motion blur is basically the same as the MBlur_2 channel we made in AOV, and the motion blur adjusted by Nuke is basically the same. Of course, I also found a bug in the Arnold renderer, its motion blur channel does not solve the shadow of motion blur, there can be seen in a big picture that cloud rendering attached below.
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