Cutting Room Floor: Parallel vs Serial Motion Mechanisms

This is part of my series of material that I edited out from my books prior to publishing. I usually cut whole sections like this when I realize they’re redundant or don’t fit well in the context. This is copy/pasted directly from my draft files with no additional editing, just for fun. 

I have spent an unreasonable amount of time writing about series vs parallel robots. It’s a popular distinction in academia and industry, but doesn’t have all that much applicability to 3D printers. We use a lot of hybrid mechanisms that fall somewhere between series and parallel. It took me a while to figure out what I really wanted to say on the subject, so I ended up with multiple deleted excerpts. I think the explanations I ended up using in Volume 1 get across the necessary information without going overboard.

A convention in XYZ mechanism design is the distinction between parallel mechanisms and series mechanisms. The difference lies in whether the actuator for any particular axis travels on another axis. For example, the X motor in a Cartesian bridge gantry may move on the Y axis. This is a common arrangement due to its simplicity, but series mechanisms have two significant downsides. First is the moving mass of the actuator: motors tend to be high-mass components which are inconvenient to accelerate/decellerate during axis travel. Second is the necessity of running flex wires for electrical control of a moving actuator. This creates an unnecessary failure point.

Subtractive CNC machines generally use very strong series mechanisms for motion control. Motion speeds tend to be slow, and the material removed per tool pass can be quite large. This means forces from tool cutting action tend to be very high relative to the mechanism speeds and self-acceleration forces. This encourages large machine mass and sturdy construction to achieve precise tool paths. Whereas 3D printers lay down very little material per tool pass, and do not need to provide any force except that required for tool acceleration. This encourages the use of low-mass designs with high speed and high cycle life.

3D printers perform best when heavy components like motors are stationary. This requires the use of parallel motion mechanisms. There are many subsequent implications for gantry design and mechanism selection. 

Here’s another section about series vs parallel mechanisms. I think I wrote this at least a year later.

Serial arm robots utilize “open kinematic chains” that sum position error at each joint, and thereby have linearly more error with larger reach. Whereas parallel mechanisms utilize “closed kinematic chains” that can scale up to large sizes with greater rigidity and less position error.

3D printing applications should use parallel mechanisms or minimize the number of serial actuators to minimize accumulation of error through multiple actuators. Where serial mechanisms are used, they should utilize closed kinematic chains whenever possble, such as by rigidly supporting linear actuators at each end and operating actuators in redundant pairs. For example, a brige gantry driven on each side will be dramatically stiffer than a cantilevered arm of the same dimensions.

[diagram of bridge gantry with redundant Y drive]

[diagram of cantilevered arm]

MOVE THIS SECTION ELSEWHERE? Dynamics/mechanics chapter?

The typos indicate that this must have been first-draft text. I usually clean up obvious spelling errors (eg “brige”) the next day after writing a new section.