|
<< Click to Display Table of Contents >> Navigation: MotionDesigner Reference & User Interface > Motion Design Considerations > To Improve a Motion |
How to improve a motion is application specific and a function of the mechanical design.
When your motion is simple, such as a Dwell-Rise-Dwell (DRD) motion, or a Dwell Rise-Return-Dwell (DRRD) motion, it is easier to answer.
The Application Notes, that accompany each “Traditional Motion Law”, consider Dynamic-Performance, Pressure-Angle, and Drive Torque as design criteria.
When a motion is a little more complex, the question of how to improve a motion should follow the general advice, given below.
Assumptions:
•The motion is for a machine, and not for the animation of a cartoon character.
•The motion is intended for a machine where the machine elements reciprocate, oscillate, index, or the speed modulates in some way. Each machine element has mass, mass moment of inertia, stiffness, and backlash.
•The inertia forces are dominant - or at least 80% of the total load.
General Advice:
•Remove all Velocity motion-discontinuities.
•Remove all Acceleration motion-discontinuities.
•Use a minimum number of segments.
oAlways ask yourself: “Can I reduce the number of segments?”
oAlways ask yourself: “Can I delete a Dwell Segment?” - especially if it is short.
•Reduce the number of motion constraints. E.g. Do you need to control the position of a Blend-Point - possibly you only need to control its velocity.
•Try to make the maximum acceleration values of each segment similar to each other. This is not always possible or desirable. But consider balancing the motion to give similar motion durations or peak accelerations, or both.
•When machine tooling must interact in some way, consider moving a tool further than you need to, if you can also give it more time. This is often a very powerful way to reduce accelerations of tools. For example, if you increase the displacement by 20% and also increase the duration by 20%, the maximum velocity will not change, but the maximum acceleration will reduce by 20%.
•Would it help to design an asymmetrical Rise or Return motion segment, with an acceleration that is not equal to the deceleration.
•Keep an eye on the Torque Fluctuation - rapid and large torque changes can lead to drive shaft over-run.
A particular motion-law might:
•have more flexibility to control the motion-derivatives at the Blend-Points - usually Flexible-Polynomial.
•suit the mechanical system. For example, the motion-law might have a “low peak maximum velocity”, which can improve the Pressure-Angle
•give a good dynamic response to the mechanical system. For example, the motion-law might have a good response to system backlash, and/or low drive stiffness.
•agree with a company preference(!) - for example Modified Sinusoid is often a company preference.
Note:
I nearly always design a complex motion (hat is not a basic DRD or a DRRD type motion) with Flexible-Polynomial segments. Flexible-Polynomial give me almost all of the flexibility I need.