﻿ Step 13.x: Dynamic Forces

# Step 13.x: Dynamic Forces

### Dynamic-Forces - General Notes

Kinetostatic Force Model

Kinetostatic forces are those forces that arise at joints, cams, springs, and gears because of the motion imposed by a perfect power source on the Parts.

There are always differences between the model given by your computer and the real, or physical, model. Identifying the differences is not always easy.

You might investigate the source of errors with different models.

 1 Tolerance Model

A 'tolerance model' explores the statistical inaccuracies between a payload's position in a real and modelled machine, by analysing all possible differences between the length of parts, position of joints, and surface cam-profiles. It explains the reason that when you assemble two or more machines, that you believe to be identical, are, in-fact, different. They can also have significant performance differences.

 2 Dynamic Model

Dynamic modeling explores the motion and force interactions between the machine elements in the mechanical system.

 • Motion of Power Source

MechDesigner uses the exact motion of a Power Source to calculate the Torque required to move the kinematic-chain. However, the real motion of a real Power-Source will not be exactly as expected. For example, even when the Power Source should rotate at constant velocity, it is often the case that when a mechanism is accelerating an inertia, the Power Source (motor) will tend to reduce its speed. At another angular position of the same mechanism, when parts are decelerating, the load may tend to drive the Power Source (motor) and thus increase its speed. Thus, the motor's speed is not constant, even if we give define it as constant speed. For this reason, the 'Power-Source' we use in MechDesigner is sometimes called a 'Fictitious Power Source'.

 • Motion of the Physical System

Even with a Fictitious Power Source, the physical system does not respond perfectly, because of the interplay of elasticity/stiffness, backlash, inertia.

 • Motion Interplay

There is a complex interplay between the response of the mechanical system to the motion actually given to it by a Power Source, and the response of the Power Source to the forces it experiences as the mechanical system moves.

 • Control Loop

Frequently, a response of a servomotor to a control loop is not perfect, even with the most sophisticated control strategy. For example, a PID loop is an 'Error Driven' Control Strategy - it only provides a signal to the servomotor when there is an error between the motion command and the motion response.

Dynamic Modelling

There are many definitions of a Dynamic Model. In the context of MechDesigner, I would take it to mean:

'...to find the motion and the forces experienced by a real mechanical system, in response to the motion provided by a Power Source.' MechDesigner does not provide this modelling.

MechDesigner calculates those inertia, gravitational, centrifugal, and Coriolis forces that arise in response to the idealised motion provided by a fictitious power source, imposed on Rigid Parts.

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