6.1 Global slipping
When external loads or forces include horizontal components, maintaining the posture without slipping requires the horizontal floor reaction to balance them. Static friction can provide this reaction up to a certain maximum value. Therefore, to prevent slipping, the magnitude of the horizontal resultant that must be supported by the contact cannot exceed the maximum available static-friction force, as shown in Figure 12.
Ergoniza Intelligent calculates this maximum force using the Coulomb friction model:
\[F_{fric,max} = \ \mu \bullet F_{normal}\]
Equation 1
where Ffric,max is the maximum available static-friction force, Fnormal is the magnitude of the total normal reaction at the supports, and μ is the coefficient of static friction between the footwear and the floor. The normal reaction depends on body weight and the vertical components of external forces, accounting for their direction. The coefficient of friction is set under Model settings, on the right side of the interface, using the Coefficient of friction control.
When the preceding criterion indicates slipping, static equilibrium cannot be maintained with the selected coefficient of friction. Slipping may occur before the supports develop the reactions required to cause tipping. Ergoniza Intelligent nevertheless assesses and displays the two conditions separately: the possibility of slipping and the location of the center of pressure relative to the base of support. This lets the user determine whether, even if slipping were prevented—for example, by using footwear or a surface with a higher coefficient of friction—the posture would remain unstable because of tipping.
Figure 12: Calculation of slipping on the floor
The preceding criterion considers the overall friction capacity of the supports. During bipedal stance, however, the horizontal and vertical reaction forces are distributed between both feet. Even if the total friction capacity is sufficient, the horizontal reaction required at one support may exceed its individual capacity. Ergoniza Intelligent also checks this condition and, where possible, redistributes the reactions between the supports. The procedure and its interpretation are explained in Section 7.3, Partial slipping.