Influence of jerk on balance recovery

Management: STRMTG
Production: IFSTTAR- Laboratoire de BioMécanique des Chocs
June - December 2014

The aim of this study was to understand the influence of jerk (time derivative of acceleration) on people’s balance, and the suitability of the requirements of standard NF EN 13 452-1 “Braking systems for urban and suburban public transport”.
A simple mechanical model was used to identify the impact of a sharp change in jerk on balance. It corresponds to people’s ability to catch their balance using three strategies: ankle strategy, hip strategy and taking at most one step used to their maximum capacity to catch their balance (short step preparation time, maximum step length, etc.). Please note that no supports (e.g. handrails) were taken into account.

First phase

The model was subjected to a disturbance corresponding to a tramway decelerating. All the steps to regain balance were then analysed to determine the % of steps needed to regain balance. This % of steps was used as an indicator to compare deceleration profiles (an indicator to be used only in relative terms) with the imbalance generated.

The relationship between “change in jerk” and “change in % of steps to regain balance” is not linear: decreasing the jerk between 4 and 8 m/s3 does not provide a significant gain, whereas below 4 m/s3, there is immediately significant improvement.
The study also confirms that the standard threshold of 8 m/s3 is indeed a maximum not to be exceeded.

Second phase

The second aim of the study was to define an "ideal" deceleration profile for equivalent stopping distances, and to determine the benefits of using maximum service braking (FU1) rather than FU3 braking.

As far as the "ideal" deceleration profile is concerned, the parameters set by the standard leave no margin for limiting jerk within the maximum stopping distance. Furthermore, a decrease in jerk implies deceleration over a longer time and therefore a higher potential impact speed.

In the event of non-acknowledgement of the driver’s activity control actuator, the request to perform FU1 braking instead of FU3 braking significantly reduces the % of cases where balance cannot be regained in one step.

When braking in the platform clearance zone (tramway in acceleration phase), the model is tilted forward: at constant deceleration increase time, this initial tram acceleration has a strong impact on imbalance. The use of FU1 instead of FU3 braking in this zone significantly reduces the risk of imbalance.