How to quantify the severity of multiple shock events

[Shockdesigner331]

Hello all. Here's my situation:
I've got a customer who manufactures seats for big rigs. The drivers of these rigs occasionally will see large shock events (~2-3 g's). The customer wants to install a system of accelerometers that continously records accelerometer readings from the seat. The goal is to come up with a system that will inform the driver when he's had too much input for the day. It can't be just a simple "count the number of events greater than 1.5 g's" since multiple events of say .75 g's quickly add up to more cumulative damage than a few 1.5 g events. Would it be possible to take a PSD of the time history data perhaps? Maybe update this every few minutes or so? (I understand the DRI and Se from OSHA codes but I don't think I can apply them for a limit switch...) If anyone has ever done anything like this or could help me out with the fundamentals I would be very grateful. Thanks.

 

[CESSNA1]

Shockdesigner331: This looks like a ride quaity issue. There is an ISO Spec that addresses ride quality you may want to read it. I have one that is 2-3 years old. If you want I will send you a copy. Reply to dphall@sbcglobal.net with your mailing address. Is the customer looking for broadband G level (That is the easiest) or frequency domain? It sounds like he wants frequency domain. If that is the case then energy imparted to the driver becomes the issue. It is frequency dependent. The seat system can be an absorber, a magnifier or just a transmitter for the driver. I also assume cost is an issue. Some other people may have some input on this.

Good luck
Regards
Dave

 

[Shockdesigner331]

Thanks for the response Dave. I don't think that it's a ride quality issue as much as it is a safety issue. (Unless the ISO spec covers personal safety) These are rather large trucks (think mining) and ocassionally they will hit rather large pot holes. This has led to driver injuries in the past. This new system is an attempt to mitigate cumulative damage from the shocks.

So I guess that energy imparted to the driver is the way to go. I'm just not sure as to how to keep track of the total energy that the driver sees.

As far as the monitoring system, cost does not seem to be an issue. They have the seats instrumented, but now need to figure out the programs to create for the data recorders/processors to keep track of the input seen by the driver.

 

[GregLocock]

From memory for many ride type events the total vibration energy dose measured via a weighting filter is the best indicator for comfort, damage or fatigue. Papers by Mike Griffin at ISVR are a good place to start.

However, if you are concerned with a specific mechanism (eg back injuries) you may have trouble finding an appropriate filter.

Make sure that you understand the interaction with the transfer function of the seat itself, ideally you should be measuring the acceleration of the seat pad itself, not the seat rail.

Occupational Health and Safety should have ths covered for mines and farmers.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.