# Bridge Mass Properties

Posted by Dodd Stacy at 1/16/2001 12:02 AM EST

Engineer on team #95, Lebanon Robotics Team, from Lebanon High School and CRREL/CREARE.

We measured our bridge properties, and here is what we found, for those teams who are taking a mathematical approach to their design effort. Our bridge weighs 214 lbf, including the weights but not including the sexy aluminum diamond side plates (not our style), and we hadn’t yet put on the two steel lip plates at the bridge ends. More important, we measure the bridge CG height - including the weights - at about 6" above the top of the bridge deck.

The measurement technique involved balancing the bridge on the end lip and measuring the small angle of the deck from vertical with a level and protractor, so the error may be significant, but there it is for what it’s worth. Put your robot weight and its CG height into play with an inverted pendulum, and see how fast the system will pitch forward and how fast you have to correct your position, if your robot wants to balance on the bridge. Good luck, all.

Dodd

Posted by Dodd Stacy at 1/16/2001 12:16 AM EST

Engineer on team #95, Lebanon Robotics Team, from Lebanon High School and CRREL/CREARE.

In Reply to: Bridge Mass Properties
Posted by Dodd Stacy on 1/16/2001 12:02 AM EST:

I forgot to include, for the bridge tenders out there, we neasure a vertical force at the end lip of 9 lbf, +/- 1, to raise or lower the end of the bridge from the balanced position. From the totally tilted position, the force required to move the bridge is very small, 1 lbf or so, less than the accuracy of the crude spring scale we were using.

I personally hope that FIRST will apply some grease to the steel angles making up the “pivot” of the bridge. It is so crude and involves such significant sliding under heavy loads that I think it will start galling and sticking without lube and become very erratic in its friction and breakaway characteristics.

Dodd

Posted by Eric Reed at 1/16/2001 10:55 AM EST

Coach on team #481, NASA Ames / De Anza High School, from De Anza High School and It could be you!.

In Reply to: Bridge Numbers PS
Posted by Dodd Stacy on 1/16/2001 12:16 AM EST:

Hi Dodd,

For a non-physics person…what does this mean? I think your message answers the following two questions, but can you make it easy for me:

How much weight placed at the lip of the bridge would begin to move the bridge from tilted position?

How much weight placed at the lip of the bridge would begin to move the bridge from balanced position?

Also, I assume that loading the bridge with, say, a 100 lb robot and two 70 lb goals would raise the weights required to move the bridge. Is that intuition correct?

Thanks,

Eric.

Posted by Dodd Stacy at 1/16/2001 11:32 AM EST

Engineer on team #95, Lebanon Robotics Team, from Lebanon High School and CRREL/CREARE.

In Reply to: Re: Bridge Numbers PS
Posted by Eric Reed on 1/16/2001 10:55 AM EST:

: Hi Dodd,

: For a non-physics person…what does this mean? I think your message answers the following two questions, but can you make it easy for me:

: How much weight placed at the lip of the bridge would begin to move the bridge from tilted position?

Very little, a pound or two.

: How much weight placed at the lip of the bridge would begin to move the bridge from balanced position?

: Also, I assume that loading the bridge with, say, a 100 lb robot and two 70 lb goals would raise the weights required to move the bridge. Is that intuition correct?

Exactly. The forces should scale with the total weight. I think 240 lbs of robot and goal on the 214 lb bridge will take (454/214)x10 lbs or about 20 lbf, give or take, to begin movement from the balanced position, with the robot exactly centered (lengthwise). This is going to be awfully tender to the robot’s position along the length of the bridge, though. Less than 6" off center, and the balanced bridge will begin tipping. Stop it if you can! Dodd

: Thanks,

: Eric.

Posted by Eric Reed at 1/16/2001 2:22 PM EST

Coach on team #481, NASA Ames / De Anza High School, from De Anza High School and It could be you!.

In Reply to: Re: Bridge Numbers PS
Posted by Dodd Stacy on 1/16/2001 11:32 AM EST:

Thanks, Dodd…this is good news, we were afraid that the bridge would not be sensitive enough.

Eric - 481

Posted by Dodd Stacy at 1/16/2001 4:54 PM EST

Engineer on team #95, Lebanon Robotics Team, from Lebanon High School and CRREL/CREARE.

In Reply to: Re: Bridge Numbers PS
Posted by Eric Reed on 1/16/2001 2:22 PM EST:

: Thanks, Dodd…this is good news, we were afraid that the bridge would not be sensitive enough.

: Eric - 481

Don’t worry, Eric. It will be very sensitive unless/until the angle iron pieces start to friction weld themselves together. For manipulating the bridge from the floor, it’s basically a teeter-totter. The only significant forces to deal with will be from the weight of bots or goals on the bridge.

And if you want to balance your bot on the bridge, get ready to dance!

I should be careful, though. The bridge pivot is so primitive and full of hysteresis, and the “over center” aspect of the elevated CG is slight enough (which is why it’s sensitive) that other teams may be producing different findings. Can others add their experience, please?

Dodd

Posted by Joe Johnson at 1/16/2001 8:29 PM EST

Engineer on team #47, Chief Delphi, from Pontiac Central High School and Delphi Automotive Systems.

In Reply to: It’s a Sensitive Kinda Bridge
Posted by Dodd Stacy on 1/16/2001 4:54 PM EST:

Dodd,

We have built a teeter-totter but did not go the extent
of analysis that you did.

Thanks for your detailed measurements and reporting.

You show once again why I always tip my hat to the guys
who where Team #95 on their shirts.

Keep up the good work.

Joe J>