delta_t data in Robot controller

[GreenDice]

Does anyone know the definition of the delta_t data byte that is sent from the Master uP to the PBasic uP?

Thanks in advanced.

[Joe Johnson]

Delta_T is the number of packets missed since the last time your PBasic program successfully got data from the Master CPU (which gets its data from the Operator Interface).

It was an attempt by FIRST to allow a little bit of "real time" control by giving the robot programs knowledge of how much time has passed since they maded it through the loop successfully.

The data is better than nothing, but it leaves a lot to be desired as a real time clock.

From time to time, I have used Delta_T or its twin packet_num to implement a counter that serves as a clock. It has some uses but it is very limited because the best resolution that you get is 25msec which is not all that great at times.

Hope this helps.

Joe J.

P.S. Typical Delta_T's should be 0, 1 or 2. The most I would recommend you live with is 4 -- absolutely MAX. Note that at 6 some rather drastic things start happening -- namely, the Master CPU thinks you are lost in a loop somewhere and turns off all your outputs -- this is not a good thing, believe me. If you consistently run at 5 for Delta_T you are only one radio interference problem from not working at all -- to my mind a very dangerous thing to risk. JJ

[Dan 550]

When your Delta-T gets up there and the CPU turns off all your outputs while on the ramp, some funky stuff happens, specifically, confusion. This happened to 2 teams in a match my team had last year when both robots were on the bridge, heavier one in front. Logically, they rolled backward and achieved a nice velocity before they both got themselves working again, but at that point they became intertwined by a piston-mounted aparatus and a piece of extrusion, locked. the tug of war that resulted was not good, and no amount of teaming could knock them apart. That was a bad thing...

[Scott358]

At the kick-off workshop in NH, the Innovation First team expalined that the controller will sense a "problem" if it see's more than 4 data packets missing (which Joe states as a recommendation), and shut down for a time.

They also said there is more info on the Innovation First site (although I haven't gone there yet for this).

Regards,
Scott358
Festo Corp
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