What is the Thunderborg controller behaviour under load?

Submitted by antoan on Sat, 08/14/2021 - 11:00

Forums:

Hello,

I would be very grateful you could clarify the following for me:

1) Should I expect a difference in wheel RPM, for a given motor throttle level, when the Monsterborg is carrying some extra weight versus when it is not:

For example:

Lets say we take a measurement of wheel RPM on one side (Motor1 for e.g), while the robot is suspended in air, at motor throttle level 0.5, of a standard Monsterborg build.

If we then place then place the Mosterborg on the ground, add an extra payload weight of 1kg on top and drive normally, at the same motor throttle level (0.5), should we expect the controller to drive the motors such that they eventually reach the same RPM?

I am trying to determine whether there is there some sort of internal set point implementation, in the Thunderborg controller which keeps wheel RPM consistent with the motor throttle level, under varying load?

2) Can you provide me with rough estimates for the Monsterborg maximum velocity and acceleration?

Thanks in advance,

Antoan

piborg's picture

piborg

Sun, 08/15/2021 - 10:39

Hi Antoan,

The RPM of the wheels will indeed vary under different loads.

When you set a power level on the ThunderBorg (such as 0.5) it is effectively setting the voltage to the motors as this percentage of the battery voltage. This in turn controls how fast the motors turn, lower voltage equates to a lower speed.

There are actually a few things which will affect the exact speed of the MonsterBorg in practice:

  • Set power level
  • Current battery voltage (battery voltages drop as they go flat)
  • Weight of the MonsterBorg + payload
  • Incline of the terrain (uphill is harder)
  • Grip on the terrain
  • Steering - since we slow down once side to turn

Since the MonsterBorg itself has no way of measuring either motor RPM, wheel RPM, or physical speed it has no way to compensate for all these variables.

As for the best possible speeds MonsterBorg gets about 2.21 m/s in a straight line on freshly charged AAs (about 13.5V) on an ideal flat surface. The AAs loose this boost in voltage fairly rapidly and settle to ~12V for ~2 m/s for most of their charge.

We have never really tried to measure the acceleration, it is quick enough that it would be hard to measure - takes less than a second to reach full speed :)

Our simulator for the Formula Pi racing uses an estimate for the acceleration of 10 m/s², this seems to mimic real-world behaviour fairly well for the Formula Pi track.

antoan

Mon, 08/16/2021 - 16:27

Really appreciate this, thank you.

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