There is an error in estimating the time using laser engraving software LaserGRBL

When using laser engraving software LaserGRBL, if the estimated time is inaccurate, please carefully read the following explanation:

Each segment has its own programmed execution speed and its own length (length of the segment, or lenght of the arc).

The tracing movements G1, G2, G3, are performed at the speed F set in the GCode, the jump movements – not tracing – are performed at the maximum speed set by the machine (grbl parameters $ 110, $ 111).

All these data are known, so it is possible to calculate the estimated time for the job as the sum of theoretical execution time of each segment (segment length * theoretical executing speed).

In reality no segment will really run at the programmed speed as the tracing of a segment is made up of a phase in which the laser head accelerates, a phase in which the speed is linear, a phase in which it decelerates.

The accelerations and decelerations are necessary to be able to obtain the changes of direction (between non-consecutive segments) and the changes of speed (between tracing and non-tracing movements) without loosing precision.

These accelerations and decelerations are controlled by the grbl parameters $120, $121. These data are also known, however the formulas to take them into account when “estimating” the times can be very complex. In fact grbl (the firmware) chooses from time to time, in real time, how and how much to accelerate or decelerate, based on the angle of incidence of the next segment. If they coincide or almost, it may not even decelerate, if they are at 90 ° or 180 ° it must decelerate almost to a stop before starting off with an acceleration ramp.

This complexity makes it very difficult to take these data into account when “estimating required time” which is therefore based only on theoretical linear constant speeds.

The projected time, on the other hand, is calculated runtime, during the execution of the work, as a projection between the percentage of the work already done, and the missing work, compared to the time already passed. This projection is quite accurate and refines itself while the working progress, but it can be very accurate after just a few minutes.

Normally there is not (there shouldn’t be) such a huge difference between estimated time and projected time, because the acceleration and deceleration phases should have little effect on the total job.

However, if your configuration of the parameters mentioned above ($110-$112 and $120-$121) is limiting, this can result in a slow execution, and therefore in a big difference between the two times.

All these parameters are set in the grbl configuration, and are stored inside the control board. These are the acceleration and maximum speed parameters.

Well balanced values are usually supplied from the factory. You can try to modify them, but beware that this could lead to the machine having less accurate results.