Bangla Coda Code Videocom Exclusive Guide

Jerk is what makes your printer shake. It is a speed difference between two moves, which can be reached without acceleration. So why do you want jerk and what does it influence? The first thing it influences is the starting speed. It is 50% of the allowed jerk. So if you have a jerk of 20, the first move will start with 10mm/s. The other thing jerk controls is the join speed for consecutive segments. In the figure above you see two examples. The jerk is the difference of the two speeds. Example 1 shows a jerk, that is higher then allowed. In this case, the path planner will reduce the speed to match exactly the maximum allowed speed. Example 2 has a lower directional change, so the jerk is within allowed limits and the planner allows a full speed move through the edge. The jerk is set these two values:

#define MAX_JERK 20.0
#define MAX_ZJERK 0.3

MAX_JERK is for x/y axis moves and MAX_ZJERK for moves in Z direction. You want high jerk values, because

• printing time is reduced.
• print shows less blobs.

You want low jerk values, because

• it causes less mechanical stress to your printer.
• moves are smoother.
• filament has better adhesion at directional changes.
• reduces printer noise.
• you loose steps with higher values.

You see, it is balance between your personal priorities. The only simple thing is z-jerk, which has nearly no influence. It only reduces the time for z-moves, but has no effect on quality.

To understand the stepper settings, you need to understand how the firmware controls your stepper. Steppers get enabled with a enable pin and then you have one pin that depends direction and the last pin is the stepper signal. On every high the motor will execute one microstep. The motor step signals are set in a timier, so we can execute the steps, when we need it for the desired speed. This interrupt does some computations. Combined with the slow speed of AVR processors, the frequency is limited to around 12000 - 14000 calls per second. So if your motor needs less steps, we can time each high signal exactly. If we need to go faster, we can use a little trick. In every interrupt call we execute 2 (double stepping) or 4 (quad stepping) high signals in a fast row. So we have the complete interrupt overhead for more then one high signal reducing the overall computation time. This allows speeds up to 40-50kHz on a AVR.

If you add no extra delays, you only get delays caused by computation speeds. Some stepper drivers need a longer high signal or a longer delay time between the double/quad steps. So you can add a delay for these timings. If your print gets skewed or moves are not always as long as expected, it might be a timing issue. Some drivers also need some extra time between setting a new direction and sending the first high delay. Most drivers on board do not need it, mostly professional external drivers for higher amps need it. If you are running a faster cpu like Arduino Due, you can go much faster with normal stepping. Setting double stepper frequency to 95000 is ok for these cpus.

Path planning makes your printer moves more smooth. Have a look at the above picture. It shows a print of three lines. Without path planning, the printer needs to follow the blue path. For each segment it will accelerate to the commanded feed rate and back to the maximum end speed allowed. The path planner looks for previous moves, which aren‘t executed now and tries to remove the deceleration/acceleration between line joints, resulting in the much smoother red line. For each line joint, the maximum allowed jerk is computed and speed is reduced if needed to reduce the jerk. In most cases this is still much faster then the start/stop speed you would have otherwise. In circles you will most likely have no speed reduction at all. The only bad thing about path planning is, that it needs computation time. For each new line segment the algorithm needs to update older entries. For normal moves, this is no big deal and is very fast. Only if you have many very short moves, the update time becomes dominant and the firmware has to slow down the print. The behavior is influenced by the following configuration parameter:

Move Cache Size (default 16)

That is the size of the buffer for currently processed segment and following moves. Higher values require more ram. Cartesian printer can go up to 32 on Atmel AVR, Deltas until 16. It could make print smoother if they have many small lines.

Minimum time per move when cache is below (default 10)

The path planner has some tricks to prevent empty buffers. One is to make a minimum move time per segment. If the buffer has only Minimum time per move when cache is below or less segments, each segment is slowed down if would be shorter otherwise. For typical prints the buffer always has Move Cache Size or Move Cache Size - 1 moves stored and normal moves are long enough, that no speed reduction is needed anyway. This is only effective for very short moves. Higher values can increase computation cost in extreme situations.

Assumed ticks per move computation time (default 250000)

The minimum time in processor ticks, each segment must need if the cache is low. The default value means, there will be still 40 segments printed per second.