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- Velocity, Acceleration, Jerk and Junction Deviation
- Junction Deviation Explained and Visualized
- Grbl v1.1 Configuration
- Configuration options
Subscribe to RSSSmoothie reads G-code instructions, and converts those into movement, typically by turning motors. While that might sound pretty trivial to do, the laws of physics actually make this a bit more challenging that one might expect. This page explains how to configure the different motion control parameters you can tune in Smoothie. Acceleration Or "the increase of speed". You experience it everyday. When you ask Smoothie to move a certain distance at a certain speed, it starts at a speed of 0 not moving. It needs to accelerate to that speed. Similarly, the axis which is controlled itself has a given weight that needs to be moved. The faster you accelerate, the more force is required to accelerate the mass to the target speed. This means that for any given machine, you must tune your acceleration. And that acceleration's value is a function of the torque of your motors, and the weight of whatever needs to move. You set the acceleration value by modifying the acceleration value in your configuration file :. Acceleration Force is mass times acceleration. If you feel like your machine is too slow, you increase acceleration. If your machine starts losing steps, losing it's position, or shakes too much, you reduce acceleration. Note that you do not need to reset your Smoothieboard to try new values. For example, M S sets acceleration to it takes a few seconds for this to take effect after the command is sent. On some machines, your Z axis is very different from the others and has different requirements and capabilities. On those machines, you can set the acceleration for Z separately, by editing the gamma. But what about if you move forward, then need to move somewhat to the right? Do you really want to decelerate to a speed of zero before moving to the right?
Velocity, Acceleration, Jerk and Junction Deviation
Most people know that 3D printers can move in X, Y, and Z. These movements are tuned using the parameters Velocity, Acceleration, and Jerk now being replaced with Junction Deviation. These parameters can truly make or break your print quality so understanding what they do is critical. It is intuitive that a printer with movements that are slow and controlled will likely have better print quality than one with high speeds and jerky movements. In order to understand how each of the parameters work we can think about a common scenario of applying the brakes in a car. Imagine you are driving down a road at 45 miles per hour velocity, Zone A and the stop light in front of you turns red. When you hit the brakes your car begins to slow down negative acceleration, Zone C. Eventually you will have decelerated all the way to 0 miles per hour and you will no longer be moving Zone E. The concepts of velocity and acceleration are easy to understand because we deal with them everyday. Jerk on the other hand is referring to how quickly you experience a change in acceleration. For many this is much more difficult to understand. If you look at our oversimplified scenario in the graph above, jerk is represented by points circled B and D. If you think about the moment you applied the brakes you likely feel a sudden change in acceleration and again at the exact moment your car comes to a complete stop. This is because the magnitude of your deceleration suddenly changes from zero to some value at points A-B and then from some value to zero at points D-E. This is jerk. The graph above assumes we are not controlling jerk at all which means we apply the brakes in the beginning abruptly at point B and then come to a complete stop abruptly at point D. Now lets image instead of constant deceleration we gradually press the brake at the beginning and then slow down gently to a complete stop. The new graph would look something similar to what you see below. Notice how the kinks in the red line have disappeared and instead are smooth contours. We have essentially reduced the areas with high jerk values. It is also important to notice that the slope of the line acceleration has not been changed. This means that acceleration and jerk are completely independent of each other. The speed in which your print head or extruder is moving at any given time. By setting M, you are limiting the maximum speed the printer is allowed to move during a print. The limit on how fast you can change velocity. In the example graphs, it is the slope of the line at any given point. The steeper the line, the greater the acceleration. The rate at which acceleration is changing. If you are looking at the slope of the line, that is acceleration. If you are looking at how quickly the line changes from one slope to the next you are referring to jerk. Marlin 2. The concepts above still hold true, however, jerk is now no longer one fixed value. Since it has went from a static value to dynamic it can now be expressed using the following expression:. Additionally, it will help prolong the life of your machine by eliminating unnecessary strain on all of its components caused by poor calibration. Great guide and explanation.
Junction Deviation Explained and Visualized
Wow that singular diagram of what Junction Deviation is makes the whole thing make so much more sense. Great work. If I can give some constructive feedback, you probably shouldn't call jerk a velocity. Calling it a velocity isn't really right, its a bound on how quickly you can change acceleration not how quickly you can accelerate. Post a Comment. Computing Junction Deviation for Marlin Firmware. October 11, As part of developing my own 3D printer firmware, I also keep an eye on what is happening in other firmware. One feature that is causing confusion in the Marlin community is the junction deviation setting. Up until recently, Marlin used the jerk method hence forth referred to as "archaic jerk" it inherited from Grbl for computing corning speed junction velocity. With the option now in Marlin to use junction deviation instead of jerk, there are many people who want to know what are good settings for junction deviation to insure they get reasonable movement while printing. In this post I will give an equation for converting the jerk values into junction deviation and my derivation of this equation. Plugging these numbers into the above equation goes like this:. So for this example the junction deviation would be set to 0. The smaller the junction deviation number, the more the machine will slow down when encountering corners. Making this number too small may slow down the print speed and cause over extrusion in the corners. Making the number larger will increase the speed at which the machine traverses corners. Setting this number too high may cause vibrations in the mechanism that produce ringing in the print surface or in extreme cases a stepper motor to loose position. If you only came here for the basic equation, you do not need to read any further. For those who would like to learn where the above equation came from, keep on reading. To setup this problem, we are going to imagine a situation where the printer traverses two moves that are at right angles to each other:. This is a convenient situation as it is the case where one axis suddenly stops moving, and the other axis suddenly starts moving. When using the archaic jerk functionality the first axis will decelerate down to the the jerk velocity in the corner and then instantly stop, at the same moment the second axis will suddenly jump up to the jerk velocity in its direction and accelerate back up to printing speed. To simplify this, we can say that with a 90 degree corner, the junction velocity is the same as the jerk setting. Next we can take that same corner move and add the virtual arc that junction deviation uses to compute the junction velocity:.
Grbl v1.1 Configuration
Grbl should respond with:. These either immediately change Grbl's running behavior or immediately print a report of the important realtime data like current position aka DRO. Grbl should respond with a list of the current system settings, as shown in the example below. All of these settings are persistent and kept in EEPROM, so if you power down, these will be loaded back up the next time you power up your Arduino. In prior versions of Grbl, each setting had a description next to it in parentheses, but Grbl v1. This was done to free up precious flash memory to add the new features available in v1. However, most good GUIs will help out by attaching descriptions for you, so you know what you are looking at. To manually change e. If everything went well, Grbl will respond with an 'ok' and this setting is stored in EEPROM and will be retained forever or until you change them. Everything else is the same. Stepper drivers are rated for a certain minimum step pulse length. Check the data sheet or just try some numbers. You want the shortest pulses the stepper drivers can reliably recognize. If the pulses are too long, you might run into trouble when running the system at very high feed and pulse rates, because the step pulses can begin to overlap each other. We recommend something around 10 microseconds, which is the default value. Every time your steppers complete a motion and come to a stop, Grbl will delay disabling the steppers by this value. ORyou can always keep your axes enabled powered so as to hold position by setting this value to the maximum milliseconds. The stepper idle lock time is the time length Grbl will keep the steppers locked before disabling. Depending on the system, you can set this to zero and disable it. On others, you may need milliseconds to make sure your axes come to a complete stop before disabling. This is to help account for machine motors that do not like to be left on for long periods of time without doing something. Also, keep in mind that some stepper drivers don't remember which micro step they stopped on, so when you re-enable, you may witness some 'lost' steps due to this. This setting inverts the step pulse signal. By default, a step signal starts at normal-low and goes high upon a step pulse event. When inverted, the step pulse behavior switches from normal-high, to low during the pulse, and back to high. Most users will not need to use this setting, but this can be useful for certain CNC-stepper drivers that have peculiar requirements.