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Setting Stepper Motor Current (vref) and what it has to do with Belt, Wheel and Extruder tension (among other things)

Forums The Beast V1, V2 and Elevator 3D Printer Support Setting Stepper Motor Current (vref) and what it has to do with Belt, Wheel and Extruder tension (among other things)

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  • #1154
    dan
    Participant

    We get asked this question a lot so I thought we’d offer some detailed advice. This is a work in progress, feel free to comment / share your thoughts and ideas. I’ll update this document as required.

    Forward :

    The point of setting your stepper driver current correctly is to

    A)Maximize the life of your stepper drivers

    B)Prevent Motors from skipping steppers, ruining your prints

    C)Reduce excessive heat and wear on your stepper motors

    D)Ensure each motor has enough power to reliably complete its task.

    Glossary :

    Stepper Driver – those little rectangular thingies you plugged into your electronics board with heatsinks on top (you did attach the heatsinks didn’t you?).

    Stepper Motor – The black motors supplied with your build used to moving things around. Most motors provided with the Beast V2 are rated to 2 Amp (huge for a Nema 17 motor), meaning unless you do something really bad to them, (unresolved short between motor wires, drop them, disassembled them, rip out cables etc.) they should not die.

    “Vref” – is a term used to obtain a reference voltage for stepper drivers, allowing users to know exactly how much power they are providing to their stepper motors. Adjusting the current of a stepper driver is easy, just turn the small potentiometer with a tiny flat head screw driver. • Be very careful though. Do not short your screw driver to any of the terminals of the stepper driver as you may cause damage to your stepper driver. Also, some stepper drivers have a definite end position meaning they turn max 270 deg. Other rotate indefinitely returning to 0% after 100%. Basically if your stepper driver stops turning, don’t force it. If you break it, they’re cheap and readily available – just search ebay : DRV8825.

    Potentiometer (“Turnpot / “Pot”) -Look closely at your stepper drivers, see that little Philips head screw? Turning it left or right will adjust the power (vref) the stepper driver provides to the stepper motor.

    “Skipping steps” Your motor being told to move to a position, but not being able to get there. If the lack of movement is caused by an overheating stepper driver, no sound is accompanied. If the lack of movement is caused by insufficient power from the stepper driver, an audiable grinding or intermittent clunking can be heard. Observing the stepper motor when this occurs, it should be evident the motor is not moving to the intended position, flicking back and forth quickly. Skipping steps can also be caused by moving or accelerating a motor too fast. Often mis configured extruders will make a quick “buzz buzz” sound when performing retraction movements. This is because they are skipping steps.

    Before I go too much further,

    • If you have a PURPLE stepper driver, it has limited throw left and right. Adjust it ANTI CLOCK WISE to increase power. DO NOT force it beyond it minimum and maximum positions – it will break if your do.

    • If you have a BLACK stepper driver, it can rotate 360 deg and has no definite stop positions. CLOCKWISE rotation will increase power. Rotating beyond 100% will cause its power output to return to 0% (and visa versa).

    Theory :

    Measuring” Vref” varies from stepper driver to stepper driver, but for the DRV8825 with which you were supplied, it is simply a matter of setting your multimeter to measure DC voltage and placing the red probe of a Multi meter on the small (Philips head) turnpot on top of your DRV8825 and the other probe carefully on any common earth pin / wire / screw hole on your motherboard. The screws holding in the black wires from your PSU is a good place. This will give you a voltage reading usually between 0v and 2.5v depending on the position of the turnpot.

    Now that you know how to measure VREF. Forget it. Forget about all the information you may have learnt because it has absolutely no relevance to YOUR specific printer. The motors you have been supplied are way over specced and should easily be able to handle any current the DRV8825 can provide (2Amp/Phase). Most of the theory you will read in regards to setting VREF refers to setting an ideal value based on your motors specification, without accounting for real world factors such as friction, load and ambient heat. Given that each axis of a 3D printer has different load variables, each stepper driver therefore should have different VREF settings to account for those variables. IE, the load of a Z axis is far greater than that of an X axis and therefore the Z axis will require a higher VREF value that X for example.

    Many 3D Printer manufacturers that sell fully assembled units can readily supply their customers with (slightly) useful VREF values to set stepper drivers to because they know how tight their belts and wheels are – after all, they assembled the printer. Their values are of no use to you and your printer unless your load and friction variables are the same as theirs. Infact using their figures may very well cause you over or under load your stepper drivers resulting in avoidable skipped steps.

    Having been assembled by you , the wheels and belts on your 3D Printer are not tensioned to a known value. And therefore the VREF of your steppers cannot be set to a specific value we provide. We explain to customers that wheels should be tightened only enough to reduce all slop in the gantry while allowing for the freest possible gantry movement, and the belts to be “taught”, not tight, not crazy tight, just taught. Once the belt is attached, If the motor shaft is held in place, you should not be able to move the moving gantry back and forward at all ie, there should be no “slop”. Plucking the belt, it should vibrate for between 4-7 seconds as a rule of thumb. This indicates the belt is sufficiently taught. If it stops vibrating within 1-2 seconds – LOOSEN YOUR BELTS – they’re over tight and you risk damaging your motors due to the excessive heat they will generate.

    You can see that there is a reasonable level of variability in the elements of your printer which cause friction. For this reason, telling you a specific VREF for each stepper driver isn’t suitable. In order to get the correct setting for either X, Y, Z or Extruders, you’ll need to run some experiments.

    The process for X and Y stepper drivers is pretty much the same, turn your printer off, manually move all axes away from the home (front left) position, home your printer (option in the LCD) and while the “in question” axis is moving, carefully adjust the stepper drivers current (without shorting anything) from its lowest position, increasing current until the motor / axis begins to move smoothly – NO FURTHER. Now, reset the test, this time, increase the power enough to allow the motor to move smoothly while you obstruct the movement gently with a limp hand. This is to simulate the unwanted plastic artifact entering the Z space and causing more load on the motor and stepper driver. Don’t apply any more force with your hand than what you can estimate would be caused if a hot nozzle ran over an unwanted lump in the print.

    For the Z axis, first make certain your lead screws are lubricated turn the printer off. Place a 1KG weight (full spool of filament) on the bed to simulate the heaviest expect print – (go heavier if you want) and drag your bed DOWN maybe to half way (NEVER DRAG THE BED UPWARDS UNLESS YOU LIKE RE-LEVELING BED). Turn your printer on, home the Z axis and adjust the stepper driver while your bed is homing, providing only enough power to allow your bed to move smoothly without skipping steps.

    Extruders (yes this need an entire sub heading).

    For the extruders, here’s where things get tricky. No single printer pre assembled printer manufacturer can give an exact figure for VREF on extruder drives, be they direct drive or otherwise UNLESS you can only use that manufacturers proprietary filament AND they control the printing variables such as speed and temperature.

    Inconsistent filament diameter is the major issue here. Filament which approaches 2mm in diameter will cause excessive friction in bowden tube or on extruder drive gears. Filament which is less than 2mm even if it is accurate before entering the bowden tube or directly to the hotend (in direct drive systems) can easily be squashed by an improperly tensioned extruder idler. In fact, even if your idler doesn’t squash your filament (it may be really hard filament), excessive idler tension will result in more stepper driver power required to make it pull filament through. Blockages in the hotend can also result excessive load on the extruder stepper motor.

    The following is possibly the most important thing you’ll learn in 3D printing so pay attention :

    Before you 3D Print anything, read and follow the process below to get a baseline or reference point for reliable extrusion BEFORE YOU GET YOUR FIRST NOZZLE BLOCK.

    Theory :

    VREF

    If a extruder stops extruding, often people turn to the extruder idler and the extruder stepper driver, with the thought “if I stop the extruder from slipping by tightening the idler more, then increase the power to the extruder idler, my troubles will go away”. What they don’t realise, is that if the extruder and extruder stepper driver had been properly tuned in the first place, they can all but eliminate both from the trouble shooting process every time. In my experience, for my Beast V1, I NEVER touch my stepper driver power or idler tension. On the V2, I NEVER touch my stepper driver power, only adjusting the idler tension when printing with soft plastics like PTFE.

    An extruder which has its VREF set correctly is like a DOCTOR for your 3D Printer. Trusting a well tuned extruder and extruder stepper driver can help you diagnose faults with amazing accuracy, but you need to trust it first.

    So how do we trust the Extruder and Extruder stepper driver?

    Firstly, this is done best with a brand new hotend, one that has been properly assembled. It imperative, especially with V6 hotends that require assembly that you follow the hotend assembly instructions for to the LETTER. Most important is that the nozzle is tightened against the threaded heatbreak NOT the HEATER BLOCK. IMO, its also important that you use thermal paste on all threaded components. Some recommendations such as those from e3D suggest its best placed only on the interface between the heartbreak and heat-sink. In my experience, its best placed on the nozzle thread too.

    When you’re confident you have a properly assembled hotend with all the thermal paste applied and no gaps in the construction, its time to tune the stepper driver and extruder idler.

    Before we start, please note, your extruder idler should be rechecked / reset when ever you change filament types or brands as the hardness of the filament will also change and with it the required idler tension to prevent filament deformation while maintaining maximum grip on your filament

    Setting the idler tension vs stepper driver power :

    Ultimately you do not want the idler to slip EVER. When the idler slips, it chews the filament. When filament gets chewed, it gets deformed. If this deformed filament works itself into the bowden tube or hotend, expect the friction in the bowden tube or hotend to increase and a block to follow shortly thereafter. So set the idler tension ONLY tight enough so that when / if a blockage occurs, the stepper motor skips steps due to a lack of power rather than progress to chew the crap out of your filament. Conversely, set your stepper driver power LOW enough to ensure that when a blockage occurs for any reason, temporary or otherwise , the motor skips steps, (relieving the pressure on the blockage before the filament gets chewed) but HIGH enough to cope with all unobstructed printing at a given speed.

    For this process to work, both idler tension and stepper power will need to be adjusted during a print.

    • Set your extruder idler tight enough to leave small “teeth marks” in your filament as it leaves the extruder. You do not want the filament to deform as its leaving the extruder, so for the moment, just use your best judgement.

    • Start with your extruder VREF high to about 1.1v

    • Best to start with an object with largish bottom surface. Start the print and deliberately Baby Step (from your LCD) the nozzle into the print surface to obstruct extrusion.

    • While this is happening, quickly adjust your stepper driver power downwards until you can hear (and see) the motor skip steps. As soon as it does, baby step upwards a few mm off the bed to relieve the pressure and allow around 10mm of filament to be extruded completely unobstructed, you should notice the extruder motor regains its ability and continues to extrude without skipping steps. If it does not, slowly increase stepper driver power only until it regains its ability to extrude without skipping steps.

    • Next, start a new print, but this time, start printing a few mm off the bed to ensure the nozzle is completely unobstructed. Gradually loosen the extruder idler tension to the point at which it begins to slip on the filament, then re-tighten 1 full turn.

    • Now repeat the process. Remember, your goal is to have the idler as loose as possible without slipping at normal printing speeds AND your extruder motor to skip steps only when a blockage occurs.

    • Start a test print. Periodically check the motor temperature. If your motor is too hot to easily hold your hand on – its too hot and likely overpowered.

    #1205
    joshdp21
    Member

    Hi everyone!

    I am still chasing my underextrusion problem on my Beast V1.2. I replaced my hotend with a spare and the clogging was resolved (yay). But I was still seeing basically 0 extrusion, and looking at the extruder gear- it did not appear to be turning while running a test print. I thought it may be that it was just slipping due to the back pressure caused by the initial layer. I took the DCV reading across the pot screw and ground for my extruder motor and was only getting a reading of 0.78V. Turning the pot did not seem to change anything. Please see that attached picture. Am I measuring the voltage correctly? (Note: I took the reading while a test print was running)

    Thanks for any help!

    #1206
    joshdp21
    Member

    joshdp21 wrote:


    Hi everyone!

    I am still chasing my underextrusion problem on my Beast V1.2. I replaced my hotend with a spare and the clogging was resolved (yay). But I was still seeing basically 0 extrusion, and looking at the extruder gear- it did not appear to be turning while running a test print. I thought it may be that it was just slipping due to the back pressure caused by the initial layer. I took the DCV reading across the pot screw and ground for my extruder motor and was only getting a reading of 0.78V. Turning the pot did not seem to change anything. Please see that attached picture. Am I measuring the voltage correctly? (Note: I took the reading while a test print was running)

    Thanks for any help!

    #1207
    dan
    Participant

    Josh. Sorry I missed your post. Our server got corrupted, preventing email notifications. Let me know if you’ve solved this. If you get zero extrusion, first you need to figure out if the extruder is actually working. This can be easily tested by removing your Bowden tube from the hotend, preheating your hotend (allowing the extruder to move) and using your lcd to move the extruder say 100mm. Does the extruder move unhindered?

    If it does, is there any torque? If you were to hold the filament before it enters the extruder, does the extruder pull or does it skip with the slightest of resistance?

    Commonly, one thing people often overlook is that when an blockage occures, you absolutely NEED to remove any filament from the Bowden tube. Upon blockages, possibly from the hotend, the extruder can chew and deform the filament inside the Bowden tube, making it impossible for the extruder to do its thing. Often people clear the source of the blockage only, completely overlooking the other EFFECT of the blockage which can be deformed filament. Removing the filament from the tube will often reveal very badly deformed filament.

    If the extruder seems fine, and Bowden not itself blocked, it’s time to look at your hotend. Primarily hotend construction, look for gaps in construction, specifically between the heartbreak and nozzle and or the Bowden tube and heartbreak. Make sure your Bowden is cut square and when you insert it into the hotend (and extruder) ensure you pull up on the neck of the push fit adapter while pushing down on the tube. This prevents gaps from forming during retract movements.

    Another tip if printing with low melt plastics such as PLA and PETG is to consider the effect of the seldom mentioned variable of “heatwork”. Heatwork is the effect of temperature over time. When printing with a heated bed, consider that the filament is likely at ambient temperature before it enters the hotend. This means less hotend heat is required to achieve a viscous material. It also means that filament will soften in the “cold zone” at a lower hotend temperature than it would if the heated bed was turned off. Again, simple solution is to reduce the hotend temperature by up to 20-30 degrees while printing with a heated bed. In my experience, PLA which prints fine at 205 deg without a heated bed running can be reliably printer at 175 with a heated bed active. At some point I’ll devise an experiment to show and graph how heatwork effects filament viscosity in Bowden tube as I’m sure the amount to reduce hotend temperature isn’t linear nor directly equivalent to the increase in ambient temperature caused by the heated bed. Also worth considering with the V2 which has a moving heated bed is that heatwork will have less effect on the viscosity of material as the bed moves further and further from the nozzle during printing. This means adjusting hotend temperature upwards as the z height increases is advisable.

    Let me know where you’re at with this so I can assist further.

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