How to set max current with Aref potmeter on Bigtree driver

[Oegtsgeest]

I use different steppers NEMA14, 17, 23. I need to set max but safe currents to achieve max torque.
Using a Aref potmeter is a handy tool for that due to the fixed relation voltage torque.

The question however is what settings to use? Now I use enableAnalogCurrentScaling() , useExternalSenseResistors() and setRunCurrent(100).
Turning the potmeter, I do not see the voltage over a shunt resistor in a stepper phase vary.
What do I have to change in the setup?

In the text under Readme it says under Autoscaling "Use enableAnalogCurrentScaling() to allow VREF, the analog input of the driver, to be used for current control." But this does not seem to work. I am puzzled

[TrMC1]

Im planning on using the TMC2209 and this library for a project so I did some research. Im not particularly good at coding and I didnt try anything yet with the module but maybe I can still help after reading through the code and the datasheet a bit. Also I am writing this a bit for myself to not forget some things, so please forgive me for a lengthy reply.

In the datasheet (https://www.analog.com/media/en/technical-documentation/data-sheets/tmc2209_datasheet_rev1.09.pdf) it says on page 54 that any Vref (set by an external voltage divider) between 2,5V and 5V gives max current wheras 0-2,5V gives linear current scaling as long as I_scale_analog is enabled.

According to the TMC2209 datasheet I_scale_analog is enabled by default but in the library it says it is disabled by default. So the library changes this setting. It does that by calling the setup function. There are several versions of this function depending on the µC you are on (line 23+). They all call the initialize function (line 677+). There the setOperationModeToSerial function is called (line 745+). In it global_config_.i_scale_analog = 0; is set and written to the registry of the TMC2209 with writeStoredGlobalConfig(). All these functions are defined in TMC2209.cpp. Unlike global_config_.i_scale_analog = 0 these functions dont change whether an external or an internal sense resistor is used. On page 22 of the datasheet it says that all default register values are 0 unless noted otherwise. On page 23 it says internal_rsense defaults to the OTP setting and on page 27 it says that the OTP setting in this case is 0, which is the global default, and which sets the default of internal_rsense to 0 aswell. 0 means that an external resistor is used. So unless that is manually changed it shouldnt be necessary to change that setting in your code.

You do need to change I_scale_analog though. Except for the setup function I didn't see another instance in the library where I_scale_analog is set to 0. So as long as you change it to 1 after the setup function was called you should be good. The function enableAnalogCurrentScaling() does exactly that. So it changes the variable on the µC and then writes it into the registry of the TMC2209. This should allow you to alter the current with the voltage divider on the module.

That being said. The datasheet gives you advice what the current will be on page 53 and 54. You are using the TMC2209 with a library so have it connected via UART which means the following formula is important to you. Irms= (CS+1)/32Vfs/(Rsense+20mohm)/(2^0,5) and Vfs'=VfsVref/2,5V in the range of 0 to V5vol/2 (which is 0-2,5V).

Having a look at the schematic (https://github.com/bigtreetech/BIGTREETECH-TMC2209-V1.2/blob/master/Schematic/TMC2209-V1.2.pdf). Rsense should be 110mOhm (R2 for coil A and R4 for coil B) and the voltage divider sets anywhere from 0 to 2,5V to Vref. In the datasheet you can find that CS actually stands for the value of IHOLD in case you want to calculate the maximum current when standing still or for IRUN in case you want to calculate the current when moving. Both IHOLD and IRUN can be anything from 0 to 31. 31 representing the maximum current. Vfs can be set to two different values. One setting is intended to use with a large Rsense and one with a smaller one. Since all current has to flow over Rsense you want it small, but then the voltage drop across Rsense will be small aswell and thus the current senseing will be inaccurate. Either way vsense sets which Vfs will be used. If vsense=0 Vfs will be set to VSRTL which defined as 325mV and if vsense=1 it will be set to VSRTL which is defined as 180mV.

Going back to the library. In TMC2209.h in line 420 to 428 the default settings for the current are set. Most notably IRUN is set to 31 which gives 1,77A when putting everything in the formula. IHOLD of 16 gives 0,94A. All the percent values in the library are merely another way to express the current range of 0-31 which the TMC2209 expects. The percent values are recalculated to the 0-31 range in TMC2209.cpp line 971 with PERCENT_MIN and PERCENT_MAX setting upper and lower bounds for the percentage you select. setRunCurrent(100), which you used, pretty much only sets to up to 1,77A for IRUN and up to 0,94A for IHOLD depending on the voltage divider practically speaking.

Now with that information you can easily change to maximum current by adjusting IHOLD_DEFAULT and IRUN_DEFAULT (line 426 and 427 in TMC2209.h) to whatever value you want without enabling the voltage divider. These currents are the maximum currents the TMC2209 will run through your motor under any circumstances.

That still doesnt answer why you cant adjust the current for your motor. Everything you did should allow you to alter the currents.

Looking at the datasheet the TMC2209 has a function called CoolStep. I didn't put much time into this chapter but in a nutshell it measures stuff and adjusts the current magically to fit the need. In the example files CoolStep is enabled. So if you really really really want to control the current you would need to disable CoolStep increasing the power consumption probably without gaining torque or for testing you could put some mechanical load on the stepper motor because then CoolStep hopefully increases the current on its own.

To my understanding it is still important to set current limits one way or another as a safety measure to avoid destructively high currents for the motor under heavy load. To my understanding you can do this with CoolStep disabled using the voltage divider or by calculating it with the formula and setting some values in the software.

Please give me some feedback if you tried this.