L6482: Right value of R sense, for 3A stepper motors?

[val_ltd]

Posted on August 15, 2017 at 11:49

Hello,

I am using 'L6482H discovery board' design example for L6482 chip, for making my own pcb board, using the same mosfets, etc.

At this moment, my board runs really well with

2A

stepper motors and 80 mili-ohm R sense resistors. Both commands, Move (DIR, N_STEP) and Run (DIR, SPD),  run very well.

But when I use Run (DIR, SPD) command with

3A

stepper motors, stepper motors almost do not have any torque, and it is easy to block the axis. Only at very low speed they have some  torque.

I understand I have to change my R sense values

, but I am not sure the way of calculating this value for 3A stepper motors.

Please, what is the way of calculating the right value of R sense?

Thanks in advance,

Valentin

#l6482h #l6482 #r-sense

[Enrico Poli]

Posted on August 24, 2017 at 15:34

Hello,

No need to change the sense resistor value. You can increase the motor current through the TVAL_xxx registers.

Example:

Target current 2 A --> TVAL_xxx = 2 A x 80 mOhm = 160 mV

Target current 3 A --> TVAL_xxx = 3 A x 80 mOhm = 24

0 mV

[val_ltd]

Posted on September 02, 2017 at 10:54

Thank you so much,

I have just tested it, and the result was right.

I miss a document that explains these practical concepts, to help us to decide the configuration values.

[val_ltd]

Posted on September 11, 2017 at 20:33

Hello Enrico,

please, I have a doubt about TVAL_xxx

If I do:

Target: motor pap with nominal current

3 Amps

:

TVAL_xxx = 3 A x 80 mOhm = 240 mV.   

So, I calculate TVAL register value:

TVAL_xxx = 240 mV / 7.8 mV ---->TVAL_xxx = 0x1E (register value)

And I use this value for all TVAL parameters:

TVAL_HOLD = iTVALxx

TVAL_RUN

=  TVALxx;TVAL_ACC =  iTVALxx;TVAL_DEC =  TVALxx;

1) With TVAL_RUN = TVAL_xxx = 0x1E motor runs with very powerfull movement. It seems right.

2) Motor get too high temperature even when it is stopped. You can burn yourself if you touch it. I know there is a retention current, but, Is it a right value for TVAL_HOLD parameter?

3) Are TVAL_ACC and TVAL_DEC values, usually, the same value that TVAL_RUN, or less ?

Is there a rule to decide all these values, from TVAL_RUN...?

Thanks in advance,

[Enrico Poli]

Posted on September 12, 2017 at 17:27

Hello Ramirez,

In most of the cases TVA_RUN, TVAL_ACC and TVAL_DEC are set to the same value. The target value depends on the motor characteristics and to torque you want to apply during the motion.

If the mechanical load connected to the motor shows a particularly high inertia, the acceleration and deceleration currents (TVAL_ACC and TVAL_DEC) could be higher that the constant-speed current (TVAL_RUN). This because the inertial cause an 'extra-torque' opposing to the speed variation.

The holding current (TVAL_HOLD) is normally significantly lower than the operating one because the friction and the detent torque help the motor to keep the position. Typically an holding current of 10% of the operating current is ok.

[val_ltd]

Posted on October 09, 2017 at 13:42

Thank you so much, Enrico.

Now everything is running right.

In order to finalize this project, I have to get ready the circuit to use Step-clock input (STCK pin 32 of L6482), as well.

I have three doubts:

- If I use this input, does torque regulation run? i.e. L6482 regulates the torque using R Sense etc, when I use STCK input instead of commands?

(Is it possible to use this mode for CNC systems, with software like Mach3, Mach4 which send square waveforms specific for controlling pap motors, or is it necessary to do anything more? )

- Does input STCK any special requeriments, related to the pulses (length, frecuency, amplitude, waveform...)

- Is there any data sheet explaining in detail the way of using this mode?

Thanks in advance,

Valentin Ramirez

[Enrico Poli]

Posted on October 27, 2017 at 13:57

When step-clock mode is used the current control is still operative and the current is set according to TVAL_HOLD register value.

The STCK input is a standard digital input, the only limit is in the high and low times that must be more than 300 ns in order to be properly detected by the internal logic.

Sorry, but I have not expertise in the CNC software you listed; I can only say that the step-clock mode operates in the same way of the usual stepper motor drivers bypassing the motion engine integrated into the device.

[val_ltd]

Posted on November 29, 2017 at 08:00

Thank you so much Enrico,

with your help, now everything is running right,

Valentin Ramirez