> i need some help to control stepper motor torque, any suggestion?
Do you try to use stepper motor in current mode?
If you want to control a stepper's torque, one way is to use Pulse width Modulation (PWM) on the voltage going to the motor. This will limit the current going to the motor and is sometimes called a chopper circuit.
This assumes you are using a simple step controller (Not microstepping). You could use a PWM device that allows you to write to a port (Or hardware IO selectors) to set the PWM ratio (On time vs. off time). Something like a PIC or an AVR microcontroller has hardware to do this. There are probably standalone PWM IC's to do this as well.
If you do it with microstepping you will need to use a microstepping controller that has a scaling factor for the current going to the motor since the microstepping IC is controlling the currents to the motor's windings with its own PWM circuit. This is how the microstepper gets in between steps, by controlling current in the windings to be somewhere "in between" steps.
Having said all of this, there are most likely commercially available stepper drivers that offer a torque limiting feature, either analog or
Stepper motor torque is not something that you should control. The torque is whatever it needs to be to maintain position, given the loads that it has to deal with.
Maybe you mean to control the current through the stepper motor? If so, then that can be done several ways. One way is to use resistors in series with the motor windings. This is cheap and simple, but it wastes lots of power and requires a higher-than-normal voltage. Another way is with a chopper closed-loop current control. This requires more circuitry, but it is the most efficient in its use of power.