![]() The motor is your choice and there are many ranges. ![]() I recommend GCODE PANEL (open source written by a pro) which has constants for dozens of variables with USB to any Windows OS with a Gcode interpreter uploaded to any Arduino with a CNC shield for a Windows laptop. Choose a good operating system with control over every variable. (steps with measured initial conditions)Ĭhoose an EMI robust twisted pair cables and current sense method (Hall or 50 mV shunts). Record and store the Z=V/I and P=V*I results for each case. Start by making a list of assumptions and measurement specs by verifying the motor no- load impedance with alternating pulse steps with constant acceleration and variable current limits and fixed voltage over a variable position range with velocity limits. Half-stepping also reduces noise but also reduces torque and speed.Īssumptions are the core saturation limits are known and do not occur from the imbalanced full bridge which would reduce L or excessive temp rise and also reduces L with forced air cooling for high duty factor work. Damping can be mechanical or electrically dampened also to reduce acoustic noise effects by synchronizing the step interval and reducing the settling time of resonance. There is another time constant with almost no load and a 2nd order torque spring-mass response that changes with load inertia. There is an electrical time constant for the T= (RdsOn(2)+ DCR)/L for a locked rotor that has no BEMF with some thermal sensitivity. Since both could change a multiplier is needed to compute V(t)/I(t)=Z(t) and compare it with the expected response under all conditions. This also affects the current step shape from the applied voltage or current limit and the voltage feedback from each applied step. ![]() We know the unloaded open-loop velocity creates a back EMF voltage (BEMF) just as DC BLDC motors have an unloaded kV/RPM while RPM or velocity reduces with the mechanical load. Thus it is a question of safety margins and damage on how you choose to sense position error by expensive sensor feedback or by the design of current profile feedback from repetitive motions with smart fault detection and recovery. Trial and error methods can also work well if known for different mass loads and limited by a=F/m, but in robotic work, this can sometimes be unknown. ![]() For safety switches can used for each end or current limited and stopped before damage. Home switches may be optical or mechanical if precise. Then a manual recal to the home switch to rezero. The normal response is to halt when a fault occurs. The challenge is, therefore, to recover from slipped steps by sensing motor impedance by current, and thus BEMF changes with velocity so as to slow down after recovering from a missed step and then adjust the limits so as not to slip after some worst-case calibration. When moving near maximum velocity or acceleration or a jerk with transient friction, the risk of slipping steps increases. Other methods might use an optical rotary encoder at great expense, but this is not used even in my Mercedes SUV tailgate which has a smart stepper control.Įach step will vary in current according to mass, acceleration, velocity, phase control, friction, R/L slew rate, mechanical resonance, electrical resonance, and active current limit.Ī phase current monitor for feedback, therefore, can be able to compare the expected current shape with the actual one on a step-by-step basis. I have not done this, but you need to compare the expected transient current for each step to ensure a slip back has not occurred. The electronic method also used in sensorless vector-controlled BLDC motors is to measure impedance from the vector transformation of voltage and current. The clue for optimal performance is to have a smooth velocity change with a controlled-acceleration change or jerk level. In clever designs, I have seen rotary oil-filled brass in plastic disk wheels attached to the stepper motor to add mechanical damping. In most cases, increasing current limit and acceleration reduces damping from transient impedance changes and natural mechanical resonance. Thus it is possible to seek times for performance without skipped steps unless there is an unexpected force exceeding torque to make the step transition. But it could also mean it's slow and noisy. This means you have excess torque for a known target friction and/or acceleration a=F/m with known momentum p=mv and stored energy E=0.5mv^2. That is done by design or testing and with trim on the parameters in order to prevent skipped steps. ![]() If you design an open-loop stepper system with end-stop feedback for "datum" or zero position calibration, with care, there is no need for step-by-step feedback. I would question the assumptions of your homework. ![]()
0 Comments
Leave a Reply. |