Hi all, i had a response like this: "With stepper driver we can start ramp with 1000 step/sec , ( 1msec/step) at half-step , full current, bipolar driving, 24volts power supply and chopper method."
I have some questions:
If i have motor like this:
Rate Current: 2,3A
Rate Voltage: 3,1v
Phase resi.: 1,25ohm
Phase ind.: 1,8mH
And i use this:
With 24v 2.2A supply.
The motor will fry? It will provide me 1000 step/sec?
And, how i can start faster up to 2000step/sec or more, what is limit? And what calculation i need to do if i apply a weight to motor and inertia consideration?
In principle that will work, and you will need to carefully set the current limit of the driver in order to not exceed the 2.3A rating on your motor, and to give the driver a heatsink. Make sure you have excellent (low inductance) capacitors on the input to the motor driver and low inductance (short, twisted) motor leads if you want to support high step rates.
Ignoring back-EMF etc, 21-24V over 1.8mH gives dI/dt of about 12kA/sec. To completely reverse the coil current (from 2.3A to -2.3A) will therefore take 0.4ms in the absolute most optimistic (motor stationary) case. My guess is that the stepper will be able to reach 1000, probably 2000 steps/second and once you go much faster than that, your torque will start to drop off because the driver will no longer be able to slew the coil current up to the full 2.3A in each direction. It will probably drop off a lot earlier than 2000 steps/sec depending on the motor's physical construction, due to back-EMF, and you will need to measure that curve or get it from the manufacturer.
Reaching that speed requires that you accelerate the motor smoothly. How fast you can accelerate depends on the torque/speed curve of the motor and the mechanical torque that the motor must supply to whatever it is carrying. And also on how clever your motor driver chip is and how its current feedback loop is arranged. If you try to accelerate too fast, the motor will drop steps, stagger and stall/buzz.
If you double the speed, you have only half as much time for the current to slew AND you face higher back-EMF, so the current will reach a peak value of less than half of what it would at low step speeds, and you get less than half the torque. At some point, the back-EMF will approach the driving voltage and the motor will develop no torque - that's your no-load max speed. It is likely to be much higher than 2000 steps/second.
raacampbell:
My experience indicates that there is an optimal current. If you go beyond it then the motor will skip steps during fast motions.
This occurs if you ramp the current up a long way and then try stepping rapidly. In the very short step times, the driver doesn't have time to slew the current back down to zero properly and out to the same current in the opposite sign, so you end up with a current bias in the windings, which means uneven torque. Also if you saturate the motor's core, things will go badly for you. I would expect that careful acceleration would solve the former problem.
I went back and experimented based on your suggestions. The thing that made the most difference was not moving in full steps. In full step mode, I have to keep the current low (<50 mA) or I get missed steps during acceleration or deceleration. This occurs even if the ramp is low. So I suspect that's a resonance issue. If I switch to half steps, I can easily use currents of 250 mA or more and accelerate far faster and to much higher speeds.
raacampbell:
I went back and experimented based on your suggestions. The thing that made the most difference was not moving in full steps. In full step mode, I have to keep the current low (<50 mA) or I get missed steps during acceleration or deceleration. This occurs even if the ramp is low. So I suspect that's a resonance issue. If I switch to half steps, I can easily use currents of 250 mA or more and accelerate far faster and to much higher speeds.
Microstepping is almost always needed to get best performance from a stepper motor,
as it reduces the amplitude of vibration and resonances.
I just kept the current low with the pot on the Big Easy Driver and read off the current used by hooking it up to a PSU that reports this value. As you guys say, micro-stepping is the way to go.