dobrinski написа:Малките бели кутийки са ферити. А за съжаление драйвера има само full and half step. Някой дали подкара схема на драйвер с 1/4 или1/8 стъпки за ток от поне 5А?След доста изпилени нерви с други драйвери прежалих 96лв. за един драйвер CW5045 до 4,5А от EXOROR. Имат и CW8060 до 6А , но струва цели 153лв. Имат доста варианти за избор на стъпки. Пробвах го на 1 , 1/2, 1/4 и 1/8 стъпки и винаги има един интервал между бързия и бавния ход, където се чуди как да работи. Най вероятно там е mid-band resonance. Сега чакам някакви приходи за останалите два драйвера. Имам два излишни SMBP2A5-FIX
mika написа:MrMurdjo, според Mariss, микростъпките не могат да подтиснат "mid-band resonance". Това е точно обратното на твърдението на Pandurko, или пък аз нещо не съм разбрал?И аз така го разбирам. Но на практика не се проявява с такава сила. Може би се дължи на факта, че фазовото закъснение, за което говори не се проявява върху ъглово отклонение от цяла стъпка и не достига тези 180 градуса, когато моторът заклинва и валът му само трепти. Но това са само мои предположения и не е задължително да са верни. Не притежавам задълбочени познания и не съм изучавал в пълен обем и систематично тези неща, нито пък практически съм се занимавал, освен в любителската си практика.
During microstepping, one tries to generate sinusoidal current waveforms in the two motor windings (one sinus, one cosine). This is done by controlling the duty cycle of the switching of one of the legs of the so-called H-bridge. When positive currents need to be controlled, one switches one leg of the H-bridge, with negative currents, the other one. (see Wikipedia: H-bridge, but take attention, I speak of 3 basic states of an H-bridge instead of 2)
To increase a current (either pos or neg) the switching powerMosfets of one leg of the H-bridge are connecting a winding very shortly (microseconds) to the powersupply, let say 80DC. This technique is called Pulse Width Modulation, PWM for short (see Wikipedia: Pulse-width_modulation). A series resistor is connected between ground and the H-bridge; in this series resistor runs the same current as through a motor winding, generating a voltage (a few tenth's of a Volt) that is used to represent the winding current to the closed-loop current-control electronics.
During the period that the powerMosfet(s) does not connect to the 80VDC, the lower 2 powerMosfets are both switched on (the 3rd state) to allow the recirculating current of a winding to be maintained until the next cycle. No current is then flowing in the series resistor. Please understand that we talk here about extremely fast switching speeds of about 100 nanosec., usually with repetitions of about 20 KHz until 70KHz or higher. (Do not confuse these speeds with the much lower speeds that control the sinusoidal microstepping waveforms; these are around 1..2 KHz when the stepper motor runs really high speed).
So far, we have some notion what happens when a current is too low in the opinion of the current feedback-loop circuit and the feedback-loop will then increase the PWM time (duty cycle). But what happens when we arrive at the decreasing part of a sinus? Nothing!
As long as the recirculating current does not fade away by ohmic losses, almost nothing happens. You might think of increasing the ohmic resistance of motor windings, but this creates only more problems. Only after the demanded current from the microstepping sinus reverses polarity, something happens; then the other pole of the H-bridge is being switched. Only then is a reverse voltage applied to the winding, that will rapidly decay the recirculating current and then increase in the wanted pos or neg direction. The beautiful sinus/cosine that you wanted ideally as the current waveform, is extremely distorted. I have never understood why everybody accepts this very poorly designed circuits, but they are the majority of all lower-priced stepper motor drives.
Of course, the best way is to reverse the active leg of the H-bridge as soon as the current need to decrease. This will cause a reverse voltage on a winding to decrease the current. We now speak of so-called four quadrant drive technology. The principle is very old, but only applied in high end motor drives because of its difficulty to design.
When trying to build a reasonably fast and precise CNC with stepper motors, focus on low inductance motors, high voltage power supplies and four-quadrant drive technology. The results of these three measures are spectacular, as I know from experience.
mika написа:MrMurdjo, според Mariss, микростъпките не могат да подтиснат "mid-band resonance". Това е точно обратното на твърдението на Pandurko, или пък аз нещо не съм разбрал?Да така е.
Сподели форума:
Бъди информиран. Следвай "Направи сам" във Facebook:
Намери изпълнител и вдъхновения за дома. Следвай MaistorPlus във Facebook:
