On Sunday, October 02, 2011 12:37:01 PM Kirk Wallace did opine: > On Sat, 2011-10-01 at 12:14 +0100, andy pugh wrote: > ... snip > > > I have an idea for a way to use a second opto to discharge the cap > > through the same resistor during the low portion of the PWM, which > > should be linear at all frequencies, but have never bothered trying > > it. > > After having more experience with getting my PWM to VFD input working, > it comes to my mind that what is behind the VFD input is an ADC, which I > think takes a snap shot (sample/hold) at a frequency. I seem to recall > one VFD manual stating the sample frequency at something around 500 Hz, > and others in the tens of kilohertz. This seems to indicate that the > input will sample whatever voltage happens to be on the input at a > given, but regular, instant in time. With raw PWM this is typically > either 0 or 10 Volts. With filtered PWM, the voltage could be anywhere > along the charge/discharge ramps. It seems with a push/pull on the > input, it may actually make things worse by making the input more like > raw PWM. If my thinking is correct, ideally each PWM cycle would be used > as a data point to drive a stable voltage output for that particular > cycle. Even then, it might be better to block a sample that happens > right on a cycle change or filter only close to that point to smooth the > change. > > Or, I could be completely wrong.
Even if it is an ADC that works on S&H, the bi-directional opto pair driving a series resistor charging a capacitor to integrate the pwm can easily reduce the 'ripple' to nearly zip if the pwm is driven at a frequency well above the S&H frequency. That would be key to smooth control, and quick enough that motor rotor inertia would be a bigger factor in real time control than the TC of the RC filter. Cheers, Gene -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Administration: An ingenious abstraction in politics, designed to receive the kicks and cuffs due to the premier or president. -- Ambrose Bierce ------------------------------------------------------------------------------ All of the data generated in your IT infrastructure is seriously valuable. Why? It contains a definitive record of application performance, security threats, fraudulent activity, and more. Splunk takes this data and makes sense of it. IT sense. And common sense. http://p.sf.net/sfu/splunk-d2dcopy2 _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
