On Thu, 28 Mar 2013, Stuart Stevenson wrote: > Date: Thu, 28 Mar 2013 07:05:01 -0500 > From: Stuart Stevenson <stus...@gmail.com> > Reply-To: "Enhanced Machine Controller (EMC)" > <emc-users@lists.sourceforge.net> > To: dengv...@charter.net, > "Enhanced Machine Controller (EMC)" <emc-users@lists.sourceforge.net> > Subject: Re: [Emc-users] tach feedback > > Gentlemen, > > When a control requests a speed from a motor it will send a signal to the > drive ex 500 rpm. > The drive wants to put out 500 rpm of current to the motor but it cannot > because the motor is not turning. The drive knows this because the tach > feedback is zero (the motor is not turning). > Therefore, the drive uses the tach feedback and the tuning parameters to > increase the current to the motor until the tach feedback reaches the > voltage necessary for the control to recognize the motor is running at the > requested speed. The drive uses the tach feedback to adjust the current to > keep the motor running at the requested speed in the case of varying load > conditions. > The drive has no knowledge of where the tach voltage is coming from. When > the requested speed and the tach feedback reach the equilibrium state the > drive does not shut down. The drive continues to output the current > necessary to run the motor at the requested speed. Why would the drive not > immediately output the requested current level if the requested speed and > the tach feedback is matched from the start? > I ask this because I want to remove the drive tuning from the system. > Everyone says the drive/motor servo tuning is the best but if that is the > case why is there tuning capability in LinuxCNC? Obviously, it is not good > enough or no effort would be expended in making it better. > I will try this on one of my systems. > Maybe I need to work on a pseudo tach feedback that I generate to get the > desired result. > thanks > Stuart > > > On Wed, Mar 27, 2013 at 11:59 PM, dave <dengv...@charter.net> wrote: > >> On Wed, 2013-03-27 at 22:43 -0500, Stuart Stevenson wrote: >>> Gentlemen, >>> If I am wanting to remove the servo tuning from a velocity loop would I >>> be able to jumper the +/- 10 volts to the tach feedback into the drive >> and >>> to the speed request into the drive. >>> Would this cause the drive to not respond? >>> Would this cause the drive the immediately output the full requested >>> current to the motor? >>> thanks >>> Stuart >>> >> Whoa! >> tach feedback is negative.... if it matches the input drive then you do >> not accelerate...i.e constant speed. >> Think of it as a summing junction on an op amp with the other input >> being the velocity voltage. >> I think I have that right even at my age. ;-) >> >> This is what make velocity drives so easy to tune as opposed to torque >> drives. The tach feedback is a loop inside the velocity input. >> >> Dave >>
In addition to what Dave has said, the tachometer input is almost never scaled the same as the velocity command input and may or may not have the same polarity, so the most likely result of tying both tach and command inputs to the velocity command source is a fast runaway. If you did manage to match the tach and velocity command scaling and polarity you will still get a (slow) runaway (that is unresponsive to the command input) because they will not match exactly and the velocity control loop in the drive is typically a PI loop, so a small input error will keep increasing the current (the I terms contribution) until you have a full runaway. Many Velocity mode drives do have a way of running in torque (current) mode, which sounds like what you want to try. Tuning torque mode drives is often tricker (and very different) than tuning velocity mode drives as you need to close the velocity loop in LinuxCNC (via the D term). You may have to run at a faster servo thread rate to get acceptable performance from a torque mode drive since the higher bandwidth velocity loop is now linuxCNCs responsibility Peter Wallace Mesa Electronics ------------------------------------------------------------------------------ Own the Future-Intel® Level Up Game Demo Contest 2013 Rise to greatness in Intel's independent game demo contest. Compete for recognition, cash, and the chance to get your game on Steam. $5K grand prize plus 10 genre and skill prizes. Submit your demo by 6/6/13. http://p.sf.net/sfu/intel_levelupd2d _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
