Javier, this is really a great post and you seem to have done a lot of related work in the past.
If so, what is your thinking about the best (easiest to tune) mode for the servo amplifier: tachometer mode, torque mode, voltage mode? If I have so many problems with tachometer mode, should I perhaps switch to torque mode? or voltage mode? In both of them, feedback is internal to the drive itself. - Igor On Tue, Sep 14, 2010 at 12:07 PM, jros <[email protected]> wrote: > > > > > > I would consider the BIAS parameter along the FF0 FF1 and FF2. > > Then all the the terms can be considered Feed Forward terms of the > controller, and the part of the control they deal with is conceptually > called "feed forward control". > > The feed forward capabilities implemented by those terms can be enough > to implement feed forward compensation of mayor dynamical effects in the > context of Cartesian machines. If the dynamics is nonlinear, accurate > feed forward terms would in general be nonlinear. > > The meaning of these parameters are dependent in the meaning of the > output of our controller. > > For example if the output of your controller is torque, > then the feed forward terms can be used to compensate (examples): > > BIAS: Torque that is needed in an axis to compensate the gravity > effects. > > FF0: Torque that is needed to compensate the forces induced by springs > (if present at all), for example, belows covering linear guides can > induce a elastic component (also viscous etc). > > FF1: Torque that is proportional to velocity, for example due to viscous > friction > > FF2: Torque tat is proportional to acceleration, this is almost always > torque used to compensate the inertia. > > A interesting torque that EMC ca not compensate for is Coulomb type > friction, that would be like a BIAS but dependent with the sense of > motion (v/abs(v)). > > If the output of the controller is velocity you should judge the meaning > yourself. > > Finally when assigning values to these parameters, I think that a good > idea is to be conservative. > > Gravity effects, and inertia effects are likely to be constant along the > life of the machine (ignoring variable loads), but this is not the case > of friction (viscous or coulomb), elastic component can be pretty stable > if there is not malfunction or broken elastic elements. > > I recommend only to compensate the part of the torque that you know is > going to be there always. I'm not sure but overcompensating a frictions > not a good idea so don't use friction compensation or compensate only > the minimum friction that you are expecting. Also values of friction can > only be estimated after experimentation (identification), this can be > the case of stiffness, but usually gravity and inertial terms can be > estimated from cad data (part weight and -additionally- gravity center > position in the case of gravity compensation). > > Feed forward compensation can be in general a good idea, as it > theoretically removes work from the standard PID controller what > theoretically should make things work better/smoother. > > In the case of nonlinear dynamics, you can still use those terms to > compensate the first term in the tailor series expansion of the above > mentioned torques. > > If you implement your own pid module, and use a dynamic model of your > machine to accurately compute the output of the FF? terms you ar making > essentially what is know as CTC (computed torque control), that is the > generalized version (for non Cartesian architectures) of the > implementation present in EMC, that in my opinion is almost the best > (except for Coulomb terms) implementation without introducing particular > characteristic of the machine, what turns out to be the case. > > I hope this helps, > > Javier > > > > > ------------------------------------------------------------------------------ > Start uncovering the many advantages of virtual appliances > and start using them to simplify application deployment and > accelerate your shift to cloud computing. > http://p.sf.net/sfu/novell-sfdev2dev > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users > ------------------------------------------------------------------------------ Start uncovering the many advantages of virtual appliances and start using them to simplify application deployment and accelerate your shift to cloud computing. http://p.sf.net/sfu/novell-sfdev2dev _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
