Hi, I'm absolutely new to control systems and would highly appreciate some good advises from some veterans or anyone who has something that could help me get going. Well, the task for me is this: Having a shaker (works exactly like a loadspeaker) and a DC accelerometer that must run in a digital control loop. Sample freq is 4000 Hz. Shaker is attached to the DAC through an amplifier (no filter). Accelerometer is attached to the ADC (no filter). the 3 dB frequency range for the system must be 0 - 100 Hz (e.g. the shaker must be able to follow a 30 Hz sinus). The control signal is digital and can be a deflection, velocity or acceleration signal depending on what is easiest or best to use. Where should I start? What sort of control system should I use? Thanks Ole
Advise to a shaker/accelerometer control system
Started by ●September 18, 2006
Reply by ●September 18, 20062006-09-18
Ole wrote:> Hi, > > I'm absolutely new to control systems and would highly appreciate some good > advises from some veterans or anyone who has something that could help me > get going. > > Well, the task for me is this: > Having a shaker (works exactly like a loadspeaker) and a DC accelerometer > that must run in a digital control loop. > Sample freq is 4000 Hz. > Shaker is attached to the DAC through an amplifier (no filter). > Accelerometer is attached to the ADC (no filter). > the 3 dB frequency range for the system must be 0 - 100 Hz (e.g. the shaker > must be able to follow a 30 Hz sinus). > The control signal is digital and can be a deflection, velocity or > acceleration signal depending on what is easiest or best to use. > Where should I start? > What sort of control system should I use? > > Thanks > Ole > >You already have some of the steps done (out of sequence, unfortunately, but it appears that you'll be able to get by). 1 Collect all of the requirements -- you have a few here, go collect the rest. 2 Determine a plant design -- this is done for you, as is so often the case. So hope that it's an adequate plant design. 3 Model the plant and sensor response. Pay attention to: - the circuit between DAC and coil -- is the DAC output a voltage command or a current command? - the mechanical coupling between the coil and the frame - how much friction? - how much stiction? - how much viscous damping? 4 Determine the sampling rate -- this is already done, cross your fingers! 5 Make a straw-man control system, and use it to measure the plant/ drive/sensor response. 6 Use your theoretical plant model, refined by your measured plant response, along with your requirements to find an adequate control system. 7 Test your control system to make sure it meets requirements. All through this process, document everything that you do. Chances are high that a simple PID control system will be adequate for your needs, possibly even a PD or PI. Chances are also high, however, that you will be challenged by either friction or integrator windup that will require some clever nonlinearities in your controller to compensate. Depending on your level of experience you may want to check out my book (see link below). I have a chapter on nonlinear control in there where I tried to treat the most common nonlinear problems (like stiction and windup) in a down-to-earth way that reflects commonly successful solutions. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●September 18, 20062006-09-18
Thanks, Is it possible to purchase an electronic copy of your book? (I live in Denmark and it takes a while to purchase books from "overthere" and I would like to get running immediately). Best regards Ole "Tim Wescott" <tim@seemywebsite.com> wrote in message news:TYOdnW2nbcKtJpPYnZ2dnUVZ_vydnZ2d@web-ster.com...> Ole wrote: >> Hi, >> >> I'm absolutely new to control systems and would highly appreciate some >> good advises from some veterans or anyone who has something that could >> help me get going. >> >> Well, the task for me is this: >> Having a shaker (works exactly like a loadspeaker) and a DC accelerometer >> that must run in a digital control loop. >> Sample freq is 4000 Hz. >> Shaker is attached to the DAC through an amplifier (no filter). >> Accelerometer is attached to the ADC (no filter). >> the 3 dB frequency range for the system must be 0 - 100 Hz (e.g. the >> shaker must be able to follow a 30 Hz sinus). >> The control signal is digital and can be a deflection, velocity or >> acceleration signal depending on what is easiest or best to use. >> Where should I start? >> What sort of control system should I use? >> >> Thanks >> Ole > You already have some of the steps done (out of sequence, unfortunately, > but it appears that you'll be able to get by). > > 1 Collect all of the requirements -- you have a few here, go collect > the rest. > 2 Determine a plant design -- this is done for you, as is so often the > case. So hope that it's an adequate plant design. > 3 Model the plant and sensor response. Pay attention to: > - the circuit between DAC and coil -- is the DAC output a voltage > command or a current command? > - the mechanical coupling between the coil and the frame > - how much friction? > - how much stiction? > - how much viscous damping? > 4 Determine the sampling rate -- this is already done, cross your > fingers! > 5 Make a straw-man control system, and use it to measure the plant/ > drive/sensor response. > 6 Use your theoretical plant model, refined by your measured plant > response, along with your requirements to find an adequate control > system. > 7 Test your control system to make sure it meets requirements. > > All through this process, document everything that you do. > > Chances are high that a simple PID control system will be adequate for > your needs, possibly even a PD or PI. Chances are also high, however, > that you will be challenged by either friction or integrator windup that > will require some clever nonlinearities in your controller to compensate. > > Depending on your level of experience you may want to check out my book > (see link below). I have a chapter on nonlinear control in there where I > tried to treat the most common nonlinear problems (like stiction and > windup) in a down-to-earth way that reflects commonly successful > solutions. > > -- > > Tim Wescott > Wescott Design Services > http://www.wescottdesign.com > > Posting from Google? See http://cfaj.freeshell.org/google/ > > "Applied Control Theory for Embedded Systems" came out in April. > See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●September 18, 20062006-09-18
Ole wrote: (top posting fixed)> "Tim Wescott" <tim@seemywebsite.com> wrote in message-- snip -->> >>Depending on your level of experience you may want to check out my book >>(see link below). I have a chapter on nonlinear control in there where I >>tried to treat the most common nonlinear problems (like stiction and >>windup) in a down-to-earth way that reflects commonly successful >>solutions. >>> Thanks, > > Is it possible to purchase an electronic copy of your book? (I live in > Denmark and it takes a while to purchase books from "overthere" and I > would like to get running immediately). > I don't think there are any electronic copies available. I do know that there is at least one UK book seller who has taken this up -- but I don't recall the name. If you can get it significantly faster from the UK it may be worth some digging to find out who has it. I do know that you can go to the Amazon site and get some previews -- at one point you could read the whole book if you were patient. It appears that they've put this detailed preview behind a customer log-in now; I haven't verified that you can still preview it once you log in. Also, you can go to my web site and look in the 'Articles' section -- some of those articles have been significantly expanded and incorporated into the book, and would at least have the kernel of the knowledge that you'd need (in particular if you're concerned with friction and backlash there's a paper that addresses that subject as thoroughly as I know how while remaining practical). -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●September 18, 20062006-09-18
Ole wrote:> Hi, > > I'm absolutely new to control systems and would highly appreciate some good > advises from some veterans or anyone who has something that could help me > get going. > > Well, the task for me is this: > Having a shaker (works exactly like a loadspeaker) and a DC accelerometer > that must run in a digital control loop. > Sample freq is 4000 Hz. > Shaker is attached to the DAC through an amplifier (no filter). > Accelerometer is attached to the ADC (no filter). > the 3 dB frequency range for the system must be 0 - 100 Hz (e.g. the shaker > must be able to follow a 30 Hz sinus). > The control signal is digital and can be a deflection, velocity or > acceleration signal depending on what is easiest or best to use. > Where should I start? > What sort of control system should I use?Ole, A voice-coil shaker is no more likely to have backlash than a woofer. The voice-coil signal probably needs little or no filtering, since the table's inertial itself provides substantial filtering. It should not be difficult to precede the amplifier with a simple filter if image frequencies are a problem. I presume that the control system's is there to maintain appropriate frequency, acceleration, or some profile with time. How does that information get to the ADC? Is there an LVDT or accelerometer? Both? To measure amplitude, you will want to find the peak of the position signal. If the input is quite sinusoidal, quadrature sampling can make that easy, but not as easy as a very high sample rate. Sampling at 20 times the table frequency would match the peak reading to the true peak within 1.52%, worst case. (Ditto for acceleration read from an accelerometer.) No anti-alias would be needed, since the system itself would then reduce the image frequencies to an acceptable level In any event, follow Tim's advice (and read the papers on his website). Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●September 18, 20062006-09-18
Ole wrote:> Thanks, > > Is it possible to purchase an electronic copy of your book? (I live in > Denmark and it takes a while to purchase books from "overthere" and I would > like to get running immediately). > > Best regards > Ole >I checked with Elsevier, who replied: "He can order directly from our web site and the order will be fulfilled from our warehouse in the UK. www.books.elsevier.com/computereng" I hope this helps. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●September 18, 20062006-09-18
Jerry Avins wrote: P.S. If the control system's purpose is removing inherent non-linearities from the voice coil and suspension, the task is more difficult than what I described. That's because the controller's bandwidth must be much higher. I still advise a sample rate of 10 to 20 times the highest shaker frequency (5 to 10 times Nyquist's theoretical minimum) for that case. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●September 18, 20062006-09-18
"Ole" <ole@blabla.com> wrote in message news:450ea262$0$13981$edfadb0f@dread15.news.tele.dk...> Hi, > > I'm absolutely new to control systems and would highly appreciate some > good advises from some veterans or anyone who has something that could > help me get going. > > Well, the task for me is this: > Having a shaker (works exactly like a loadspeaker) and a DC accelerometer > that must run in a digital control loop. > Sample freq is 4000 Hz.That is easy enough> Shaker is attached to the DAC through an amplifier (no filter). > Accelerometer is attached to the ADC (no filter).Good. Filters would just add phase delay.> the 3 dB frequency range for the system must be 0 - 100 Hz (e.g. the > shaker must be able to follow a 30 Hz sinus).This will be the hard part. Designing 100 Hz systems is not easy if there is much mass. If the amplitude doesn't have to change then consider a mechanical cam.> The control signal is digital and can be a deflection, velocity or > acceleration signal depending on what is easiest or best to use.I would use position feedback and the accelerormeter. A submicron glass scale would be good.> Where should I start?Start with the requirements and then the mechanical design. The mechanical design must be right or the motion controller doesn't have a chance. What is the amplitude of the stroke and moving mass? Be aware that the acceleration increases with the square of the frequency.> What sort of control system should I use?I would buy a motion controller that has both a position and accelerometer interface. The motion controller should also have gains for higher order derivatives and for feed forwards. There is no way you will achieve your goals with just a PID, PI or PD. A PI has no chance at all because there there will be no stability without the derivatives. You must have feed forwards if you want to track accurately and you should have higher order derivatives that use the acceleration feedback so there are gains for errors in acceleration and jerk. If you can't find a local motion controller that can do the job then consider this: http://www.deltamotion.com/products/motion/rmc70/index.php The pictured control has a Profibus DP interface but for research and testing I recommend Ethernet because one can get real time graphs of the data like this ftp://ftp.deltacompsys.com/public/jpg/Cosine%20Wave%208Hz%200.1rev.PNG Peter Nachtwey Delta Computer Systems, Inc.
Reply by ●September 18, 20062006-09-18
Peter Nachtwey wrote:> "Ole" <ole@blabla.com> wrote in message > news:450ea262$0$13981$edfadb0f@dread15.news.tele.dk... > >>Hi, >> >>I'm absolutely new to control systems and would highly appreciate some >>good advises from some veterans or anyone who has something that could >>help me get going. >> >>Well, the task for me is this: >>Having a shaker (works exactly like a loadspeaker) and a DC accelerometer >>that must run in a digital control loop. >>Sample freq is 4000 Hz. > > > That is easy enough > > >>Shaker is attached to the DAC through an amplifier (no filter). >>Accelerometer is attached to the ADC (no filter). > > > Good. Filters would just add phase delay. > > >>the 3 dB frequency range for the system must be 0 - 100 Hz (e.g. the >>shaker must be able to follow a 30 Hz sinus). > > > This will be the hard part. Designing 100 Hz systems is not easy if there > is much mass. > If the amplitude doesn't have to change then consider a mechanical cam. > > >>The control signal is digital and can be a deflection, velocity or >>acceleration signal depending on what is easiest or best to use. > > > I would use position feedback and the accelerormeter. A submicron glass > scale would be good. > > >>Where should I start? > > > Start with the requirements and then the mechanical design. The mechanical > design must be right or the motion controller doesn't have a chance. > What is the amplitude of the stroke and moving mass? > Be aware that the acceleration increases with the square of the frequency. > > >>What sort of control system should I use? > > I would buy a motion controller that has both a position and accelerometer > interface. The motion controller should also have gains for higher order > derivatives and for feed forwards. > > There is no way you will achieve your goals with just a PID, PI or PD. A PI > has no chance at all because there there will be no stability without the > derivatives. You must have feed forwards if you want to track accurately > and you should have higher order derivatives that use the acceleration > feedback so there are gains for errors in acceleration and jerk. > > If you can't find a local motion controller that can do the job then > consider this: > http://www.deltamotion.com/products/motion/rmc70/index.php > The pictured control has a Profibus DP interface but for research and > testing I recommend Ethernet because one can get real time graphs of the > data like this > ftp://ftp.deltacompsys.com/public/jpg/Cosine%20Wave%208Hz%200.1rev.PNG >How versatile are those things, and what sort of sampling rate can they sustain? -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●September 19, 20062006-09-19
Peter Nachtwey wrote:> ... The mechanical > design must be right or the motion controller doesn't have a chance.Amen! ...> Be aware that the acceleration increases with the square of the frequency.Of course, you imply constant displacement here. ... Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
