simulating a digital control loop
in [Design]
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From: Jim Thompson on 30 Mar 2010 12:01 On Tue, 30 Mar 2010 08:29:36 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Tue, 30 Mar 2010 15:20:12 GMT, Jan Panteltje ><pNaonStpealmtje(a)yahoo.com> wrote: > >>On a sunny day (Tue, 30 Mar 2010 07:02:15 -0700) it happened John Larkin >><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in >><pd04r5dj17cfb89gf4m25png9elos4vf12(a)4ax.com>: >> >>>I woke up in the middle of the night and decided to count parts. >> >> >>I once dream t I was an electron, moving through the ciruits. >>Poor electrons, the things we do to them. >> >>I woke up at 3:30 last night because computers started beeping... alarms... >>power glitch, rare here, but the server stuff did not re-start. >>Have to add a big relay and a watchdog some day. >>Finised the millivolt measurement part of my scope-PIC. >>You can move a cursor over the waveform and it displays the voltage at that point. >> >> >>> I've >>>been working on this circuit so much in the last three days that I can >>>literally see it in my head and stroll around and count the parts. I >>>got 68 parts in each channel. Times 12 channels, plus VME interface, >>>13th control processor, power supplies, LEDs, and BIST, that will be >>>about 1000 parts. That's a lot; I should pare it down some if I can. >> >>>I need a current-dumper for when I'm operating in voltage-regulator >>>mode, to give some minimum load to the source follower. I can use an >>>LND150 depletion fet: ground the gate, connect the source to a uP port >>>pin, then the drain becomes the dump. Saves me two parts. >> >>BJTs are cool. >> >> > >I could also do it with a BCX70J NPN transistor, with a 1M resistor >from a port pin to the base, ground the emitter, and beta limit. That >would be a more predictable sink than the dmos fet. Certain geezers >wouldn't approve. > >John You just love tweaking "Certain geezers" don't you. But you're an old geezer yourself. AND you're as dumb as a stump. Any 2nd year college student can tell you a more reliable, and cheaper, way to accomplish your minimum load using a port pin. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | The only thing bipartisan in this country is hypocrisy
From: Jim Thompson on 30 Mar 2010 13:12 On Tue, 30 Mar 2010 08:15:29 -0700, Tim Wescott <tim(a)seemywebsite.now> wrote: >pnachtwey wrote: >> On Mar 29, 9:11 am, John Larkin >> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >>> I'm designing a gadget that uses an ARM uP, with 12-bit mux'd ADC and >>> a 10-bit DAC, to essentially make a current and voltage regulated >>> power supply. The ADC will measure voltage and current and the DAC >>> will control a fairly soft source-follower series-pass mosfet. The >>> customer load could be most anything. >>> >>> I think it would be time-effective to simulate the control loop while >>> the PC board is being fabbed, so we don't have to play with dynamics >>> as much in the critical delivery path. >>> >>> I can simulate it as an analog loop using LT Spice, as I'm familiar >>> with that and could get it done quickly. It would be handy if I could >>> also use the same model in digital mode, which would add sampling >>> delays and maybe even quantization. >>> >>> Any thoughts on how to do this? >> Why not use Matlab, Scilab, Mathcad etc? >> I do simulations for motion control all the time. I use a Trunc() >> function in Mathcad to simulate quantizing. I also have functions for >> generating noise. > >In my experience the general-purpose math package route works very well >with motion control -- things are generally far more linear and (with >the exception of friction) if they're nonlinear they generally have just >a few salient nonlinearities so you know what you have to model. > >Circuits -- and switching amplifiers specifically -- tend to have lots >of nonlinearities that may or may not matter, many more parts whose >dynamics may or may not matter (or matter under different operating >conditions, because of those nonlinearities), and they're just plain >more opaque in operation. Add to that the fact that the world is >flooded with excellent circuit simulation packages, and the attraction >of simulating one's digital controller inside the circuit simulator >becomes very strong. PSpice has CEIL, FLOOR and INTQ (as well as many others), so "plants" can be behaviorally modeled within an electronic controller. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | The only thing bipartisan in this country is hypocrisy
From: Tim Wescott on 30 Mar 2010 13:36 Jim Thompson wrote: > On Tue, 30 Mar 2010 08:15:29 -0700, Tim Wescott <tim(a)seemywebsite.now> > wrote: > >> pnachtwey wrote: >>> On Mar 29, 9:11 am, John Larkin >>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >>>> I'm designing a gadget that uses an ARM uP, with 12-bit mux'd ADC and >>>> a 10-bit DAC, to essentially make a current and voltage regulated >>>> power supply. The ADC will measure voltage and current and the DAC >>>> will control a fairly soft source-follower series-pass mosfet. The >>>> customer load could be most anything. >>>> >>>> I think it would be time-effective to simulate the control loop while >>>> the PC board is being fabbed, so we don't have to play with dynamics >>>> as much in the critical delivery path. >>>> >>>> I can simulate it as an analog loop using LT Spice, as I'm familiar >>>> with that and could get it done quickly. It would be handy if I could >>>> also use the same model in digital mode, which would add sampling >>>> delays and maybe even quantization. >>>> >>>> Any thoughts on how to do this? >>> Why not use Matlab, Scilab, Mathcad etc? >>> I do simulations for motion control all the time. I use a Trunc() >>> function in Mathcad to simulate quantizing. I also have functions for >>> generating noise. >> In my experience the general-purpose math package route works very well >> with motion control -- things are generally far more linear and (with >> the exception of friction) if they're nonlinear they generally have just >> a few salient nonlinearities so you know what you have to model. >> >> Circuits -- and switching amplifiers specifically -- tend to have lots >> of nonlinearities that may or may not matter, many more parts whose >> dynamics may or may not matter (or matter under different operating >> conditions, because of those nonlinearities), and they're just plain >> more opaque in operation. Add to that the fact that the world is >> flooded with excellent circuit simulation packages, and the attraction >> of simulating one's digital controller inside the circuit simulator >> becomes very strong. > > PSpice has CEIL, FLOOR and INTQ (as well as many others), so "plants" > can be behaviorally modeled within an electronic controller. In this case John is treating the circuit as a plant and simulate the (digital) controller -- but it's the same difference. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
From: Jan Panteltje on 30 Mar 2010 15:48 On a sunny day (Tue, 30 Mar 2010 08:29:36 -0700) it happened John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in <ev54r59vseoa0rmogu5edugn57meuhaopt(a)4ax.com>: >On Tue, 30 Mar 2010 15:20:12 GMT, Jan Panteltje ><pNaonStpealmtje(a)yahoo.com> wrote: > >>On a sunny day (Tue, 30 Mar 2010 07:02:15 -0700) it happened John Larkin >><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in >><pd04r5dj17cfb89gf4m25png9elos4vf12(a)4ax.com>: >> >>>I woke up in the middle of the night and decided to count parts. >> >> >>I once dream t I was an electron, moving through the ciruits. >>Poor electrons, the things we do to them. >> >>I woke up at 3:30 last night because computers started beeping... alarms... >>power glitch, rare here, but the server stuff did not re-start. >>Have to add a big relay and a watchdog some day. >>Finised the millivolt measurement part of my scope-PIC. >>You can move a cursor over the waveform and it displays the voltage at that point. >> >> >>> I've >>>been working on this circuit so much in the last three days that I can >>>literally see it in my head and stroll around and count the parts. I >>>got 68 parts in each channel. Times 12 channels, plus VME interface, >>>13th control processor, power supplies, LEDs, and BIST, that will be >>>about 1000 parts. That's a lot; I should pare it down some if I can. >> >>>I need a current-dumper for when I'm operating in voltage-regulator >>>mode, to give some minimum load to the source follower. I can use an >>>LND150 depletion fet: ground the gate, connect the source to a uP port >>>pin, then the drain becomes the dump. Saves me two parts. >> >>BJTs are cool. >> >> > >I could also do it with a BCX70J NPN transistor, with a 1M resistor >from a port pin to the base, ground the emitter, and beta limit. That >would be a more predictable sink than the dmos fet. Certain geezers >wouldn't approve. LOL But you could ground the base, and drive the emittor negative via a resistor. Very predictable current if you have negative supply available. >John >
From: John Larkin on 30 Mar 2010 18:21
On Tue, 30 Mar 2010 19:48:39 GMT, Jan Panteltje <pNaonStpealmtje(a)yahoo.com> wrote: >On a sunny day (Tue, 30 Mar 2010 08:29:36 -0700) it happened John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in ><ev54r59vseoa0rmogu5edugn57meuhaopt(a)4ax.com>: > >>On Tue, 30 Mar 2010 15:20:12 GMT, Jan Panteltje >><pNaonStpealmtje(a)yahoo.com> wrote: >> >>>On a sunny day (Tue, 30 Mar 2010 07:02:15 -0700) it happened John Larkin >>><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in >>><pd04r5dj17cfb89gf4m25png9elos4vf12(a)4ax.com>: >>> >>>>I woke up in the middle of the night and decided to count parts. >>> >>> >>>I once dream t I was an electron, moving through the ciruits. >>>Poor electrons, the things we do to them. >>> >>>I woke up at 3:30 last night because computers started beeping... alarms... >>>power glitch, rare here, but the server stuff did not re-start. >>>Have to add a big relay and a watchdog some day. >>>Finised the millivolt measurement part of my scope-PIC. >>>You can move a cursor over the waveform and it displays the voltage at that point. >>> >>> >>>> I've >>>>been working on this circuit so much in the last three days that I can >>>>literally see it in my head and stroll around and count the parts. I >>>>got 68 parts in each channel. Times 12 channels, plus VME interface, >>>>13th control processor, power supplies, LEDs, and BIST, that will be >>>>about 1000 parts. That's a lot; I should pare it down some if I can. >>> >>>>I need a current-dumper for when I'm operating in voltage-regulator >>>>mode, to give some minimum load to the source follower. I can use an >>>>LND150 depletion fet: ground the gate, connect the source to a uP port >>>>pin, then the drain becomes the dump. Saves me two parts. >>> >>>BJTs are cool. >>> >>> >> >>I could also do it with a BCX70J NPN transistor, with a 1M resistor >>from a port pin to the base, ground the emitter, and beta limit. That >>would be a more predictable sink than the dmos fet. Certain geezers >>wouldn't approve. >LOL > >But you could ground the base, and drive the emittor negative via a resistor. >Very predictable current if you have negative supply available. > > Sorry, no negative supply. So the depletion mode fet, with port pin driving the source, works with zero additional parts. The BCX70 would actually have a more repeatable sink current (the fet has a 3:1 Idss spec, whereas the BCX70J beta spread is under 2:1) but needs the base resistor. We are very tight for space, so every part matters. John |