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Embedded Systems October 2000 Vol13_11

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Unfortunately the resulting system is ve ry "slipper'y," and can be a chal- lenge to control. In addition , th e electrical requireme nts to build a good curre nt-output amplifier and non-contact transducer interface can be challenging. You can expect th at if you are doing a proj ect like thi s YOLl are a membe r of a fairly talented team (or you' re working on a short- lived project). The equations of motion for u1is system are fairly simple. The force on u1e stage is proportional to the drive command alone, so the acceleration of t.h e system is exactly proportional to the drive. The step response of this system by itself is a parabola, as shown in Figure 5. As we will see later this makes the control problem more challenging because of the sluggishness with which the stage starts moving, and its entl1u- siasm to keep moving once it gets going. Stage Position transducer Temperature control The third example plant I'll use is a heater. Figure 6 shows a diagram of an example system. The ves el is heated by an electric heater, and the temper- ature of its contents is sensed by a tem- perature-sensing device. Thermal systems tend to have very complex responses. I'm going to ignore quite a bit of detail and give a very approximate model. Unless your performance requirements are severe, an accurate model isn't necessary. Figure 7 shows the step response of What do the leading silicon vendors know about BIOS? hey know that the right BIOS is key to the success of em- bedded designs-and that con fi g- urability is key to the right BIOS. T hat's why AMD, Intel, and STMicro ship General Software's Embedded BIOS pre- installed on their embedded platform evaluation boards. With over 400 configuration options, Embedded BIOS offe rs the advanced configurability YOLl need to run your custom target environ- ment without edi ting the core BIOS source code. Contact us today for detailed information and a free sample BIOS binary for your standard reference design. Embedded BIOS™ ADAPTATION KIT Full source code automatica lly configured with over 400 p.mlln crcrs using BIOSrarr'" expert sYMem CO E BIOS FEATURES: ROM/RAM/Flash disks. Setup sysrcm, console re-d i rc~crion , manufactu ring mode, WinCE loader. conflgur:tble PC!. integrared debugger, modular G1. lIolits to chipset, board. and CPU-lcvelll1odli les CHIP ETS: ALI- Abddin V. Finali AMD- 186. 5000. SC400. SC520 INTE 386EX. 430HX/TX. 44 0BX. 810.840 NSC-Geode eXm. eXlv STMicro-STPC fami ly IDEAL FOR: Windows 95/98/CE/NT Embedded. DOS. Li nux , and all x86-based operat ing SYS(CIllS SOFTWARE GENERA[ www.gensw.com.sales@gensw.com • 800·850·5755 • 425·454·5755 90 OCTOBER 2000 Embedded Systems Programming Controllers The eleme nts of a PID controller pre- sented here eiLher take their input from the measured plant output or from tl1e error signal, which is the dif- ference between the plant output and th e system command. I'm going L wl-ite th e control code using floating point to keep implementation details out of the discllssion. It's lip to you to adapt this if YOll are going to imple- ment your controller witl1 in teger or othe r tixed-point arithmetic. I'm going to assume a function call as shown below. As the di scuss ion evolves, you' ll see how the data struc- ture and the internals of the f'unction shapes up. doubLe UpdatePIO(SPid * pid, doubLe error, doubLe position) the system to a change in v,(. I've used time constants ofr] = O.ls and r2 = 0. 3s. The response tends to settle out to a constant temperature for a given drive, but it can take a great deal of time doing it. Also, witl10ut lots of insula- tion, tl1ermal systems tend to be very sensitive to outside effects. This effect is not shown in the figure, but we'll be investigating it later in the article. O

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