4th RTL Workshop: A Digital PID Controller Using RTAI

Click here to learn
about this Sponsor:

Home | News | Articles | Polls | Forum

Keywords: Match:

by Rodrigo da Silva Guerra, Hermes Jose Goncalves, and Walter Fetter Lages

Abstract

This work presents an implementation of a digital PID (Proportional+ Integral+Differential) controller in a PC machine running RTAI. PID controllers are well known and have a wide range of applications in the control of analog systems such as servomotors. In order to achieve the desired performance the system output (axis position) is feed-back to the PID controller which generates the system input (motor voltage), in a closed loop. The controller computes the error between the desired system output (reference) and the actual system output. Then, the system input, which is the PID controller output, is generated as weighted sum of the error, its integral and its differential. The weighting coefficients are known as controller gains and referred as Kp, Ki, and Kd. The values of the controller gains determines the performance of the closed loop system and even its stability. The design of PID controllers is outside the scope of this paper, however it should be said that its values are very dependent on the sampling rate at which system output is read and the new value for system input generated. Therefore it is highly desirable to have an accurate sampling rate. For typical mechanical systems, the adequate sampling rate is in the range of milliseconds. Thus, well implemented PID controllers are usually hard real time systems. The PID controller described in this work uses the PC parallel port as an analog I/O interface. Just two bits are used. One to read the system output and the other to generate the system input. As the system input and output are analog variables, a technique called Pulse Width Modulation (PWM) is used to generate (and read) analog values using digital signals. This technique is based on varying the duty cycle of a rectangular wave accordingly to the analog value to be represented. By using PWM, an analog interface can be built without the use of conventional A/D and D/A converters. However, the switching rate of the PWM signal should be fast enough to be filtered by the system under control. For typical mechanical systems that means a few hundreds of microseconds. The implemented system can be characterized by four main tasks: Two PWM tasks which generate the input and output signals, a PID task responsible for the controller in a proper sense and a user interface task. Except for the user interface task, each one of the main tasks has its own time requirements which must be accurately met. The user interface was implemented as a plain Linux process using the NCurses library. Real time tasks were provided by threads under LXRT hard real time mode. The implementation of PWM under RTAI using the timing functions provided by RTAI are presented and the convenience of using these functions for actual real time control is discussed. In Particular, the semantics of rt_sleep() is analyzed in details and it is shown that the current semantics can lead to an confusing interpretation of what a periodic task is and what a task period is. Furthermore the current semantics of rt_sleep() is incompatible with the the semantics of rt_busy_sleep(), which can lead to even more confusion. A new semantics for rt_sleep() is suggested and the corresponding implementation presented.

Read full paper (PDF download)

(Click here for further information)

7 Advantages of D2D Backup
For decades, tape has been the backup medium of choice. But, now, disk-to-disk (D2D) backup is gaining in favor. Learn why you should make the move in this whitepaper.

4 Legal Reasons to Control Internet Access
The Internet is obviously a valuable resource for many organizations. However, many are exposed to legal liability concerns because they fail to control Internet access. Learn if you're safe in this white paper.

Rapidly Resolve J2EE Application Problems
Whether you are in the process of building J2EE applications or have J2EE applications already running in production, you must ensure that they deliver the expected ROI. Learn how in this white paper.

Load Testing 2.0 for Web 2.0
There are many unknowns in stress testing Web 2.0 applications. Find out how to test the performance of Web 2.0 in this white paper.

Build Better Games Online
For the game infrastructure providers, life is complex. Making money from games has become more complicated. Why? Find out in this white paper.

Building a Virtual Infrastructure from Servers to Storage
This white paper discusses the virtual storage solutions that reduce cost, increase storage utilization, and address the challenges of backing up and restoring Server environments.

Gaining Faster Wireless Connections with WiMAX
Welcome to what is quickly becoming the hyperconnected world where anything that would benefit from being connected to the network will be connected. Learn more in this white paper.

Is Your Desktop a Security Threat?
The new wave of sophisticated crimeware not only targets specific companies, but also targets desktops and laptops as backdoor entryways into those business’ operations and resources. Learn how to stay safe in this white paper.

Increasing SAN Reliability by 100 Percent
Storage area networks (SAN) are a strong part of storage plans. Learn how to increase your reliability and uptime by 100 percent in this case study.

Got a HOT tip? please tell us!

Click here for a profile of each sponsor:

PLATINUM SPONSORS
<a href="http://ad.doubleclick.net/jump/entzd.linuxdevices/desktop;kw=timesys;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?"" style="border-width:0px;height:150px;width:30px;"><img src="http://ad.doubleclick.net/ad/entzd.linuxdevices/desktop;kw=timesys;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?" style="border:0; height: 150; width:30;" /></a>		<a href="http://ad.doubleclick.net/jump/entzd.linuxdevices/desktop;kw=windriver;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?"" style="border-width:0px;height:150px;width:30px;"><img src="http://ad.doubleclick.net/ad/entzd.linuxdevices/desktop;kw=windriver;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?" style="border:0; height: 150; width:30;" /></a>

<a href="http://ad.doubleclick.net/jump/entzd.linuxdevices/desktop;kw=versalogic;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?"" style="border-width:0px;height:150px;width:30px;"><img src="http://ad.doubleclick.net/ad/entzd.linuxdevices/desktop;kw=versalogic;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?" style="border:0; height: 150; width:30;" /></a>		<a href="http://ad.doubleclick.net/jump/entzd.linuxdevices/desktop;kw=winsystems;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?"" style="border-width:0px;height:150px;width:30px;"><img src="http://ad.doubleclick.net/ad/entzd.linuxdevices/desktop;kw=winsystems;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?" style="border:0; height: 150; width:30;" /></a>

<a href="http://ad.doubleclick.net/jump/entzd.linuxdevices/desktop;kw=virtuallogix;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?"" style="border-width:0px;height:150px;width:30px;"><img src="http://ad.doubleclick.net/ad/entzd.linuxdevices/desktop;kw=virtuallogix;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?" style="border:0; height: 150; width:30;" /></a>		<a href="http://ad.doubleclick.net/jump/entzd.linuxdevices/desktop;kw=trolltech;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?"" style="border-width:0px;height:150px;width:30px;"><img src="http://ad.doubleclick.net/ad/entzd.linuxdevices/desktop;kw=trolltech;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?" style="border:0; height: 150; width:30;" /></a>

<a href="http://ad.doubleclick.net/jump/entzd.linuxdevices/desktop;kw=montavista;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?"" style="border-width:0px;height:150px;width:30px;"><img src="http://ad.doubleclick.net/ad/entzd.linuxdevices/desktop;kw=montavista;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?" style="border:0; height: 150; width:30;" /></a>		<a href="http://ad.doubleclick.net/jump/entzd.linuxdevices/desktop;kw=technicalsolutions;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?"" style="border-width:0px;height:150px;width:30px;"><img src="http://ad.doubleclick.net/ad/entzd.linuxdevices/desktop;kw=technicalsolutions;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?" style="border:0; height: 150; width:30;" /></a>

GOLD SPONSORS
<a href="http://ad.doubleclick.net/jump/entzd.linuxdevices/desktop;kw=ultimate;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?"" style="border-width:0px;height:150px;width:30px;"><img src="http://ad.doubleclick.net/ad/entzd.linuxdevices/desktop;kw=ultimate;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?" style="border:0; height: 150; width:30;" /></a>		(Become a sponsor)

ADVERTISEMENT
(Advertise here)

Check out the latest Linux powered...

mobile phones!

other cool
gadgets

BREAKING NEWS

• Linux video camera geo-tags, writes to SATA drives
• Garmin Nav devices run Gnome Linux
• Ten LiMo phones this month?
• It's a Yankee Doodle Linux phone
• Wind River to host "Developer Day"
• Dev boards gain Linux support
• 802.11n zooms ahead
• Low-power mini-ITX board runs Linux
• Pico-ITX board bears twins
• Mass-market WiFi router invites Linux hackers
• LiMo phone specialist buys app stack
• "PDA phone" runs Linux
• ST, NXP spin phone chip JV
• Military-grade USB key supports Linux
• USB Linux systems expand

Linux-Watch headlines:
• Microsoft tactics push India toward Linux
• Bell, SuperMicro sued over GPL
• "Business intelligence" software goes GPL
• Will Atom bomb?
• LF Summit videos posted
• Linux gains "embedded" maintainers
• Virtualization on tap in SLES and RHEL upgrades
• Linux gets security black eye
• Verizon chooses Linux "platform of choice"
• Hats off to Fedora 9

Also visit our sister site:

Sign up for LinuxDevices.com's...

news feed

Home \| News \| Articles \| Polls \| Forum \| About \| Contact

	Ziff Davis Enterprise Home \| Contact Us \| Advertise \| Link to Us \| Reprints \| Magazine Subscriptions \| Newsletters Tech RSS Feeds \| White Papers \| ROI Calculators \| Tech Podcasts \| Tech Video \| VARs \| Channel News
	Baseline \| Careers \| Channel Insider \| CIO Insight \| DesktopLinux \| DeviceForge \| DevSource \| eSeminars \| eWEEK \| Enterprise Network Security \| LinuxDevices \| Linux Watch \| Microsoft Watch \| Mid-market \| Networking \| PDF Zone \| Publish \| Security IT Hub \| Strategic Partner \| Web Buyer's Guide \| Windows for Devices
	Developer Shed \| Dev Shed \| ASP Free \| Dev Articles \| Dev Hardware \| SEO Chat \| Tutorialized \| Scripts \| Code Walkers \| Web Hosters \| Dev Mechanic \| Dev Archives \| igrep
Use of this site is governed by our Terms of Service and Privacy Policy. Except where otherwise specified, the contents of this site are copyright © 1999-2008 Ziff Davis Enterprise Holdings Inc. All Rights Reserved. Reproduction in whole or in part in any form or medium without express written permission of Ziff Davis Enterprise is prohibited. Linux is a registered trademark of Linus Torvalds. All other marks are the property of their respective owners.

Home \| News \| Articles \| Polls \| Forum \| About \| Contact

	Ziff Davis Enterprise Home \| Contact Us \| Advertise \| Link to Us \| Reprints \| Magazine Subscriptions \| Newsletters Tech RSS Feeds \| White Papers \| ROI Calculators \| Tech Podcasts \| Tech Video \| VARs \| Channel News
	Baseline \| Careers \| Channel Insider \| CIO Insight \| DesktopLinux \| DeviceForge \| DevSource \| eSeminars \| eWEEK \| Enterprise Network Security \| LinuxDevices \| Linux Watch \| Microsoft Watch \| Mid-market \| Networking \| PDF Zone \| Publish \| Security IT Hub \| Strategic Partner \| Web Buyer's Guide \| Windows for Devices
	Developer Shed \| Dev Shed \| ASP Free \| Dev Articles \| Dev Hardware \| SEO Chat \| Tutorialized \| Scripts \| Code Walkers \| Web Hosters \| Dev Mechanic \| Dev Archives \| igrep
Use of this site is governed by our Terms of Service and Privacy Policy. Except where otherwise specified, the contents of this site are copyright © 1999-2008 Ziff Davis Enterprise Holdings Inc. All Rights Reserved. Reproduction in whole or in part in any form or medium without express written permission of Ziff Davis Enterprise is prohibited. Linux is a registered trademark of Linus Torvalds. All other marks are the property of their respective owners.