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  14  <h1>
  15      <a href="https://people.skolelinux.org/pere/blog/">Petter Reinholdtsen</a>
  16
  17  </h1>
  18
  19 </div>
  20
  21
  22     <h3>Entries from July 2022.</h3>
  23
  24     <div class="entry">
  25       <div class="title">
  26         <a href="https://people.skolelinux.org/pere/blog/Automatic_LinuxCNC_servo_PID_tuning_.html">Automatic LinuxCNC servo PID tuning?</a>
  27       </div>
  28       <div class="date">
  29         16th July 2022
  30       </div>
  31       <div class="body">
  32         <p>While working on a CNC with servo motors controlled by the
  33 <a href="https://en.wikipedia.org/wiki/LinuxCNC">LinuxCNC</a>
  34 <a href="https://en.wikipedia.org/wiki/PID_controller">PID
  35 controller</a>, I recently had to learn how to tune the collection of values
  36 that control such mathematical machinery that a PID controller is.  It
  37 proved to be a lot harder than I hoped, and I still have not succeeded
  38 in getting the Z PID controller to successfully defy gravity, nor X
  39 and Y to move accurately and reliably.  But while climbing up this
  40 rather steep learning curve, I discovered that some motor control
  41 systems are able to tune their PID controllers.  I got the impression
  42 from the documentation that LinuxCNC were not.  This proved to be not
  43 true.</p>
  44
  45 <p>The LinuxCNC
  46 <a href="http://linuxcnc.org/docs/html/man/man9/pid.9.html">pid
  47 component</a> is the recommended PID controller to use.  It uses eight
  48 constants <tt>Pgain</tt>, <tt>Igain</tt>, <tt>Dgain</tt>,
  49 <tt>bias</tt>, <tt>FF0</tt>, <tt>FF1</tt>, <tt>FF2</tt> and
  50 <tt>FF3</tt> to calculate the output value based on current and wanted
  51 state, and all of these need to have a sensible value for the
  52 controller to behave properly.  Note, there are even more values
  53 involved, theser are just the most important ones.  In my case I need
  54 the X, Y and Z axes to follow the requested path with little error.
  55 This has proved quite a challenge for someone who have never tuned a
  56 PID controller before, but there is at least some help to be found.
  57
  58 <p>I discovered that included in LinuxCNC was this old PID component
  59 at_pid claiming to have auto tuning capabilities.  Sadly it had been
  60 neglected since 2011, and could not be used as a plug in replacement
  61 for the default pid component.  One would have to rewriting the
  62 LinuxCNC HAL setup to test at_pid.  This was rather sad, when I wanted
  63 to quickly test auto tuning to see if it did a better job than me at
  64 figuring out good P, I and D values to use.</p>
  65
  66 <p>I decided to have a look if the situation could be improved.  This
  67 involved trying to understand the code and history of the pid and
  68 at_pid components.  Apparently they had a common ancestor, as code
  69 structure, comments and variable names were quite close to each other.
  70 Sadly this was not reflected in the git history, making it hard to
  71 figure out what really happened.  My guess is that the author of
  72 <a href="https://github.com/LinuxCNC/linuxcnc/blob/master/src/hal/components/at_pid.c">at_pid.c</a>
  73 took a version of
  74 <a href="https://github.com/LinuxCNC/linuxcnc/blob/master/src/hal/components/pid.c">pid.c</a>,
  75 rewrote it to follow the structure he wished pid.c to have, then added
  76 support for auto tuning and finally got it included into the LinuxCNC
  77 repository.  The restructuring and lack of early history made it
  78 harder to figure out which part of the code were relevant to the auto
  79 tuning, and which part of the code needed to be updated to work the
  80 same way as the current pid.c implementation.  I started by trying to
  81 isolate relevant changes in pid.c, and applying them to at_pid.c.  My
  82 aim was to make sure the at_pid component could replace the pid
  83 component with a simple change in the HAL setup loadrt line, without
  84 having to "rewire" the rest of the HAL configuration.  After a few
  85 hours following this approach, I had learned quite a lot about the
  86 code structure of both components, while concluding I was heading down
  87 the wrong rabbit hole, and should get back to the surface and find a
  88 different path.</p>
  89
  90 <p>For the second attempt, I decided to throw away all the PID control
  91 related part of the original at_pid.c, and instead isolate and lift
  92 the auto tuning part of the code and inject it into a copy of pid.c.
  93 This ensured compatibility with the current pid component, while
  94 adding auto tuning as a run time option.  To make it easier to identify
  95 the relevant parts in the future, I wrapped all the auto tuning code
  96 with '#ifdef AUTO_TUNER'.  The end result behave just like the current
  97 pid component by default, as that part of the code is identical.  The
  98 <a href="https://github.com/LinuxCNC/linuxcnc/pull/1820">end result
  99 entered the LinuxCNC master branch</a> a few days ago.</p>
 100
 101 <p>To enable auto tuning, one need to set a few HAL pins in the PID
 102 component.  The most important ones are <tt>tune-effort</tt>,
 103 <tt>tune-mode</tt> and <tt>tune-start</tt>.  But lets take a step
 104 back, and see what the auto tuning code will do.  I do not know the
 105 mathematical foundation of the at_pid algorithm, but from observation
 106 I can tell that the algorithm will, when enabled, produce a square
 107 wave pattern centered around the <tt>bias</tt> value on the output pin
 108 of the PID controller.  This can be seen using the HAL Scope provided
 109 by LinuxCNC.  In my case, this is translated into voltage (+-10V) sent
 110 to the motor controller, which in turn is translated into motor speed.
 111 So at_pid will ask the motor to move the axis back and forth.  The
 112 number of cycles in the pattern is controlled by the
 113 <tt>tune-cycles</tt> pin, and the extremes of the wave pattern is
 114 controlled by the <tt>tune-effort</tt> pin.  Of course, trying to
 115 change the direction of a physical object instantly (as in going
 116 directly from a positive voltage to the equivalent negative voltage)
 117 do not change velocity instantly, and it take some time for the object
 118 to slow down and move in the opposite direction.  This result in a
 119 more smooth movement wave form, as the axis in question were vibrating
 120 back and forth.  When the axis reached the target speed in the
 121 opposing direction, the auto tuner change direction again.  After
 122 several of these changes, the average time delay between the 'peaks'
 123 and 'valleys' of this movement graph is then used to calculate
 124 proposed values for Pgain, Igain and Dgain, and insert them into the
 125 HAL model to use by the pid controller.  The auto tuned settings are
 126 not great, but htye work a lot better than the values I had been able
 127 to cook up on my own, at least for the horizontal X and Y axis.  But I
 128 had to use very small <tt>tune-effort<tt> values, as my motor
 129 controllers error out if the voltage change too quickly.  I've been
 130 less lucky with the Z axis, which is moving a heavy object up and
 131 down, and seem to confuse the algorithm.  The Z axis movement became a
 132 lot better when I introduced a <tt>bias</tt> value to counter the
 133 gravitational drag, but I will have to work a lot more on the Z axis
 134 PID values.</p>
 135
 136 <p>Armed with this knowledge, it is time to look at how to do the
 137 tuning.  Lets say the HAL configuration in question load the PID
 138 component for X, Y and Z like this:</p>
 139
 140 <blockquote><pre>
 141 loadrt pid names=pid.x,pid.y,pid.z
 142 </pre></blockquote>
 143
 144 <p>Armed with the new and improved at_pid component, the new line will
 145 look like this:</p>
 146
 147 <blockquote><pre>
 148 loadrt at_pid names=pid.x,pid.y,pid.z
 149 </pre></blockquote>
 150
 151 <p>The rest of the HAL setup can stay the same.  This work because the
 152 components are referenced by name.  If the component had used count=3
 153 instead, all use of pid.# had to be changed to at_pid.#.</p>
 154
 155 <p>To start tuning the X axis, move the axis to the middle of its
 156 range, to make sure it do not hit anything when it start moving back
 157 and forth.  Next, set the <tt>tune-effort</tt> to a low number in the
 158 output range.  I used 0.1 as my initial value.  Next, assign 1 to the
 159 <tt>tune-mode</tt> value. Note, this will disable the pid controlling
 160 part and feed 0 to the output pin, which in my case initially caused a
 161 lot of drift.  In my case it proved to be a good idea with X and Y to
 162 tune the motor driver to make sure 0 voltage stopped the motor
 163 rotation.  On the other hand, for the Z axis this proved to be a bad
 164 idea, so it will depend on your setup.  It might help to set the
 165 <tt>bias</tt> value to a output value that reduce or eliminate the
 166 axis drift.  Finally, after setting <tt>tune-mode</tt>, set
 167 <tt>tune-start</tt> to 1 to activate the auto tuning.  If all go well,
 168 your axis will vibrate for a few seconds and when it is done, new
 169 values for Pgain, Igain and Dgain will be active.  To test them,
 170 change <tt>tune-mode</tt> back to 0.  Note that this might cause the
 171 machine to suddenly jerk as it bring the axis back to its commanded
 172 position, which it might have drifted away from during tuning.  To
 173 summarize with some halcmd lines:</p>
 174
 175 <blockquote><pre>
 176 setp pid.x.tune-effort 0.1
 177 setp pid.x.tune-mode 1
 178 setp pid.x.tune-start 1
 179 # wait for the tuning to complete
 180 setp pid.x.tune-mode 0
 181 </pre></blockquote>
 182
 183 <p>After doing this task quite a few times while trying to figure out
 184 how to properly tune the PID controllers on the machine in, I decided
 185 to figure out if this process could be automated, and wrote a script
 186 to do the entire tuning process from power on.  The end result will
 187 ensure the machine is powered on and ready to run, home all axis if it
 188 is not already done, check that the extra tuning pins are available,
 189 move the axis to its mid point, run the auto tuning and re-enable the
 190 pid controller when it is done.  It can be run several times.  Check
 191 out the
 192 <a href="https://github.com/SebKuzminsky/MazakVQC1540/blob/bon-dev/scripts/run-auto-pid-tuner">run-auto-pid-tuner</a>
 193 script on github if you want to learn how it is done.</p>
 194
 195 <p>My hope is that this little adventure can inspire someone who know
 196 more about motor PID controller tuning can implement even better
 197 algorithms for automatic PID tuning in LinuxCNC, making life easier
 198 for both me and all the others that want to use LinuxCNC but lack the
 199 in depth knowledge needed to tune PID controllers well.</p>
 200
 201 <p>As usual, if you use Bitcoin and want to show your support of my
 202 activities, please send Bitcoin donations to my address
 203 <b><a href="bitcoin:15oWEoG9dUPovwmUL9KWAnYRtNJEkP1u1b">15oWEoG9dUPovwmUL9KWAnYRtNJEkP1u1b</a></b>.</p>
 204
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 206       <div class="tags">
 207
 208
 209         Tags: <a href="https://people.skolelinux.org/pere/blog/tags/3d-printer">3d-printer</a>, <a href="https://people.skolelinux.org/pere/blog/tags/debian">debian</a>, <a href="https://people.skolelinux.org/pere/blog/tags/english">english</a>, <a href="https://people.skolelinux.org/pere/blog/tags/linuxcnc">linuxcnc</a>, <a href="https://people.skolelinux.org/pere/blog/tags/robot">robot</a>.
 210
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 217     <div id="sidebar">
 218
 219
 220
 221 <h2>Archive</h2>
 222 <ul>
 223
 224 <li>2023
 225 <ul>
 226
 227 <li><a href="https://people.skolelinux.org/pere/blog/archive/2023/01/">January (3)</a></li>
 228
 229 <li><a href="https://people.skolelinux.org/pere/blog/archive/2023/02/">February (1)</a></li>
 230
 231 <li><a href="https://people.skolelinux.org/pere/blog/archive/2023/04/">April (2)</a></li>
 232
 233 <li><a href="https://people.skolelinux.org/pere/blog/archive/2023/05/">May (3)</a></li>
 234
 235 <li><a href="https://people.skolelinux.org/pere/blog/archive/2023/06/">June (1)</a></li>
 236
 237 <li><a href="https://people.skolelinux.org/pere/blog/archive/2023/08/">August (1)</a></li>
 238
 239 <li><a href="https://people.skolelinux.org/pere/blog/archive/2023/09/">September (1)</a></li>
 240
 241 <li><a href="https://people.skolelinux.org/pere/blog/archive/2023/10/">October (1)</a></li>
 242
 243 <li><a href="https://people.skolelinux.org/pere/blog/archive/2023/11/">November (4)</a></li>
 244
 245 <li><a href="https://people.skolelinux.org/pere/blog/archive/2023/12/">December (1)</a></li>
 246
 247 </ul></li>
 248
 249 <li>2022
 250 <ul>
 251
 252 <li><a href="https://people.skolelinux.org/pere/blog/archive/2022/02/">February (1)</a></li>
 253
 254 <li><a href="https://people.skolelinux.org/pere/blog/archive/2022/03/">March (3)</a></li>
 255
 256 <li><a href="https://people.skolelinux.org/pere/blog/archive/2022/04/">April (2)</a></li>
 257
 258 <li><a href="https://people.skolelinux.org/pere/blog/archive/2022/06/">June (2)</a></li>
 259
 260 <li><a href="https://people.skolelinux.org/pere/blog/archive/2022/07/">July (1)</a></li>
 261
 262 <li><a href="https://people.skolelinux.org/pere/blog/archive/2022/09/">September (1)</a></li>
 263
 264 <li><a href="https://people.skolelinux.org/pere/blog/archive/2022/10/">October (1)</a></li>
 265
 266 <li><a href="https://people.skolelinux.org/pere/blog/archive/2022/12/">December (1)</a></li>
 267
 268 </ul></li>
 269
 270 <li>2021
 271 <ul>
 272
 273 <li><a href="https://people.skolelinux.org/pere/blog/archive/2021/01/">January (2)</a></li>
 274
 275 <li><a href="https://people.skolelinux.org/pere/blog/archive/2021/02/">February (1)</a></li>
 276
 277 <li><a href="https://people.skolelinux.org/pere/blog/archive/2021/05/">May (1)</a></li>
 278
 279 <li><a href="https://people.skolelinux.org/pere/blog/archive/2021/06/">June (1)</a></li>
 280
 281 <li><a href="https://people.skolelinux.org/pere/blog/archive/2021/07/">July (3)</a></li>
 282
 283 <li><a href="https://people.skolelinux.org/pere/blog/archive/2021/08/">August (1)</a></li>
 284
 285 <li><a href="https://people.skolelinux.org/pere/blog/archive/2021/09/">September (1)</a></li>
 286
 287 <li><a href="https://people.skolelinux.org/pere/blog/archive/2021/10/">October (1)</a></li>
 288
 289 <li><a href="https://people.skolelinux.org/pere/blog/archive/2021/12/">December (1)</a></li>
 290
 291 </ul></li>
 292
 293 <li>2020
 294 <ul>
 295
 296 <li><a href="https://people.skolelinux.org/pere/blog/archive/2020/02/">February (2)</a></li>
 297
 298 <li><a href="https://people.skolelinux.org/pere/blog/archive/2020/03/">March (2)</a></li>
 299
 300 <li><a href="https://people.skolelinux.org/pere/blog/archive/2020/04/">April (2)</a></li>
 301
 302 <li><a href="https://people.skolelinux.org/pere/blog/archive/2020/05/">May (3)</a></li>
 303
 304 <li><a href="https://people.skolelinux.org/pere/blog/archive/2020/06/">June (2)</a></li>
 305
 306 <li><a href="https://people.skolelinux.org/pere/blog/archive/2020/07/">July (1)</a></li>
 307
 308 <li><a href="https://people.skolelinux.org/pere/blog/archive/2020/09/">September (1)</a></li>
 309
 310 <li><a href="https://people.skolelinux.org/pere/blog/archive/2020/10/">October (1)</a></li>
 311
 312 <li><a href="https://people.skolelinux.org/pere/blog/archive/2020/11/">November (1)</a></li>
 313
 314 </ul></li>
 315
 316 <li>2019
 317 <ul>
 318
 319 <li><a href="https://people.skolelinux.org/pere/blog/archive/2019/01/">January (4)</a></li>
 320
 321 <li><a href="https://people.skolelinux.org/pere/blog/archive/2019/02/">February (3)</a></li>
 322
 323 <li><a href="https://people.skolelinux.org/pere/blog/archive/2019/03/">March (3)</a></li>
 324
 325 <li><a href="https://people.skolelinux.org/pere/blog/archive/2019/05/">May (2)</a></li>
 326
 327 <li><a href="https://people.skolelinux.org/pere/blog/archive/2019/06/">June (5)</a></li>
 328
 329 <li><a href="https://people.skolelinux.org/pere/blog/archive/2019/07/">July (2)</a></li>
 330
 331 <li><a href="https://people.skolelinux.org/pere/blog/archive/2019/08/">August (1)</a></li>
 332
 333 <li><a href="https://people.skolelinux.org/pere/blog/archive/2019/09/">September (1)</a></li>
 334
 335 <li><a href="https://people.skolelinux.org/pere/blog/archive/2019/11/">November (1)</a></li>
 336
 337 <li><a href="https://people.skolelinux.org/pere/blog/archive/2019/12/">December (4)</a></li>
 338
 339 </ul></li>
 340
 341 <li>2018
 342 <ul>
 343
 344 <li><a href="https://people.skolelinux.org/pere/blog/archive/2018/01/">January (1)</a></li>
 345
 346 <li><a href="https://people.skolelinux.org/pere/blog/archive/2018/02/">February (5)</a></li>
 347
 348 <li><a href="https://people.skolelinux.org/pere/blog/archive/2018/03/">March (5)</a></li>
 349
 350 <li><a href="https://people.skolelinux.org/pere/blog/archive/2018/04/">April (3)</a></li>
 351
 352 <li><a href="https://people.skolelinux.org/pere/blog/archive/2018/06/">June (2)</a></li>
 353
 354 <li><a href="https://people.skolelinux.org/pere/blog/archive/2018/07/">July (5)</a></li>
 355
 356 <li><a href="https://people.skolelinux.org/pere/blog/archive/2018/08/">August (3)</a></li>
 357
 358 <li><a href="https://people.skolelinux.org/pere/blog/archive/2018/09/">September (3)</a></li>
 359
 360 <li><a href="https://people.skolelinux.org/pere/blog/archive/2018/10/">October (5)</a></li>
 361
 362 <li><a href="https://people.skolelinux.org/pere/blog/archive/2018/11/">November (2)</a></li>
 363
 364 <li><a href="https://people.skolelinux.org/pere/blog/archive/2018/12/">December (4)</a></li>
 365
 366 </ul></li>
 367
 368 <li>2017
 369 <ul>
 370
 371 <li><a href="https://people.skolelinux.org/pere/blog/archive/2017/01/">January (4)</a></li>
 372
 373 <li><a href="https://people.skolelinux.org/pere/blog/archive/2017/02/">February (3)</a></li>
 374
 375 <li><a href="https://people.skolelinux.org/pere/blog/archive/2017/03/">March (5)</a></li>
 376
 377 <li><a href="https://people.skolelinux.org/pere/blog/archive/2017/04/">April (2)</a></li>
 378
 379 <li><a href="https://people.skolelinux.org/pere/blog/archive/2017/06/">June (5)</a></li>
 380
 381 <li><a href="https://people.skolelinux.org/pere/blog/archive/2017/07/">July (1)</a></li>
 382
 383 <li><a href="https://people.skolelinux.org/pere/blog/archive/2017/08/">August (1)</a></li>
 384
 385 <li><a href="https://people.skolelinux.org/pere/blog/archive/2017/09/">September (3)</a></li>
 386
 387 <li><a href="https://people.skolelinux.org/pere/blog/archive/2017/10/">October (5)</a></li>
 388
 389 <li><a href="https://people.skolelinux.org/pere/blog/archive/2017/11/">November (3)</a></li>
 390
 391 <li><a href="https://people.skolelinux.org/pere/blog/archive/2017/12/">December (4)</a></li>
 392
 393 </ul></li>
 394
 395 <li>2016
 396 <ul>
 397
 398 <li><a href="https://people.skolelinux.org/pere/blog/archive/2016/01/">January (3)</a></li>
 399
 400 <li><a href="https://people.skolelinux.org/pere/blog/archive/2016/02/">February (2)</a></li>
 401
 402 <li><a href="https://people.skolelinux.org/pere/blog/archive/2016/03/">March (3)</a></li>
 403
 404 <li><a href="https://people.skolelinux.org/pere/blog/archive/2016/04/">April (8)</a></li>
 405
 406 <li><a href="https://people.skolelinux.org/pere/blog/archive/2016/05/">May (8)</a></li>
 407
 408 <li><a href="https://people.skolelinux.org/pere/blog/archive/2016/06/">June (2)</a></li>
 409
 410 <li><a href="https://people.skolelinux.org/pere/blog/archive/2016/07/">July (2)</a></li>
 411
 412 <li><a href="https://people.skolelinux.org/pere/blog/archive/2016/08/">August (5)</a></li>
 413
 414 <li><a href="https://people.skolelinux.org/pere/blog/archive/2016/09/">September (2)</a></li>
 415
 416 <li><a href="https://people.skolelinux.org/pere/blog/archive/2016/10/">October (3)</a></li>
 417
 418 <li><a href="https://people.skolelinux.org/pere/blog/archive/2016/11/">November (8)</a></li>
 419
 420 <li><a href="https://people.skolelinux.org/pere/blog/archive/2016/12/">December (5)</a></li>
 421
 422 </ul></li>
 423
 424 <li>2015
 425 <ul>
 426
 427 <li><a href="https://people.skolelinux.org/pere/blog/archive/2015/01/">January (7)</a></li>
 428
 429 <li><a href="https://people.skolelinux.org/pere/blog/archive/2015/02/">February (6)</a></li>
 430
 431 <li><a href="https://people.skolelinux.org/pere/blog/archive/2015/03/">March (1)</a></li>
 432
 433 <li><a href="https://people.skolelinux.org/pere/blog/archive/2015/04/">April (4)</a></li>
 434
 435 <li><a href="https://people.skolelinux.org/pere/blog/archive/2015/05/">May (3)</a></li>
 436
 437 <li><a href="https://people.skolelinux.org/pere/blog/archive/2015/06/">June (4)</a></li>
 438
 439 <li><a href="https://people.skolelinux.org/pere/blog/archive/2015/07/">July (6)</a></li>
 440
 441 <li><a href="https://people.skolelinux.org/pere/blog/archive/2015/08/">August (2)</a></li>
 442
 443 <li><a href="https://people.skolelinux.org/pere/blog/archive/2015/09/">September (2)</a></li>
 444
 445 <li><a href="https://people.skolelinux.org/pere/blog/archive/2015/10/">October (9)</a></li>
 446
 447 <li><a href="https://people.skolelinux.org/pere/blog/archive/2015/11/">November (6)</a></li>
 448
 449 <li><a href="https://people.skolelinux.org/pere/blog/archive/2015/12/">December (3)</a></li>
 450
 451 </ul></li>
 452
 453 <li>2014
 454 <ul>
 455
 456 <li><a href="https://people.skolelinux.org/pere/blog/archive/2014/01/">January (2)</a></li>
 457
 458 <li><a href="https://people.skolelinux.org/pere/blog/archive/2014/02/">February (3)</a></li>
 459
 460 <li><a href="https://people.skolelinux.org/pere/blog/archive/2014/03/">March (8)</a></li>
 461
 462 <li><a href="https://people.skolelinux.org/pere/blog/archive/2014/04/">April (7)</a></li>
 463
 464 <li><a href="https://people.skolelinux.org/pere/blog/archive/2014/05/">May (1)</a></li>
 465
 466 <li><a href="https://people.skolelinux.org/pere/blog/archive/2014/06/">June (2)</a></li>
 467
 468 <li><a href="https://people.skolelinux.org/pere/blog/archive/2014/07/">July (2)</a></li>
 469
 470 <li><a href="https://people.skolelinux.org/pere/blog/archive/2014/08/">August (2)</a></li>
 471
 472 <li><a href="https://people.skolelinux.org/pere/blog/archive/2014/09/">September (5)</a></li>
 473
 474 <li><a href="https://people.skolelinux.org/pere/blog/archive/2014/10/">October (6)</a></li>
 475
 476 <li><a href="https://people.skolelinux.org/pere/blog/archive/2014/11/">November (3)</a></li>
 477
 478 <li><a href="https://people.skolelinux.org/pere/blog/archive/2014/12/">December (5)</a></li>
 479
 480 </ul></li>
 481
 482 <li>2013
 483 <ul>
 484
 485 <li><a href="https://people.skolelinux.org/pere/blog/archive/2013/01/">January (11)</a></li>
 486
 487 <li><a href="https://people.skolelinux.org/pere/blog/archive/2013/02/">February (9)</a></li>
 488
 489 <li><a href="https://people.skolelinux.org/pere/blog/archive/2013/03/">March (9)</a></li>
 490
 491 <li><a href="https://people.skolelinux.org/pere/blog/archive/2013/04/">April (6)</a></li>
 492
 493 <li><a href="https://people.skolelinux.org/pere/blog/archive/2013/05/">May (9)</a></li>
 494
 495 <li><a href="https://people.skolelinux.org/pere/blog/archive/2013/06/">June (10)</a></li>
 496
 497 <li><a href="https://people.skolelinux.org/pere/blog/archive/2013/07/">July (7)</a></li>
 498
 499 <li><a href="https://people.skolelinux.org/pere/blog/archive/2013/08/">August (3)</a></li>
 500
 501 <li><a href="https://people.skolelinux.org/pere/blog/archive/2013/09/">September (5)</a></li>
 502
 503 <li><a href="https://people.skolelinux.org/pere/blog/archive/2013/10/">October (7)</a></li>
 504
 505 <li><a href="https://people.skolelinux.org/pere/blog/archive/2013/11/">November (9)</a></li>
 506
 507 <li><a href="https://people.skolelinux.org/pere/blog/archive/2013/12/">December (3)</a></li>
 508
 509 </ul></li>
 510
 511 <li>2012
 512 <ul>
 513
 514 <li><a href="https://people.skolelinux.org/pere/blog/archive/2012/01/">January (7)</a></li>
 515
 516 <li><a href="https://people.skolelinux.org/pere/blog/archive/2012/02/">February (10)</a></li>
 517
 518 <li><a href="https://people.skolelinux.org/pere/blog/archive/2012/03/">March (17)</a></li>
 519
 520 <li><a href="https://people.skolelinux.org/pere/blog/archive/2012/04/">April (12)</a></li>
 521
 522 <li><a href="https://people.skolelinux.org/pere/blog/archive/2012/05/">May (12)</a></li>
 523
 524 <li><a href="https://people.skolelinux.org/pere/blog/archive/2012/06/">June (20)</a></li>
 525
 526 <li><a href="https://people.skolelinux.org/pere/blog/archive/2012/07/">July (17)</a></li>
 527
 528 <li><a href="https://people.skolelinux.org/pere/blog/archive/2012/08/">August (6)</a></li>
 529
 530 <li><a href="https://people.skolelinux.org/pere/blog/archive/2012/09/">September (9)</a></li>
 531
 532 <li><a href="https://people.skolelinux.org/pere/blog/archive/2012/10/">October (17)</a></li>
 533
 534 <li><a href="https://people.skolelinux.org/pere/blog/archive/2012/11/">November (10)</a></li>
 535
 536 <li><a href="https://people.skolelinux.org/pere/blog/archive/2012/12/">December (7)</a></li>
 537
 538 </ul></li>
 539
 540 <li>2011
 541 <ul>
 542
 543 <li><a href="https://people.skolelinux.org/pere/blog/archive/2011/01/">January (16)</a></li>
 544
 545 <li><a href="https://people.skolelinux.org/pere/blog/archive/2011/02/">February (6)</a></li>
 546
 547 <li><a href="https://people.skolelinux.org/pere/blog/archive/2011/03/">March (6)</a></li>
 548
 549 <li><a href="https://people.skolelinux.org/pere/blog/archive/2011/04/">April (7)</a></li>
 550
 551 <li><a href="https://people.skolelinux.org/pere/blog/archive/2011/05/">May (3)</a></li>
 552
 553 <li><a href="https://people.skolelinux.org/pere/blog/archive/2011/06/">June (2)</a></li>
 554
 555 <li><a href="https://people.skolelinux.org/pere/blog/archive/2011/07/">July (7)</a></li>
 556
 557 <li><a href="https://people.skolelinux.org/pere/blog/archive/2011/08/">August (6)</a></li>
 558
 559 <li><a href="https://people.skolelinux.org/pere/blog/archive/2011/09/">September (4)</a></li>
 560
 561 <li><a href="https://people.skolelinux.org/pere/blog/archive/2011/10/">October (2)</a></li>
 562
 563 <li><a href="https://people.skolelinux.org/pere/blog/archive/2011/11/">November (3)</a></li>
 564
 565 <li><a href="https://people.skolelinux.org/pere/blog/archive/2011/12/">December (1)</a></li>
 566
 567 </ul></li>
 568
 569 <li>2010
 570 <ul>
 571
 572 <li><a href="https://people.skolelinux.org/pere/blog/archive/2010/01/">January (2)</a></li>
 573
 574 <li><a href="https://people.skolelinux.org/pere/blog/archive/2010/02/">February (1)</a></li>
 575
 576 <li><a href="https://people.skolelinux.org/pere/blog/archive/2010/03/">March (3)</a></li>
 577
 578 <li><a href="https://people.skolelinux.org/pere/blog/archive/2010/04/">April (3)</a></li>
 579
 580 <li><a href="https://people.skolelinux.org/pere/blog/archive/2010/05/">May (9)</a></li>
 581
 582 <li><a href="https://people.skolelinux.org/pere/blog/archive/2010/06/">June (14)</a></li>
 583
 584 <li><a href="https://people.skolelinux.org/pere/blog/archive/2010/07/">July (12)</a></li>
 585
 586 <li><a href="https://people.skolelinux.org/pere/blog/archive/2010/08/">August (13)</a></li>
 587
 588 <li><a href="https://people.skolelinux.org/pere/blog/archive/2010/09/">September (7)</a></li>
 589
 590 <li><a href="https://people.skolelinux.org/pere/blog/archive/2010/10/">October (9)</a></li>
 591
 592 <li><a href="https://people.skolelinux.org/pere/blog/archive/2010/11/">November (13)</a></li>
 593
 594 <li><a href="https://people.skolelinux.org/pere/blog/archive/2010/12/">December (12)</a></li>
 595
 596 </ul></li>
 597
 598 <li>2009
 599 <ul>
 600
 601 <li><a href="https://people.skolelinux.org/pere/blog/archive/2009/01/">January (8)</a></li>
 602
 603 <li><a href="https://people.skolelinux.org/pere/blog/archive/2009/02/">February (8)</a></li>
 604
 605 <li><a href="https://people.skolelinux.org/pere/blog/archive/2009/03/">March (12)</a></li>
 606
 607 <li><a href="https://people.skolelinux.org/pere/blog/archive/2009/04/">April (10)</a></li>
 608
 609 <li><a href="https://people.skolelinux.org/pere/blog/archive/2009/05/">May (9)</a></li>
 610
 611 <li><a href="https://people.skolelinux.org/pere/blog/archive/2009/06/">June (3)</a></li>
 612
 613 <li><a href="https://people.skolelinux.org/pere/blog/archive/2009/07/">July (4)</a></li>
 614
 615 <li><a href="https://people.skolelinux.org/pere/blog/archive/2009/08/">August (3)</a></li>
 616
 617 <li><a href="https://people.skolelinux.org/pere/blog/archive/2009/09/">September (1)</a></li>
 618
 619 <li><a href="https://people.skolelinux.org/pere/blog/archive/2009/10/">October (2)</a></li>
 620
 621 <li><a href="https://people.skolelinux.org/pere/blog/archive/2009/11/">November (3)</a></li>
 622
 623 <li><a href="https://people.skolelinux.org/pere/blog/archive/2009/12/">December (3)</a></li>
 624
 625 </ul></li>
 626
 627 <li>2008
 628 <ul>
 629
 630 <li><a href="https://people.skolelinux.org/pere/blog/archive/2008/11/">November (5)</a></li>
 631
 632 <li><a href="https://people.skolelinux.org/pere/blog/archive/2008/12/">December (7)</a></li>
 633
 634 </ul></li>
 635
 636 </ul>
 637
 638
 639
 640 <h2>Tags</h2>
 641 <ul>
 642
 643  <li><a href="https://people.skolelinux.org/pere/blog/tags/3d-printer">3d-printer (19)</a></li>
 644
 645  <li><a href="https://people.skolelinux.org/pere/blog/tags/amiga">amiga (1)</a></li>
 646
 647  <li><a href="https://people.skolelinux.org/pere/blog/tags/aros">aros (1)</a></li>
 648
 649  <li><a href="https://people.skolelinux.org/pere/blog/tags/bankid">bankid (4)</a></li>
 650
 651  <li><a href="https://people.skolelinux.org/pere/blog/tags/betalkontant">betalkontant (9)</a></li>
 652
 653  <li><a href="https://people.skolelinux.org/pere/blog/tags/bitcoin">bitcoin (12)</a></li>
 654
 655  <li><a href="https://people.skolelinux.org/pere/blog/tags/bootsystem">bootsystem (17)</a></li>
 656
 657  <li><a href="https://people.skolelinux.org/pere/blog/tags/bsa">bsa (2)</a></li>
 658
 659  <li><a href="https://people.skolelinux.org/pere/blog/tags/chrpath">chrpath (3)</a></li>
 660
 661  <li><a href="https://people.skolelinux.org/pere/blog/tags/debian">debian (197)</a></li>
 662
 663  <li><a href="https://people.skolelinux.org/pere/blog/tags/debian edu">debian edu (159)</a></li>
 664
 665  <li><a href="https://people.skolelinux.org/pere/blog/tags/debian-handbook">debian-handbook (9)</a></li>
 666
 667  <li><a href="https://people.skolelinux.org/pere/blog/tags/digistan">digistan (11)</a></li>
 668
 669  <li><a href="https://people.skolelinux.org/pere/blog/tags/dld">dld (18)</a></li>
 670
 671  <li><a href="https://people.skolelinux.org/pere/blog/tags/docbook">docbook (32)</a></li>
 672
 673  <li><a href="https://people.skolelinux.org/pere/blog/tags/drivstoffpriser">drivstoffpriser (4)</a></li>
 674
 675  <li><a href="https://people.skolelinux.org/pere/blog/tags/english">english (455)</a></li>
 676
 677  <li><a href="https://people.skolelinux.org/pere/blog/tags/fiksgatami">fiksgatami (23)</a></li>
 678
 679  <li><a href="https://people.skolelinux.org/pere/blog/tags/fildeling">fildeling (14)</a></li>
 680
 681  <li><a href="https://people.skolelinux.org/pere/blog/tags/freeculture">freeculture (34)</a></li>
 682
 683  <li><a href="https://people.skolelinux.org/pere/blog/tags/freedombox">freedombox (9)</a></li>
 684
 685  <li><a href="https://people.skolelinux.org/pere/blog/tags/frikanalen">frikanalen (20)</a></li>
 686
 687  <li><a href="https://people.skolelinux.org/pere/blog/tags/h264">h264 (20)</a></li>
 688
 689  <li><a href="https://people.skolelinux.org/pere/blog/tags/intervju">intervju (43)</a></li>
 690
 691  <li><a href="https://people.skolelinux.org/pere/blog/tags/isenkram">isenkram (16)</a></li>
 692
 693  <li><a href="https://people.skolelinux.org/pere/blog/tags/kart">kart (23)</a></li>
 694
 695  <li><a href="https://people.skolelinux.org/pere/blog/tags/kodi">kodi (6)</a></li>
 696
 697  <li><a href="https://people.skolelinux.org/pere/blog/tags/ldap">ldap (9)</a></li>
 698
 699  <li><a href="https://people.skolelinux.org/pere/blog/tags/lego">lego (5)</a></li>
 700
 701  <li><a href="https://people.skolelinux.org/pere/blog/tags/lenker">lenker (8)</a></li>
 702
 703  <li><a href="https://people.skolelinux.org/pere/blog/tags/linuxcnc">linuxcnc (5)</a></li>
 704
 705  <li><a href="https://people.skolelinux.org/pere/blog/tags/lsdvd">lsdvd (2)</a></li>
 706
 707  <li><a href="https://people.skolelinux.org/pere/blog/tags/ltsp">ltsp (1)</a></li>
 708
 709  <li><a href="https://people.skolelinux.org/pere/blog/tags/madewithcc">madewithcc (3)</a></li>
 710
 711  <li><a href="https://people.skolelinux.org/pere/blog/tags/mesh network">mesh network (8)</a></li>
 712
 713  <li><a href="https://people.skolelinux.org/pere/blog/tags/multimedia">multimedia (46)</a></li>
 714
 715  <li><a href="https://people.skolelinux.org/pere/blog/tags/nice free software">nice free software (15)</a></li>
 716
 717  <li><a href="https://people.skolelinux.org/pere/blog/tags/noark5">noark5 (24)</a></li>
 718
 719  <li><a href="https://people.skolelinux.org/pere/blog/tags/norsk">norsk (323)</a></li>
 720
 721  <li><a href="https://people.skolelinux.org/pere/blog/tags/nuug">nuug (198)</a></li>
 722
 723  <li><a href="https://people.skolelinux.org/pere/blog/tags/offentlig innsyn">offentlig innsyn (40)</a></li>
 724
 725  <li><a href="https://people.skolelinux.org/pere/blog/tags/open311">open311 (2)</a></li>
 726
 727  <li><a href="https://people.skolelinux.org/pere/blog/tags/opensnitch">opensnitch (4)</a></li>
 728
 729  <li><a href="https://people.skolelinux.org/pere/blog/tags/opphavsrett">opphavsrett (75)</a></li>
 730
 731  <li><a href="https://people.skolelinux.org/pere/blog/tags/personvern">personvern (114)</a></li>
 732
 733  <li><a href="https://people.skolelinux.org/pere/blog/tags/raid">raid (2)</a></li>
 734
 735  <li><a href="https://people.skolelinux.org/pere/blog/tags/reactos">reactos (1)</a></li>
 736
 737  <li><a href="https://people.skolelinux.org/pere/blog/tags/reprap">reprap (11)</a></li>
 738
 739  <li><a href="https://people.skolelinux.org/pere/blog/tags/rfid">rfid (3)</a></li>
 740
 741  <li><a href="https://people.skolelinux.org/pere/blog/tags/robot">robot (17)</a></li>
 742
 743  <li><a href="https://people.skolelinux.org/pere/blog/tags/rss">rss (1)</a></li>
 744
 745  <li><a href="https://people.skolelinux.org/pere/blog/tags/ruter">ruter (7)</a></li>
 746
 747  <li><a href="https://people.skolelinux.org/pere/blog/tags/scraperwiki">scraperwiki (2)</a></li>
 748
 749  <li><a href="https://people.skolelinux.org/pere/blog/tags/sikkerhet">sikkerhet (60)</a></li>
 750
 751  <li><a href="https://people.skolelinux.org/pere/blog/tags/sitesummary">sitesummary (4)</a></li>
 752
 753  <li><a href="https://people.skolelinux.org/pere/blog/tags/skepsis">skepsis (5)</a></li>
 754
 755  <li><a href="https://people.skolelinux.org/pere/blog/tags/standard">standard (75)</a></li>
 756
 757  <li><a href="https://people.skolelinux.org/pere/blog/tags/stavekontroll">stavekontroll (7)</a></li>
 758
 759  <li><a href="https://people.skolelinux.org/pere/blog/tags/stortinget">stortinget (14)</a></li>
 760
 761  <li><a href="https://people.skolelinux.org/pere/blog/tags/surveillance">surveillance (65)</a></li>
 762
 763  <li><a href="https://people.skolelinux.org/pere/blog/tags/sysadmin">sysadmin (5)</a></li>
 764
 765  <li><a href="https://people.skolelinux.org/pere/blog/tags/usenix">usenix (2)</a></li>
 766
 767  <li><a href="https://people.skolelinux.org/pere/blog/tags/valg">valg (9)</a></li>
 768
 769  <li><a href="https://people.skolelinux.org/pere/blog/tags/verkidetfri">verkidetfri (21)</a></li>
 770
 771  <li><a href="https://people.skolelinux.org/pere/blog/tags/video">video (80)</a></li>
 772
 773  <li><a href="https://people.skolelinux.org/pere/blog/tags/vitenskap">vitenskap (4)</a></li>
 774
 775  <li><a href="https://people.skolelinux.org/pere/blog/tags/web">web (42)</a></li>
 776
 777 </ul>
 778
 779
 780     </div>
 781     <p style="text-align: right">
 782  Created by <a href="http://steve.org.uk/Software/chronicle">Chronicle v4.6</a>
 783 </p>
 784
 785   </body>
 786 </html>