1. Subject nº 5
SunDog TM STCU: A generic sun tracking control unit for concentration technologies
Mail to: Ignacio Luque-Heredia, INSPIRA, SL, C/Chile 10, Edf. “Madrid-92” Of. 16-17, 28290 Las Matas – Madrid, Spain
Ignacio Luque-Heredia, José M. Moreno, Goulven Quemere, Pedro H. Magalhães, Rafael Cervantes, José L. Auger
INSPIRA, SL, C/Chile, 10, 28290 Las Matas, Madrid - Spain
Ph.: +34 91630 45 34, Fax: +34 91630 40 87 , iluque@inspira.es
PURPOSE OF THE WORK In such a way the overall auto-calibration routine following the
We present here an electronic sun tracking control unit which tracker’s installation would be developed in two sessions, first a
named SunDogTM STCU, is prepared to perform in the sub- dawn to dusk tracking error acquisition session, followed by a
degree accuracy range, and whose general purpose auto- night session, in which best fit parameters are calculated
calibration routines enable the control of any sort of one or two allowing for occasionally long iterative optimization processes.
axis mechanical sun tracker while correcting for the From then on, open-loop tracking will proceed but now based
manufacturing, installation or collector assembly errors affecting on sun ephemeris computing corrected by the error model tuned
tracking accuracy. on the best fit parameters sought.
APPROACH SunDog’s core electronic unit was first tested in the Trell
Hybrid sun tracking strategies attempt to combine the best of Tracker™, a 4m2 laboratory precision tracker developed by
both open-loop ephemeris computing, and close-loop sun Inspira for this purpose for which in first instance a couple of
pointing sensor based controllers, while avoiding the problems units were produced to allow for testing tracking strategies in a
of both. By using an open-loop sun primary acquisition phase, competitive basis. In these, the error sources considered by the
hybrid controllers wake and stow at specified positions and calibration error model can be induced and tuned to a priori
times and do not stop tracking whichever the sunshine known values in order to check the validity of the model.
conditions, thus avoiding the problems in acquiring and locking Parallel to its testing an end product design was targeted,
onto the sun that occur with closed-loop systems. By using a providing an enclosure suitable for operation in harsh climate
close-loop error correction phase, hybrid controllers avoid the conditions and integrating quick hands on control panel. More
open-loop controller problems of needing accurate alignment, versatile man-machine interface is enabled by the SunDog
assembly, and time information. Monitor Windows® software which can be run on a PC
connected to SunDog’s serial port while remote monitoring,
These error correction routines usually take as feedback signal synchronization, and reporting is provided by SunDog’s internal
the array’s power output, thus taking advantage of the obvious web and e-mail server.
equivalence between perfect sun pointing and maximum
production. Tracking errors gathered can be somehow tabulated SCIENTIFIC INNOVATION AND RELEVANCE
and permanently updated recurring to forecasting techniques to Concentration keeps on fighting for its share in today’s booming
account for their drifts, such as is done in the EPS-TenerifeTM PV market, closer than ever to the settings required for these
sun tracking control unit produced by Inspira for the EUCLIDES technologies to develop its cost-breaking potential. Several
concentrator. An alternative approach within hybrid sun major PV manufacturers are pushing for very high concentration
tracking, and the one followed by the new SunDog, relies in the concepts devised to cost effectively drive to market the presently
modelable nature of sun tracking errors, so the set of acquired ongoing research in very high efficiency PV devices. However,
errors are now fitted by a six parameter error model by means of a reasonable amount of field experience has been acquired since
an embedded, Levenberg-Marquardt based, global optimization the onset of concentration technologies, which clearly keeps on
routine. In this way tracking errors are characterized for pointing out tracking systems as one of its most error prone
synthetic purposes, thus avoiding the need for permanent components, and one of the overloads which is delaying
vigilance of their drifts and the associated updating techniques. industrial take off. SunDog represents one of the most serious
In the other hand, the generality of the sun tracking error model efforts to our knowledge in providing the required precise,
developed and integrated in SunDog, enables its operation in reliable, and general purpose tracking control, with an extra
whichever one or two axis trackers and among the former, both concern for the high accuracies demanded by the present rising
in single or multi secondary axis concepts. trends in concentration ratio.
A chapter apart is that of the algorithms employed in SunDog RESULTS AND CONCLUSIONS
for the automatic gathering of the set of sun tracking errors, The reported work has resulted in a novel sun tracking control
which is to be fitted by the error model. These involve two unit for concentration technologies, prepared to reliably perform
dimensional search and optimization techniques, integrating in the subdegree accuracy range, compliant with any tracking
forecasting routines. An issue which is to be carefully addressed axes assembly, and integrating the auto-calibration routines
at this stage is that of the bias which is to be imposed on the required to cancel tracker’s manufacturing and installation
array when working as sun pointing sensor, which will errors. Most remarkable problems solved lie in the one hand in
ultimately determine the accuracy of the error measurements. the development of a well conditioned error model for
SunDog also contributes in this respect by obtaining up to a calibration purposes and its associated robust optimization
tenfold enhancement in the resolution of axes position feedback techniques, and in the other in the routines and techniques
provided by the encoders, by means of interpolation circuits involved in an accurate and automatic compilation of the
which nevertheless permit its backlash error-free primary axis tracking error set. The efficient integration of these set of
mounts. algorithms in a low cost electronic system is also noteworthy.

2. (left) SunDog STCU 1st. pre-industrial prototype.
(above) SunDog STCU core electronic module, including
tracking control unit and DC motor drives
(left) SunDog Monitor main window.
(below) Design drawing of the Trell Tracker
laboratory tracker for testing tracking strategies.
Front panel detail of the
Trell Tracker and its two
present installations at
University of Malaga and
Institute of Solar Energy in
Madrid, for testing of
strategies in a competitive
basis.

3. elevation −α
1.5
1
0.5
azimuth −γ
-3 -2 -1 1 2 3
elevation −α
1.5
1
0.5
azimuth −γ
-2 -1 1 2
-0.5
Transforms over a rectangular grid on the azimuth-elevation plane of the error model calibration
transform. Only primary axis tilt - zenith angle component -. transform (above) and this composed with
non-orthogonality of axes (below)
Error model fitting by the embedded Levenberg-Marquardt based optimization code. To enable local
search representation (red line) only two of the six model parameters are employed in the search space,
i.e. primary axis tilt (zenith angle component) & primary axis tilt (azimuth component). Here see how
local search starts close to saddle point and proceeds into the only global minima existent within a period.