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Several VueMetrix controllers (DPSS 0.8, DPSS 3.0, DPSS 4.0, HCT) have the capability of regulating one or two temperatures. Temperature control on these systems is accomplished by a customer-supplied Peltier (thermoelectric) cooler or TEC. Drive circuitry for one or two TECs is built into these controllers; the number of such drivers and their specified current and voltage can be found in the individual product manuals. This document describes in general the use of VueMetrix temperature controllers.
In addition to the TEC, active thermal control also requires a device to sense the temperature. For this purpose VueMetrix controllers use thermistors. The thermistor selection and calibration sections describe this.
Other controllers in the VueMetrix product line do not actively control the temperature of the laser, but instead provide the capability of monitoring the laser temperature for safety purposes. This requires only a calibrated thermistor of the appropriate type, connections between the thermistor and the controller, and the proper setting of the controller's firmware. This is explained in detail in a later section of this document.
The output voltage and current capability of the TEC driver varies from one controller to another (see table).
| Servo | Maximum voltage (V) | Maximum current (A) |
| DPSS 0.8 TEC #1 | 4.0 | 4.0 |
| DPSS 0.8 TEC #2 | 2.2 | 2.0 |
| DPSS 3.0 TEC #1 | 4.0 | 2.5 |
| DPSS 3.0 TEC #2 | 4.0 | 2.5 |
| DPSS 4.0 TEC #1 | 4.0 | 4.0 |
| DPSS 4.0 TEC #2 | 4.0 | 2.5 |
| HCT | 11.5 | 15.0 |
Programmer's note: When developing custom software to interface to a VueMetrix controller, note that the controller's command set, instead of the aforementioned percentage scale, uses an integer scale where o is maximum cooling and 30000 is maximum heating.
The TEC driver in VueMetrix controllers is a voltage source. The drive setting (0% through 100%) directly affects this voltage. The current that flows through the TEC, however, is dictated by the TEC impedance. This is, in general, a complicated function of the TEC characteristics and also the instantaneous temperature difference across the TEC. As the temperature servo operates, the extremes of the driver's performance can be reached. Therefore it is important to choose a TEC that will not exceed the rated current of the driver under the most extreme conditions.
DPSS controllers (DPSS 0.8, DPSS 3.0, DPSS 4.0) continually sense and report the status of their TEC driver circuits. This information is displayed in the WinVue TEC Utilities menu in the field labeled “TEC hardware status.” In the event the driver raises an error signal the temperature servo will report a fault, which in turn will cause the laser to shut off.
When the controller is first turned on, the behavior of the temperature servos depends on whether Autostart is enabled. Autostart will automatically turn on the temperature servos and wait until they report a “locked” condition before starting the laser. If Autostart is not in use, the temperature servos power up in the off condition until the user turns them on. Once the servos are on, they operate without any further intervention from the user as long as the controller remains on. Should the host PC (or other host computer) be disconnected the servos will continue to regulate the temperature. In the event of a temperature servo fault the laser will be shut off.
The most common operations use the Main TEC control window. Turn the servos on and off by clicking the “Enable servo” button. By default, the servos are initially off when the controller is powered up.
To automatically start the temperature servos when power is applied, use the Autostart feature.
Below the Enable button is a list of possible states of the servo, with a highlight showing the present state. The states are defined as follows:
Off. The servo is not engaged and the TEC drive circuit is off.
Seeking. The servo is engaged but has not acquired the set point temperature. The drive percentage may be changing rapidly.
Locked. The servo has acquired and is maintaining the set point temperature. The servo is considered to be Locked when the temperature has remained within a certain tolerance window for a certain amount of time. The exact parameters are product-dependent; see the chart.
| Servo Type | Temperature tolerance (degrees C) | Time to lock (s) |
| DPSS 0.8/3.0 TEC #1 | 0.1 | 1.0 |
| DPSS 0.8/3.0 TEC #2 | 0.02 | 1.0 |
| HCT | 0.5 | 10.0 |
Manual. The servo is not engaged. The drive is under user control through the TEC Utilities Window.
Fault. The servo logic has detected a fault. If the laser was on it has been shut off. If the laser is off it cannot be started until the Clear Fault button is clicked. A brief description of the fault is displayed just to the right. The possible faults and their meanings are as follows:
The set point slider is used to select the target temperature. It can be adjusted at any time regardless of whether the servo is on.
The temperature error indicator is a center-weighted bar displaying the deviation from the set point.
The current temperature and current TEC drive are indicated numerically.
Infrequently used functions related to the temperature control logic are grouped in the TEC Utility window.
The check box labeled “Do not use this TEC” should be selected when the corresponding temperature servo is not to be used. When this box is checked, Autostart will not engage the temperature servo. No faults will be reported from this TEC or from this temperature sensor.
The entry fields for Slope gain and Offset gain allow fine-tuning of the temperature servo. This is discussed in a later section.
The entry fields for Thermistor Slope and Thermistor Intercept allow the use of a thermistor other than the standard one. See the section Calibrating the Thermistor to use a different thermistor type.
The check box labeled “Enable drive setting” can be checked to test the operation of the TEC manually. Checking this box changes the display directly below to an adjustable slider. It also turns off the temperature servo. Although this is useful for troubleshooting and for initial servo set up, it must be used with caution as many laser systems can be damaged by long exposure to extreme heating or cooling.
The displays on the upper right of this window are for information only. “Temp change rate” is a number proportional to the change of temperature versus time. A negative value indicates decreasing temperature. “TEC hardware status” displays “OK” when the TEC driver circuit is functioning properly. In the event of a hardware fault the displayed information is product dependent, either showing simply “Fault” or in some cases a more detailed message.
Temperature control can be broken down into three fundamental blocks:
Temperature sensing (thermistor)
Heating and cooling (TEC)
The temperature control servo
The selection and calibration of the thermistor is described in detail elsewhere. Proper wiring of the thermistor is described in the individual product manuals. The operation of the thermistor can be checked using the temperature display on the Main TEC window.
Selection of the proper TEC is described above. Proper wiring of the TEC is described in the individual product manuals. The operation of the TEC can be checked using the temperature display on the Main TEC window, in conjunction with the drive setting function of the TEC Utility window. A drive setting of greater than 50% should result in a temperature rise; settings below 50% should result in a temperature decrease. Settings of 0% or 100% should be used only briefly, but the TEC hardware status display should continue to read “OK” even at these extreme settings.
If the TEC and the thermistor are working properly, but the response of the temperature servo is sluggish or results in faults, the temperature servo coefficients may require adjustment. See the section titled “Tuning the temperature servo” below.
The temperature servos on VueMetrix controllers use a proprietary method. They are somewhat analogous to a traditional PI servo, but there are significant differences. Two gain coefficients are used: the offset gain determines the response of the servo in proportion to the set point error (the difference between the set point and the actual temperature); the slope gain determines the response to the observed temperature versus time slope. The set point is achieved when both the set point error and the slope are zero. It is useful to think of the offset gain as determining the responsiveness of the servo, while the slope gain determines its damping. Because of the inherent non-linearity of TECs, perfect performance throughout the range is not achieved. But the algorithm combines noise immunity, good performance at the set point, and minimal overshoot when changing from one set point to another.
The controller periodically measures the temperature, performs the necessary calculations, and adjusts the TEC drive. On the DPSS series of controllers this occurs five times per second. On the HCT controller, designed to control more massive loads, it is once per second.
WinVue's Temperature Log window records and displays the controller's temperature control performance. Both the TEC drive and the actual temperature are displayed as a function of time, and the data can be exported to the system clipboard and to a spreadsheet-compatible file.
In determining the optimum offset and slope gains for a given situation, a crucial step is to understand and characterize the thermal load. For this purpose WinVue users can employ the “Measure thermal load” window. This models the load using four parameters:
Steady-state temperature at 50% drive. Normally this is the ambient temperature.
Degrees per percent drive. The amount of temperature change, in the absence of servo control, caused by a one percent change in TEC drive.
Time constant. A characteristic of the thermal mass being controlled. In general, the larger the mass the larger the time constant.
Delay. The time elapsed between a signal being applied at the TEC and a measurable response appearing on the thermistor.
These values do not have to be determined separately. As described in the next section, the Measure Thermal Load window performs the necessary measurements and calculates the parameters for you. These parameters are then used in the “Temperature Servo Simulation” window, which simulates the performance of the temperature servo. Using this method, effective values for the offset and slope gain coefficients can be found in only a few minutes.
The information here assumes a familiarity with the temperature servo theory of operation.
There are two steps involved in the servo tuning process. The first step is a characterization of the thermal load, and requires a working controller, a properly connected thermistor and TEC, and the WinVue application. The second step is to use this information to model the servo, which requires the WinVue application.
The controller will subject the system to alternating cycles of heating and cooling from the TEC and measure the response. By default, the cycles consist of heating at a TEC drive of 55% and cooling at 45%, with a period of 100 seconds. These values will work well for a typical situation. They can be changed if necessary before starting the data acquisition process.
The data acquisition process is automatic. You should allow it to run for approximately two cycles of the applied signal, i.e., two times the number entered in the Period field. Longer times will improve the model but may allow ambient effects to distort the data.
The best results will be obtained if the Period is roughly equal
to the thermal time constant of the load. Under these conditions the
acquired data will resemble a triangle with a noticeable curvature to
each segment. 
If the curvature is too pronounced a
shorter period can be used, and if there is little or no curvature a
longer period should be used. Press Stop to end the data
acquisition.
When the data acquisition is finished it can optionally be saved and recalled in a spreadsheet format.
The next step uses a model for the load, and an iterative least squares fitting process to optimize it. Typical of such techniques, a starting guess for the values of the four parameters is needed. When the Stop button is pressed to end the data acquisition, WinVue enters some reasonable numbers for the parameters. You can confirm this by clicking on the “Run model once” button.
Press the “Optimize model” button. WinVue will now iteratively refine the values of the four parameters. The progress of the fitting process can be monitored by observing the “Chi sqr of fit” (Χ2 ) field, the statistical parameter that is being optimized. The process may require a few seconds depending on the size of the data set and the speed of the host computer.
When the process is finished and a Χ2 has been obtained that exceeds 0.1 degrees, the fit is repeated automatically up to three times. In some cases this results in further improvement. Pressing the “Optimize model” button again may also result in further improvement.
Click on the “Transfer coeffs to model window” button to open the Temperature servo simulation window and automatically transfer the coefficients just determined.
The servo gain coefficients can be entered manually, or they can be obtained from the controller by clicking on the “Get coefficients” button.
Like the previous window this involves an iterative optimization process. The calculation uses three additional parameters for which WinVue has already provided defaults:
The servo “Update interval,” which defaults to 0.2 seconds for the DPSS series of controllers and 1.0 seconds for the HCT.
The starting temperature
The final temperature
The program performs a simulation of the controller's performance in changing from the starting temperature to the final temperature based on the model obtained in steps 1-5. The servo parameters are adjusted for best results (fastest convergence with minimal overshoot).
Press “Run” to run the simulation once with the displayed gain coefficients. Press “Fit” to use WinVue to perform an iterative improvement in the coefficients. Continually pressing “Fit” may result in further improvement.
The final coefficients for the best servo performance can be entered into the controller on the TEC Utilities window.
To sense the temperature requires a customer-supplied
thermistor. The controllers measure the voltage across this
thermistor using a circuit similar to the figure
. Note that the
figure is for a typical VueMetrix controller; for the DPSS series
only R1 is 6200 ohms. The voltage at point A in the diagram is
measured by the controller's microprocessor and converted to an
approximate temperature. The formula relating the voltage measured
by the microprocessor to the thermistor resistance is V = 2.5 * R1/
(R1 + Rthermistor). The circuit
is optimized for a standard 10K thermistor, i.e., one which has a
resistance of 10000 ohms at 25o C. Differences between
thermistors can be accommodated by setting the appropriate
calibration constants in the controller, however, it is strongly
recommended to use a thermistor with a nominal resistance of 10K at
25o C.
Some controllers (DPSS 0.8,DPSS 3.0, DPSS 4.0, HCT, HCV) use a simple linear formula T = I + VS to convert the measured voltage to temperature, where I is termed the “thermistor intercept” and S is termed the “thermistor slope.” These quantities can be calculated for a specific thermistor using the thermistor manufacturer's data and a knowledge of the circuit. In addition, the measured voltage at point A can be observed on the WinVue Service Window, where the full scale corresponds to 2.5V. Such controllers are shipped with a default intercept of 85.66 and a slope of -39.39 (for HCT these numbers are 73.97 and -38.89, respectively), which are appropriate constants for the VueMetrix standard thermistor.
For all other controllers with temperature monitoring, the conversion of observed voltage to temperature uses a third-order formula: T=A+BV+CV2+DV3. The four coefficients A, B, C and D can be calculated with WinVue's Thermistor Coefficients window. For the standard thermistor the values are A=109.5682, B=-129.4330, C=71.5989, and D=-17.7934. These default constants are loaded into each controller when shipped from the factory.
During operation the controllers continually check the external (laser) temperature. In the event that the temperature falls outside of a specified window, the controller will shut off the laser and report an over temperature or under temperature fault. These limits are set using WinVue from the Main Laser window by clicking on the button labeled “Limits.” Factory defaults are set to 15 and 35 degrees.
The “Limits” dialog of WinVue allows this feature to be disabled.
The figure shows the WinVue Thermistor Coefficients window. The purpose of this window is to compute the coefficients used to calculate the temperature from the measured thermistor input voltage. For DPSS, HCV and HCT controllers a simple linear formula is used:
T = I + SV
where I is the intercept and S is the slope.
For other controllers a cubic equation is used:
T = A + BV + CV2 + DV3
A, B, C, and D are the thermistor coefficients.
In both formulas T is the temperature in degrees Celsius and V is the measured voltage at the microprocessor.
To use this window you must enter data characterizing the thermistor. This can be done one of four ways, reflecting the different ways that thermistor manufacturers supply such data. The first method, and the most generic, is to construct a properly formatted text file containing resistance values as a function of temperature. Each line in this file must contain two numbers, a temperature in degrees C and a resistance in ohms, separated by a comma (see the example). To read this file into the Thermistor Coefficient window, click on the button titled “Load.” WinVue will load the contents of the file and compute the coefficients used by the controller.
The other three methods are accessed using the buttons “ABCD model,” “Beta model,” and “Steinhart-Hart model.” Manufacturers generally use one of these methods for characterizing their thermistors. Each of these buttons displays a small dialog box whose purpose is to calculate and produce a file containing temperature versus resistance data as described in the previous paragraph.
The “ABCD Model” describes the thermistor's resistance according to the formula:
R = 10000 * exp(A + B/T + C/T2 + D/T3)
This assumes a nominal resistance of 10000 ohms at 25o C. Enter the manufacturer-supplied values for A, B, C, D and click on “To Fit.” Choose a name for the file. WinVue will create a file and also compute the coefficients used by the controller.
The “Beta Model” describes the thermistor's resistance according to the formula:
R = 10000β(exp(1/T))
This assumes a nominal resistance of 10000 ohms at 25o C. Enter the manufacturer-supplied values for β and click on “To Fit.” Choose a name for the file. WinVue will create a file and also compute the coefficients used by the controller.
The Steinhart-Hart model describes the thermistor's resistance according to the formula:
1/T = A + Bln(R) + Cln(R)3
Enter the manufacturer-supplied values for A, B, and C and click on “To Fit.” Choose a name for the file. WinVue will create a file and also compute the coefficients used by the controller.
Once the coefficients are computed they are displayed in the gray boxes on the left side of the screen. On HCT controllers and on controllers that have no temperature servo, these coefficients can be loaded into the controller by clicking on the “Upload” button (this requires a valid connection to the controller). On DPSS controllers, since there are multiple thermistors, the coefficients must be entered into the controller manually through the TEC Utility window. Manual entry is also available on other types of controllers through the Utility menu (click “Calibrate Thermistor”).
0,32773.01 2,29593.32 4,26757.56 6,24224.95 8,21959.92 10,19931.45 12,18112.41 14,16479.03 16,15010.5 18,13688.52 20,12497 22,11421.77 24,10450.34 26,9571.66 28,8775.97 30,8054.64 32,7400 34,6805.23 36,6264.31 38,5771.84 40,5323.02 42,4913.59 44,4539.71 46,4197.97 48,3885.32 50,3599 52,3336.71 54,3096.21 56,2875.48 58,2672.72 60,2486.29 62,2314.72 64,2156.69 66,2011.02 68,1876.61 70,1752.51 72,1637.81 74,1531.74 76,1433.55 78,1342.6 80,1258.29
Default constants for WinVue controllers (except DPSS and HCT):