Author Topic: Ultra Precision Reference LTZ1000  (Read 968811 times)

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Offline dietert1

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Re: Ultra Precision Reference LTZ1000
« Reply #2825 on: January 26, 2020, 04:27:28 pm »
A complete answer to your question is difficult. Maybe you can first make a LTZ1000 substitute from discrete parts plus some wires? It's a minor effort but may save your real LTZ1000s life.

Regards, Dieter
 
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Offline Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #2826 on: January 26, 2020, 04:47:04 pm »
A substitute for the voltage loop part is relatively simple (transistor + Zener)
For the heater loop, one could limit the heater voltage (e.g. at the base of the heater driver transistor), so that no excessive power is applied. This could be part of the normal circuit, not just for the initial test.

Another point is checking the circuit once more.  I known trap for the young players is the unusual pin-out of the LT1013 in SO8 case.   
 
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Offline Crossphased

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Re: Ultra Precision Reference LTZ1000
« Reply #2827 on: January 26, 2020, 07:48:22 pm »
Great thanks very much guys, will make a substitute LTZ from discrete components- good idea!
 

Offline alex-sh

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Re: Ultra Precision Reference LTZ1000
« Reply #2828 on: January 27, 2020, 06:30:59 pm »
Quick temperature oven question.
I have the following temperature graph meaning there is about 0.6C fluctuation. I wonder if this is due to the oven not being properly insulated?

EDIT: The amplitude is 0.3C (max 35.1, 34.75 min). I have TE Connectivity GA10K3A1A 3976K NTC +/- 0.1C. The oven should stabilise to 0.1C
« Last Edit: January 27, 2020, 07:10:13 pm by alex-sh »
 

Online Vgkid

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Re: Ultra Precision Reference LTZ1000
« Reply #2829 on: January 27, 2020, 06:47:18 pm »
If that scale is in minutes, and from initial turn on. Give it time to stabilize.
If you own any North Hills Electronics gear, message me. L&N Fan
 

Offline alex-sh

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Re: Ultra Precision Reference LTZ1000
« Reply #2830 on: January 27, 2020, 06:50:03 pm »
Thank you. The problem is that it’s not stabilising.

 

Offline Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #2831 on: January 27, 2020, 06:53:17 pm »
It looks like a regulator that is rather close to oscillation. Much added insulation will change the system and thus have different parameters for the controller.  It may still stabilize over a long time, but it would be better to optimize the regulator tuning.
 
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Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #2832 on: January 27, 2020, 07:01:25 pm »
I wonder if this is due to the oven not being properly insulated?
I fear that the NTC for the temperature controller is too far from the heat source.

Thats why I typically use 2 temperature sensors.
One for the heater and one for the point of interest.

With best regards

Andreas
 
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Offline alex-sh

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Re: Ultra Precision Reference LTZ1000
« Reply #2833 on: January 27, 2020, 07:13:30 pm »
I wonder if this is due to the oven not being properly insulated?
I fear that the NTC for the temperature controller is too far from the heat source.

Thats why I typically use 2 temperature sensors.
One for the heater and one for the point of interest.

With best regards

Andreas

Hi Andreas,

Interesting point. I have a metal enclosure and two NTC sensors (I specified the type two posts above) - One sensor is attached to the centre of the enclosure (metal part) and another sensor is also in the middle measuring the air. Is this a wrong approach?

Thanks
Alex
 

Offline alex-sh

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Re: Ultra Precision Reference LTZ1000
« Reply #2834 on: January 27, 2020, 07:16:54 pm »
Much added insulation will change the system and thus have different parameters for the controller. 

Yeah. I have lost  |O insulation material and now have to order a new one. Any recommendation on insulation please?
I think insulation is a must. Otherwise  the metal enclosure is losing heat very quickly and it takes time to heat up the oven (four 10W resistors). Hence 0.3C oscillation
 

Offline floobydust

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Re: Ultra Precision Reference LTZ1000
« Reply #2835 on: January 27, 2020, 07:20:11 pm »
The period looks like 160 seconds or 2.7 minutes?
I've seen temperature graphs with that long of a time-constant due to the lab HVAC cycling. Make sure there is no draft or airflow from a vent influencing your test. You can put a piece of cardboard over it to see if that makes a difference.
 
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Offline alex-sh

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Re: Ultra Precision Reference LTZ1000
« Reply #2836 on: January 27, 2020, 07:23:54 pm »
The period looks like 160 seconds or 2.7 minutes?
I've seen temperature graphs with that long of a time-constant due to the lab HVAC cycling. Make sure there is no draft or airflow from a vent influencing your test. You can put a piece of cardboard over it to see if that makes a difference.

There is no aircon. However, you are correct there is a temp change / draft. It comes back to a proper insulation
 

Offline imo

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Re: Ultra Precision Reference LTZ1000
« Reply #2837 on: January 27, 2020, 07:45:07 pm »
Yeah. I have lost  |O insulation material and now have to order a new one. Any recommendation on insulation please?
Aerogel
https://en.wikipedia.org/wiki/Aerogel
« Last Edit: January 27, 2020, 07:47:27 pm by imo »
 

Offline dietert1

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Re: Ultra Precision Reference LTZ1000
« Reply #2838 on: January 27, 2020, 07:53:24 pm »
What kind of controller are you using? Oscillation is typical for an on-off controller. A linear "PI" controller will also behave like that when using excessive gain factors. Also the P and the I gain need to be near a certain ratio for fast settling time.
A well adjusted linear controller will show small/slow changes of heater output and even smaller changes of temperature. As Andreas wrote, a good oven needs a sensor close to the heater, with no air gap in between. My LTFLU oven keeps that "fast" sensor to within +/- 0.5 milliK over 24 hours, using an Arroyo TecSource 5235 as controller. The Arroyo PDF manuals are quite instructive.

Regards, Dieter

 

Offline alex-sh

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Re: Ultra Precision Reference LTZ1000
« Reply #2839 on: January 27, 2020, 08:42:34 pm »
Solved!

Attaching one NTC directly to the heater (inside the box to the wall against the source) source and putting another NTC in the middle of the box solved the problem. Thank you Andreas!

Now the heater is having an output of 210 mA rather than going from 400mA down to zero and back to 400mA. The inside temperature is now 34.6 (needs fine-tuning) within 0.1C

« Last Edit: January 28, 2020, 08:08:15 pm by alex-sh »
 
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Offline Hermann W

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Re: Ultra Precision Reference LTZ1000
« Reply #2840 on: January 29, 2020, 11:18:36 am »
I wonder if this is due to the oven not being properly insulated?
I fear that the NTC for the temperature controller is too far from the heat source.

Thats why I typically use 2 temperature sensors.
One for the heater and one for the point of interest.
There are many ways to get to the goal. But that's not a classic control technique. The actual value must be measured at the point to be regulated. Otherwise the actual value does not correspond to the setpoint. The heater may be constant, but not the test object. The example shows quite clearly that the control parameters have a too high gain - as Kleinstein said.
What kind of controller is used? Temperature control works quite well with a PI controller. The integration time constant must then be chosen to be the same as the control system time constant.
 
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Offline imo

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Re: Ultra Precision Reference LTZ1000
« Reply #2841 on: January 29, 2020, 01:37:13 pm »
How are your 2 thermistors wired, plz?
 

Offline dietert1

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Re: Ultra Precision Reference LTZ1000
« Reply #2842 on: January 29, 2020, 01:40:22 pm »
As far as i understand, once you have a rock-solid heater temperature control loop, you can stack another control loop on top, this time using the DUTs temperature sensor.

Regards, Dieter
 

Offline Hermann W

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Re: Ultra Precision Reference LTZ1000
« Reply #2843 on: January 29, 2020, 02:00:18 pm »
As far as i understand, once you have a rock-solid heater temperature control loop, you can stack another control loop on top, this time using the DUTs temperature sensor.
This is a common way when the manipulated variable (heating) requires separate control. This is not really necessary for temperature control.
 

Offline Hermann W

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Re: Ultra Precision Reference LTZ1000
« Reply #2844 on: January 29, 2020, 02:21:44 pm »
Now i know what a rocksolid heater is. This is a good example. I once did that for my wood boiler. The internal regulation was for the position of the air throttle and the external regulation for the exhaust gas temperature.
 

Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #2845 on: January 29, 2020, 09:32:32 pm »
How are your 2 thermistors wired, plz?

In my ageing box for references one NTC is on the PCB in the middle of all voltage references.
(left in the middle of the PCB)

The other is in the middle of the top heater foil (with the red wires).
The bottom heater foil (below the PCB) is in series with the top heater foil so it needs no extra NTC.

The heater is controlled by a simple P-Controller (fast) from the heat spreader NTC.
The slow contol loop with the NTC on the PCB is a I-Controller with some anti windup limitations when the current temperature is far away from setpoint. The I-Controller output gives a Offset which is added to the setpoint for the P-Controller.

Since there is some self-heating due to many references the Offset is negative (I-Out in deg C)

here a log of the heat-up phase:

Time                                         T-PCB  T-HTR   14V      17V      24V      I-Out   P-Out (hex from 0x0000 to 0x2400 or 0..10000 dez)
08:21:01;D7   0  0 9999.9 H9; 38.21; 31.79; 13881; 16974; 24006; -4.18; P2400;   
08:22:01;D7   0 12    0.0 D7; 39.34; 39.41; 13881; 16981; 24012; -4.18; P2400; 
08:23:01;D7@  1 12    0.0 D7; 40.87; 45.15; 13874; 16974; 24033; -4.18; P2400;   
08:24:01;D7@  2 12    0.0 D7; 42.55; 49.58; 13867; 16988; 24053; -4.18; P2400;   
08:25:01;D7@  3 12    0.0 D7; 44.15; 51.91; 13860; 17021; 24593; -4.18; P2400;   
08:26:01;D7@  4 12    0.0 D7; 45.24; 51.88; 13860; 16981; 24087; -4.18; P2400;   
08:27:01;D7@  5 12    0.0 D7; 46.16; 51.91; 13847; 17028; 24600; -4.18; P2400;   
08:28:01;D7@  6 12    0.0 D7; 47.02; 51.91; 13847; 17028; 24600; -4.18; P2400;   
08:29:01;D7@  7 12    0.0 D7; 47.77; 51.88; 13840; 17028; 24094; -4.18; P2400;   
08:30:01;D7@  8 12    0.0 D7; 48.45; 51.91; 13827; 17028; 24600; -4.18; P2400;   
08:31:01;D7@  9 12    0.0 D7; 49.06; 51.79; 13827; 17028; 24600; -4.18; P0000; 
08:32:01;D7@ 10 12    0.0 D7; 49.44; 50.55; 13820; 17028; 24600; -4.18; P0000;   
08:33:01;D7@ 11 12    0.0 D7; 49.71; 49.49; 13820; 17028; 24600; -4.18; P0000; 
08:34:01;D7@ 12 12    0.0 D7; 49.85; 48.49; 13813; 17028; 24600; -4.18; P0000;   
08:35:01;A4   0  0    0.1 A4; 49.98; 47.63; 13820; 17028; 24600; -4.18; P0000;   
08:36:01;A5   0  0    0.1 A5; 50.01; 46.86; 13813; 17028; 24593; -4.18; P0000;   
08:37:01;A6   0  0    0.1 A6; 50.03; 46.12; 13820; 17035; 24593; -4.18; P0000;   
08:38:01;A7   0  0    0.1 A7; 49.96; 45.49; 13813; 17035; 24593; -4.18; P0133;
08:39:01;B4   0  0    0.1 B4; 49.94; 45.17; 13820; 17035; 24600; -4.18; P0257;   
08:40:01;B5   0  0    0.1 B5; 49.92; 45.06; 13820; 17035; 24600; -4.15; P02bf;   
08:41:01;B6   0  0    0.1 B6; 49.92; 45.01; 13806; 17035; 24593; -4.13; P0302; 
08:42:01;B7   0  0    0.1 B7; 49.87; 44.99; 13820; 17035; 24593; -4.08; P0330; 
08:43:01;C4   0  0    0.1 C4; 49.85; 45.01; 13813; 17028; 24593; -4.03; P0343;   
08:44:01;C5   0  0    0.1 C5; 49.87; 45.03; 13820; 17028; 24593; -3.99; P035c;
08:45:01;C6   0  0    0.1 C6; 49.85; 45.03; 13813; 17028; 24593; -3.94; P0383; 
08:46:01;C7   0  0    0.1 C7; 49.87; 45.10; 13813; 17028; 24600; -3.90; P0373;
08:47:01;D4   0  0    0.1 D4; 49.87; 45.10; 13813; 17035; 24600; -3.86; P039a; 
08:48:01;D5   0  0    0.1 D5; 49.87; 45.15; 13813; 17035; 24600; -3.81; P0397;
08:49:01;D6   0  0    0.1 D6; 49.87; 45.19; 13813; 17021; 24600; -3.77; P0395; 
08:50:01;D7   0  0    0.1 D7; 49.89; 45.21; 13820; 17035; 24593; -3.73; P03a7;   
08:51:01;E4   0  0    0.1 E4; 49.92; 45.26; 13806; 17028; 24593; -3.71; P039e; 
08:52:01;E5   0  0    0.1 E5; 49.92; 45.28; 13813; 17028; 24600; -3.68; P03a2; 
08:53:01;E6   0  0    0.1 E6; 49.94; 45.31; 13813; 17035; 24593; -3.66; P03a7; 
08:54:01;E7   0  0    0.1 E7; 49.96; 45.31; 13813; 17035; 24600; -3.65; P03b9; 
08:55:01;F4   0  0    0.1 F4; 49.96; 45.37; 13813; 17035; 24587; -3.63; P0387;
08:56:01;F5   0  0    0.1 F5; 50.01; 45.35; 13813; 17035; 24607; -3.64; P03a7; 
08:57:01;F6   0  0    0.1 F6; 50.01; 45.40; 13813; 17021; 24600; -3.64; P037b;   
08:58:01;F7   0  0    0.1 F7; 50.05; 45.37; 13806; 17035; 24593; -3.66; P038e; 
08:59:01;G4   0  0    0.1 G4; 50.03; 45.37; 13813; 17028; 24593; -3.67; P037e;
09:00:01;G5   0  0    0.1 G5; 50.05; 45.35; 13813; 17035; 24600; -3.68; P038a;
09:01:01;G6   0  0    0.1 G6; 50.07; 45.37; 13813; 17035; 24593; -3.71; P0366; 
09:02:01;G7   0  0    0.1 G7; 50.07; 45.35; 13813; 17035; 24593; -3.73; P0364;
09:03:01;H4   0  0    0.1 H4; 50.10; 45.33; 13813; 17028; 24600; -3.76; P0362;
09:04:01;H5   0  0    0.1 H5; 50.10; 45.31; 13813; 17028; 24600; -3.80; P0359; 
09:05:01;H6   0  0    0.1 H6; 50.07; 45.28; 13813; 17035; 24600; -3.82; P0350;   

With best regards

Andreas
 
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Offline imo

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Re: Ultra Precision Reference LTZ1000
« Reply #2846 on: January 29, 2020, 09:45:31 pm »
@Andreas: has your thermostat got "digital" control (2xRth+MCU) then?
I've been looking for some 2xRth+(1-2x)opamp "linear" version I will add directly on the 10V Vref board.
« Last Edit: January 29, 2020, 09:51:00 pm by imo »
 

Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #2847 on: January 29, 2020, 10:26:52 pm »
Hello

yes it is a PIC24FV301 with 12 Bit ADC where the NTCs and the 100 Hz PWM heater output is connected.
It is only used as dumb measurement machine, multiplexer controller and heater actuator (with overtemperature limitation).

The actual control loop is on the connected PC.
Every 10 seconds the P-controller part is calculated.
The I-controller only all 60 seconds.
The long time constants and especially the anti-windup strategy are difficult in analog domain.

with best regards

Andreas
 
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Offline imo

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Re: Ultra Precision Reference LTZ1000
« Reply #2848 on: January 30, 2020, 12:23:40 am »
But volt-nuts would rather avoid digital+pwm close to their lovely reference :) Therefore the idea with analog PI even it would be difficult (we do it not because it is easy..).
« Last Edit: January 30, 2020, 12:33:23 am by imo »
 

Offline Andreas

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Re: Ultra Precision Reference LTZ1000
« Reply #2849 on: January 30, 2020, 05:36:45 am »
Hello,

yes you are right, I only use it for those ugly AD586, LTC1236 and LT1027 references for pre-ageing.
But 100 Hz PWM will be canceled out by most integrating instruments with mains suppression.

The principle can also be used in a analog controller. A inner P-Controller for the heater. And the setpoint with a offset from a I-Controller (where you have carefully limit the output signal range to avoid very long settling times).

with best regards

Andreas
 


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