Some questions on how is working the Peltier module of an Argus 3.... - Page 1
Forum section: electronics / general

author: Lambda
author_rank: Regular Contributor
timestamp: April 14, 2021, 01:56:58 pm
content: Hello.When reading the different topics here speaking about the fire brigade camera Argus 3 and especially studying the documentation provided by Bill here:http://www.fire-tics.co.uk/A3.htmhttp://www.fire-tics.co.uk/forum/viewtopic.php?f=6&t=41for instance...I was asking myself the following:I understood that the function of the embedded Peltier Module is to allow the BST sensor to be kept at a normal and optimal running temperature in case of excessive environmental temperature increasing, the sensor to be submitted to such "over heating"....The optimal temperature (from memory, please correct me in case of...) would be around 30°C, is it correct?Therefore, i imagine that in case of overheating of the sensor, the Peltier module enters into action and cool down said sensor till the required normal running temperature is reached.... with the application of a conventional feedback loop control, i guess?But during this cooling process, what is difficult for me to understand is how the Peltier module would expel the extracted calories from the sensor... indeed, generally speaking, Peltier module needs to be equipped with a quite well sized heatsink (huge radiator with cooling fan, or liquid cooling used in heat exchanger, whatever...).Indeed, when the heatsink is undersized, the Peltier module runs easily badly into a thermal runaway mode and gets globally warmer and warmer without cold face and hot face anymore...But when i observe the inside of a torn-down Argus 3, i do not see any heatsink or any kind of embedded heat exchanger (or is it one of the rear flat aluminium flange fixed on the main PCB/motherboard unit....?)http://www.fire-tics.co.uk/project2/A3_repair1.htmSo shortly worded:How the Peltier module from the Argus 3 can work properly and ensure properly its cooling function in excessive hot environment (fire brigade working environment, for instance), without falling in an undesired thermal runaway mode?As usual, and in advance, thank you for your explanations.Best regards.Stéphane

author: Fraser
author_rank: Super Contributor
timestamp: April 14, 2021, 04:59:47 pm
content: The Peltier temperature stabiliser in the ARGUS 3 BST sensor is not an extreme environmental temperature countermeasure. It is ‘just’ a temperature stabiliser for the BST die that is operating within a range of acceptable environmental temperatures specified by Raytheon. The thermal load of the actual BST die in its vacuum is not that great. As far as I am aware, E2V were not using the Peltier TEC in a novel manner as a means to keep the BST sensor within operating temperature limits during high environmental temperature fire fighting activities but Bill can, of course, answer that question better than me ! The BST sensor array is in a ceramic package, the rear of which is thermally connected to the heat-sink. The heat sink does not normally have to be that large or exotic. Bill actually commented on the folly of ISG placing a fan on the BST sensor heat sink when it was encased in a non ventilated casing. That was with regard to the ISG Wasp design.The BST TEC is similar to that used in the high stability microbolometer designs in that it provides temperature stabilisation of the sensor die, but it is not intended to compensate for extreme environmental temperatures around the sensor array. In fire fighting cameras, various approaches have been taken to maintain an acceptable temperature within the sealed camera casing whilst the camera operates in potentially high temperature environs. I have seen large thermal heat absorbers and thermal blankets used in this scenario but Bill will be able to enlighten us on the design challenges of the fire fighting camera internal thermal environment.Fraser

author: Bill W
author_rank: Super Contributor
timestamp: April 14, 2021, 09:26:44 pm
content: You are pretty close to it Stephane. The BST sensor die is kept at 30°C with a feedback loop, of Raytheon design on the kit PCB. So at turn on the sensor is actually being heated up to 30°C, although most of the initial current flow you see is to do with the loop coming to stability.As Fraser says, the thermal load is not huge being just dissipation and conduction / radiation into the vacuum package. The heat sink - the thick plate shown here :http://www.fire-tics.co.uk/project2/images/chassis.jpg(and below)is just that, a heat sink to buy 'time in the fire' that absorbs energy and heats up. The typical exposure we designed to was that in around 45 minutes at 80°C the whole camera would get up to 80°C. At 80°C the peltier could not be able to maintain a differential of 50°C and runaway would happen, with ~ 4W going into the plate. To prevent the runaway the heatsink temperature is monitored (see blue thermistor to the left of the motor) and gives an alarm at 75°C before cutting off the peltier at 80°C (by EEV circuitry) to prevent damage. The picture goes to pot of course, but hopefully the warning time was enough to get out of the fire, and operationally ties in with how fire services are used to dealing with limited air supply.Other things we, and others, considered were:Phase change wax fill, buys more time but then how to cool it back down ?Copper brick, which you swap out like a battery which was used in Cairns ViperA bit heavy !https://www.ebay.com/itm/Thermal-Management-Block-for-Cairns-Viper-Thermal-Imaging-Heat-Seeking-Camera-/380698382876Hose attachments into the waterFoiled - the 'camera' man is not the 'hose' man.A lot of 'conventional' thermal management does not work when it is the environment is heating up the electronics not the other way round.BillPS - thanks for the kick, I must get to upload the next chunk of Argus3 info

author: Lambda
author_rank: Regular Contributor
timestamp: April 16, 2021, 10:32:07 am
content: Hello.Fraser, Bill, thank you a lot for your combined explanations, i get it clearly now.At the end, for this temperature stabilisation constraints, laws of thermodynamics still rule as ultimate tyrans...I am questioning you about such technology, because i am currently considering to start a new project based on this technology, for feeding my learning curve.... i will have, by the way, to investigate in the not so far future, for one core based on such BST sensor (Bill...).And indeed, Bill, i will read with pleasure and big interest your new coming input on Argus3....Best regards.Stéphane