I have read advice on naming a company that if the name is confusing people will forget it. I don't know how many times I've had to repeat the name Omnific and spell it out. Well, based on that I decided to do my consulting under the name Tremaine Consulting Group.
I have a new website: TremaineConsultingGroup.com and a blog at TremaineConsultingGroup.com\blog
I will leave this blog in place as-is but all new postings will be at my new site.
Thanks you,
Brian
Monday, June 6, 2016
Sunday, November 30, 2014
Avoid the use of MTBF
The use of MTBF (Mean Time Between Failures) is almost always applied wrong. As a producer of a product you would want the product to last beyond the warranty period and long enough to be perceived by the customer as a 'Quality' product. One way of quantifying this is that a product will have X% reliability at Y years with C% confidence.
MTBF is the inverse of the failure rate, NOT the life-time of the product. As example a product or assembly might have a MTBF of 400,000 device-hrs. This does not mean the product life is 400,000hrs (45.6 years)! It means the failure rate is 2.5 per million hours.
Implicit in the concept of MTBF or it's inverse failure rate is the that the failure rate is constant. Think of the MTBF as meaning that in any given hour the possibility of failure is 1/MTBF. This is applicable during the flat portion of the 'Bath Tub' curve showing failure rate versus time (see figure). Quite often in a product there will be components that have a very low failure rate but have a wear out function that forms the right-hand side of the Bath Tub curve. For example, the product with a 400,000 device-hr MTBF might have a element that wears out at 20,000hrs as example.
One way of analyzing a product is to add up all the failure rates of the individual components. Commonly expressed as failures per billion hours this is referred to as the FIT rate (Failures in Time). For our example a product with a MTBF of 400,000 has a FIT rate of 2500.
Back to our example, a product with an MTBF of 400,000 that is operated 24/7 will have a failure rate of about (1 - exp(-t/MTBF)) in a year, or 2.2% annual failre rate (AFR). Because of the uniform distribution of failures a small sample of the failure rate may result in measured values ranging from 1% to 5%, but over a large population will be 2.2%. In 5 years this would be over 10% failures, or less than 90% reliability. In this light the MTBF of 400,000 no longer seems so good.
When designing a product from the start there should be a reliability target and a pro-forma reliability budget based on the target design. If the reliability target looks like it cannot be met then the architecture of the system needs to be re-thought out or the reliability target changed, understanding the business consequences.
MTBF is the inverse of the failure rate, NOT the life-time of the product. As example a product or assembly might have a MTBF of 400,000 device-hrs. This does not mean the product life is 400,000hrs (45.6 years)! It means the failure rate is 2.5 per million hours.
Implicit in the concept of MTBF or it's inverse failure rate is the that the failure rate is constant. Think of the MTBF as meaning that in any given hour the possibility of failure is 1/MTBF. This is applicable during the flat portion of the 'Bath Tub' curve showing failure rate versus time (see figure). Quite often in a product there will be components that have a very low failure rate but have a wear out function that forms the right-hand side of the Bath Tub curve. For example, the product with a 400,000 device-hr MTBF might have a element that wears out at 20,000hrs as example.
One way of analyzing a product is to add up all the failure rates of the individual components. Commonly expressed as failures per billion hours this is referred to as the FIT rate (Failures in Time). For our example a product with a MTBF of 400,000 has a FIT rate of 2500.
Back to our example, a product with an MTBF of 400,000 that is operated 24/7 will have a failure rate of about (1 - exp(-t/MTBF)) in a year, or 2.2% annual failre rate (AFR). Because of the uniform distribution of failures a small sample of the failure rate may result in measured values ranging from 1% to 5%, but over a large population will be 2.2%. In 5 years this would be over 10% failures, or less than 90% reliability. In this light the MTBF of 400,000 no longer seems so good.
Saturday, July 19, 2014
I delivered the first phase of a project to a new client. A passive test fixture for HEMT transistors that must operate at 1000Vdc and 200C continuous operation for 1000's of hours. Not super high tech but I did need to learn some new things to make this work.
The next phase is more complex with a custom embedded oven, temperature controller using a Microchip and data logging to boot with a custom Visual C++ GUI. The data logging will record temperature and Drain leakage current as the device is stressed over time.
I'll update the blog as interesting problems arise on this project.
The next phase is more complex with a custom embedded oven, temperature controller using a Microchip and data logging to boot with a custom Visual C++ GUI. The data logging will record temperature and Drain leakage current as the device is stressed over time.
I'll update the blog as interesting problems arise on this project.
Sunday, February 16, 2014
EMI Reduction of Laser Modulation
The topic discussed here involves the solution of an EMI problem caused by modulating a laser at 40Mhz. In EMI testing there were failures at multiple harmonics of the 40Mhz. The root of the problem is that the laser is a two terminal device and the case is electrically tied to the anode. A simplified LTspice circuit model of the laser driver is shown in Figure 1 showing the stray capacitance.
The stray capcitance, C1, from the laser case to the chassis ground allows for a high frequncy AC current path in the chassis of the assembly flowing back to the common ground. This current path forms a large loop which radiates EMI. The option of finding a 3-terminal laser with a grounded case was not available. So instead the circuit was re-designed to mitigate the problem. The choice here was to use the principle of a common mode choke (CMC). The modfied circuit is shown in Figure 2 with the choke driving the laser diode.
If the current flowing into the diode equals the current flowing out of the diode the flux changes in the CMC cancel and there is no impediment to the current flow. On the other hand if there is a current imbalance due to current flowing through the stray capacitance of the chassis then there is a differeintial current in the CMC. The differential current see's a high impedance of the choke and the amplitude of the current is diminished. There is no effect on the normal operation of the circuit, i.e., no reduction in the 40Mhz modulation of the laser current.
This solution very effectivley reduced the EMI problem and allowed the equipment to pass EMC testing.
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| Figure 1 - Original circuit |
The stray capcitance, C1, from the laser case to the chassis ground allows for a high frequncy AC current path in the chassis of the assembly flowing back to the common ground. This current path forms a large loop which radiates EMI. The option of finding a 3-terminal laser with a grounded case was not available. So instead the circuit was re-designed to mitigate the problem. The choice here was to use the principle of a common mode choke (CMC). The modfied circuit is shown in Figure 2 with the choke driving the laser diode.
 |
| Figure 2 - Circuit with CM choke |
If the current flowing into the diode equals the current flowing out of the diode the flux changes in the CMC cancel and there is no impediment to the current flow. On the other hand if there is a current imbalance due to current flowing through the stray capacitance of the chassis then there is a differeintial current in the CMC. The differential current see's a high impedance of the choke and the amplitude of the current is diminished. There is no effect on the normal operation of the circuit, i.e., no reduction in the 40Mhz modulation of the laser current.
This solution very effectivley reduced the EMI problem and allowed the equipment to pass EMC testing.
Monday, October 29, 2012
Laser Die Thermal FEM
While investigating laser life I spent some time involved in the thermal analysis of the elements of the laser case. Not having any published data on the thermal performance of the laser die and case led me to develop a finite element thermal model (FEM) of the package. To do this I first needed to find FEM software within my budget. After looking at several commercial packages that were crippled for evaluation I settled on an Open Source code.
The software I settled on is actually multiple packages. The heart of the software is Elmer which is a 3D solver available both as a GUI and command line solver (http://www.csc.fi/english/pages/elmer). The input I use to Elmer is a mesh in *.unv format. I generate the mesh by importing *.step files into Salome (http://www.salome-platform.org/) and defining geometry groups then meshing.
For viewing the results there are post processors built into Elmer, or you can use ParaView (http://www.paraview.org/) which is also Open Source.
The ElmerSolver is a multiphysics solver with no limitations on the number of nodes other than the computing / memory limitations of your hardware. Even further Elmer evidently is capable of parallel computing.
The software I settled on is actually multiple packages. The heart of the software is Elmer which is a 3D solver available both as a GUI and command line solver (http://www.csc.fi/english/pages/elmer). The input I use to Elmer is a mesh in *.unv format. I generate the mesh by importing *.step files into Salome (http://www.salome-platform.org/) and defining geometry groups then meshing.
For viewing the results there are post processors built into Elmer, or you can use ParaView (http://www.paraview.org/) which is also Open Source.
The ElmerSolver is a multiphysics solver with no limitations on the number of nodes other than the computing / memory limitations of your hardware. Even further Elmer evidently is capable of parallel computing.
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| Mesh |
The figure on the right shows the mesh of the laser package.
Below are the postprocessing results for the thermal analysis showing the overall package and a close up of the die.
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| Overall |
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| Die |
Elmer has been used for both steady state and transient thermal analysis and has aided greatly in the understanding of the laser thermal performance. I'm a fan of Elmer.
Sunday, July 17, 2011
Sensor Scheme with Ambient Light Correction
In studying estimation methods for control systems I once had a professor say "use all available information". This may seem trivial but it has served as very good advice over many years. In a recent system I have been designing a sensor array to detect light from a display for purposes of calibrating pixel timing versus RBG colors. This method is sensitive to ambient light corrupting the SNR of the signal. However it was observed that the measurement scheme is taking readings in the KHz time span but most ambient light is DC.
As an aside even a florescent bulb is primarily DC ambient luminance. The 60Hz AC mains is applied across a phosphor tube exciting the phosphor to emit light. But due to the persistence the luminance is primarily DC.
The scheme used for correction was to detect when the measurements are not being made. This was readily done by monitoring the control signal to an ADC. When the measurements have been idle for a period defined by a one-shot delay an analog multiplexer circuit integrates the output signal and applies a DC bias to the sensor transimpedance amplifier input. When the output is at zero DC the integrator stops moving and the output stays at zero volts. While a measurement is being made this circuit is opened and the integrator value is held during the measurement time. The result is a sample & hold feedback circuit that dynamically corrects for DC ambient light and prevents saturation of the input stages.
As an aside even a florescent bulb is primarily DC ambient luminance. The 60Hz AC mains is applied across a phosphor tube exciting the phosphor to emit light. But due to the persistence the luminance is primarily DC.
The scheme used for correction was to detect when the measurements are not being made. This was readily done by monitoring the control signal to an ADC. When the measurements have been idle for a period defined by a one-shot delay an analog multiplexer circuit integrates the output signal and applies a DC bias to the sensor transimpedance amplifier input. When the output is at zero DC the integrator stops moving and the output stays at zero volts. While a measurement is being made this circuit is opened and the integrator value is held during the measurement time. The result is a sample & hold feedback circuit that dynamically corrects for DC ambient light and prevents saturation of the input stages.
Thursday, November 4, 2010
CPC Optic
Lately I have been doing some simulation of a neat little optic device called a CPC. Short for Compound Parabolic Concentrator. This is a class of optic device known as a non-imaging optic. As the name implies it is used to concentrate a beam into a small area. It does so much more efficiently than imaging optics. Here is a picture of a CPC:
My Matlab program treats the CPC as a solid of revolution made of a material with index n2 and surrounding air index n1. The program performs ray tracing to find which rays reach the detector at the CPC exit aperture.
The figure below shows a Matlab result of several ray traces. Note the rays enter the entrance aperture and may bounce around several times, or not at all, before hitting the sensor at the exit aperture on the left.
CPC's can be made of molded plastic (Polystyrene for example) and therefore can be made very inexpensive in large volumes.
Because the incident angle can exceed the critical TIR angle, asin(n1/n2), the outer surface is metalized to preserve 100% TIR. Otherwise drop outs would occur at the sensor. This is shown in the angle scan of the next figure. In this figure a point light source is placed at 10mm in front of the entrance aperture on the CPC centerline and the angle is rotated through the full sweep to characterize the CPC.
Another item of note is that if the surface is not metalized then touching the surface will create evanescent coupling that will degrade the optical coupling
My Matlab program treats the CPC as a solid of revolution made of a material with index n2 and surrounding air index n1. The program performs ray tracing to find which rays reach the detector at the CPC exit aperture.
The figure below shows a Matlab result of several ray traces. Note the rays enter the entrance aperture and may bounce around several times, or not at all, before hitting the sensor at the exit aperture on the left.
CPC's can be made of molded plastic (Polystyrene for example) and therefore can be made very inexpensive in large volumes.
Because the incident angle can exceed the critical TIR angle, asin(n1/n2), the outer surface is metalized to preserve 100% TIR. Otherwise drop outs would occur at the sensor. This is shown in the angle scan of the next figure. In this figure a point light source is placed at 10mm in front of the entrance aperture on the CPC centerline and the angle is rotated through the full sweep to characterize the CPC.
Another item of note is that if the surface is not metalized then touching the surface will create evanescent coupling that will degrade the optical coupling
This has been my fun with CPC optics. I hope you found this interesting and maybe even useful.
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