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Opportunity General Health
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post Aug 17 2015, 06:22 PM
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I did not know about a good log for this problems. In the english Wikipedia is a small chapter of the failures of Spirit: https://en.wikipedia.org/wiki/Spirit_(rover...ar_and_failures

I am also interested in such a timeline, it could be interesting for the Wikipedia.

Here is a list about the equipment failures I know:
Opportunity:
- A) The shoulder of the instrument arm stopped working at Sol 654 (November 25, 2005). The Joint-1 azimuth motor stalled because of increased electrical resistance (Source: https://en.wikipedia.org/wiki/Opportunity_m...ulder_troubles).
As I know opportunity does not stow the arm anymore and drives in a "fisherman" position over the martian surface.
- cool.gif Also Opportunity had problems with the heater. Opportunity happens to have a heater stuck in the "on" position that draws additional power. (see http://mars.nasa.gov/mer/mission/status_op...tyAll_2007.html, Sol 1316)-
- C) One wheel cannot be steered anymore, and is locked in a specific position. But the navigators have learned, to drive with this issue.
- D) Also, Opportunity drives backwards since many year , because there always had been elevated currents in one of the wheels.
- E) The MiniTES does not work since the sandstorm 2007.
- F) The Mössbauer instrument does not work anymore, but I think there is no exact time stamp since it stopped working, it was just a permanent degregation (as expected)
- G) The amnesia events since one year (Wikipedia EN: Early September 2014)


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hendric
post Aug 17 2015, 08:19 PM
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There's a couple of advantages the MER batteries have vs typical LiIon batteries here on Earth:

1. No high-temp storage or operation. How often is a laptop/phone left in a car or trunk in the heat? And turned on without waiting for the battery to cool down? Low operating temps extend battery life and prevent self-discharge.

2. Relatively low drain vs their capacities. While fairly large batteries, their drain doesn't look that large vs their capacity. During the highest drain activities in the daytime , the solar panel provides additional current. During nighttime the primary activity might be radio comms without movement. Laptop and cellphone batteries have frequent high-drain activities while disconnected from their charging system, ie watching a video over the air.

3. Relatively slow charging cycles vs their capacities. People need fast charging for their batteries, so consumer charging circuits typically charge at max rates - 1C/Ah typically vs what looks like .2C/Ah peak for the rovers.

4. Low-vibration environments. Well, other than launch and EDL, the vibration environment is pretty benign on Mars. Having to design for launch/EDL levels of vibration might also have endurance side effects - better connections, etc. Phones/laptops are often dropped or jarred on tables, etc. Probably a minor effect.

5. Individual cells are balance-charged. Lithium chemistries really, really hate being overcharged. What typically kills cheap laptop batteries is a design that uses a single charging circuit, passing through all the cells. These cheap designs cutoff charging once all the cells have reached nominal. If a cell reaches nominal early, it gets overcharged while waiting for the slowpokes, hurting its capacity and making it hit nominal even earlier the next charge cycle. Typically when new all the cells are more-or-less matched, but over time they drift apart. This is more important for LiPoly chemistries than LiIon, but it still matters for battery lifetime over a few hundred charges. People can tear apart used laptop batteries to get the "good" remaining cells for other uses.

6. Not charging to full rating. Continuously charging a Lithium battery to its full rating will also hurt lifetime. Newer laptops can be setup to only charge to 50-80% during the week and 100% on the weekends, or on-demand. Looks like other than EDL, the MER batteries kept below ~75% capacity. Old laptops used to keep their cells at 100% all the time while plugged in for extended amounts of time, seriously compromising their performance. Smarter chargers now charge to the requested level, and shutoff until the level has dropped an amount, typically 3-5%. They also can be setup so that the system load is not run directly off the batteries, allowing the charger to provide power directly to the system without routing it through the battery first. Also, the MER charging is always done in a pretty benign environment, with batteries that are presumably pretty cool.

7. No intentional deep-discharges. This, IMO, is the biggie. Looking at the graph in the paper, the only time MER batteries were ever deep discharged was the Spirit anomaly, otherwise they are kept at about 50% DoD. The anomaly drained her batteries to the cutoff voltage (Lithium chemistries are damaged when drained completely, unlike NiCd or NiMH. Batteries typically have a self-protection circuit that cuts the ground line below a minimum voltage during discharge.) People often drain laptops and phones until the battery is "dead" with the internal cutoff circuit activated - what's worse, they often turn the phone "back on" - with the phone off the battery rises slightly above cutoff, and during boot the power consumption is much lower so the battery continues to drain until the radios are activated and then the phone dies again. These deep discharges really hurt battery lifetime.

I don't know how accurate the capacity measurements are for the MER batteries, typically a gas-gauge chip is placed in series with the battery and reports capacity basically by comparing current IN versus current OUT. Over time they need to reset their count by doing a "training cycle" - to relearn the battery capacity. Without these training cycles the reported numbers get more and more inaccurate - if you have ever had a device that went from 30% charge down to "OMG I'm gonna die" it's because its gas gauge needed to be retrained. The reported % number is based on a certain capacity, if your battery has degraded below that capacity number, as it drains the battery self-protection circuit flags a warning before it cuts off the battery, hence the OMG message. Typically there is a maximum amount the capacity can go down during each training cycle, so it might take more than one to become accurate again. Since there are two packs per MER, it's possible one battery pack could do a training cycle while the other is in use, but it doesn't seem like that's reflected in the graphs. Maybe they are estimating capacity based on the cell voltage, which is pretty inaccurate (hence the invention of the gas gauge chips). It does look like newer gas gauge chips model aging internally, not sure what was space-rated 15 years ago during MER development though. http://www.ti.com/product/bq27741-g1 is a sample gas gauge chip.

I've done some embedded HW/SW development for cellphones and mobile devices using LiIon and LiPoly batteries, but I am not a battery engineer. smile.gif


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ddeerrff
post Jun 3 2017, 02:56 AM
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Do I remember right that Oppy's wheel motors have brushes? If so, how in the world (or how on Mars) can those brushes not be totally worn away? Yet Oppy keeps on going, and going, and going..... wheel.gif wheel.gif wheel.gif
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serpens
post Jun 3 2017, 07:04 AM
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Yes, both drive and steering motors on Oppy are brushed which made good design sense given the 90 day, 700 metres driving distance mission success profile. The loss of the steering motor is possibly due to brushes as was the drive motor failure on Spirit, but given the staggering driving statistics for Opportunity the longevity of the motors is astounding. Well to be honest the Lithium Ion batteries are also performing brilliantly. I suspect that the lions share of the credit for both the performance of both batteries and drive motors accrues to the management of the MER engineering team and drivers.
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RoverDriver
post Jun 3 2017, 01:09 PM
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The wear and tear on the brushes is proportional to each actuator use. If you take the current rover odometer, divide by one wheel circumference and multiply by 1500 (the gear ratio) you get a decent number (100 million revs) which is not too shabby but not too out of specs.

Oppy RF steering actuator if I remember correctly (it happened close to 14 years ago!) the most likely theory was that the magnetic detent came unglued and got jammed into the motor.

Spirit RF (and at the end the RR) drive actuators instead were suffering from what appeared to be a contact failure between the brush and commutator as if there was a non-conductive layer deposited over time. We called that "napping motor".

This is also different from the Joint 1 actuator on Oppy's IDD. That one was attributed to a broken winding wire due to fatigue caused by thermal expansion/contraction ultimately due to the stuck shoulder heater.

All actuators on MER are brushed motors, except the stepper used on the MI dust cover mechanism. That one was built as an open loop control. Open loop is now also used on the three RAT actuators, but that is due to the loss of signal from the encoders due to wear and tear on the IDD flex cable).

Now you know almost as much as what we tell to new rover driver trainees.

Paolo

PS: questions... longevity... incept date... Eyes, I only do eyes!


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hendric
post Jun 3 2017, 06:16 PM
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I envision a ceremony where new rover drivers are given The Tome, a collection of all knowledge on the rovers, that has a page on Rover Longevity that is just a URL to this discussion. smile.gif


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Space Enthusiast Richard Hendricks
--
"The engineers, as usual, made a tremendous fuss. Again as usual, they did the job in half the time they had dismissed as being absolutely impossible." --Rescue Party, Arthur C Clarke
Mother Nature is the final inspector of all quality.
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