SD540 Motor Controller

[marshall]

Originally Posted by geezloueez

What is the lowest you have witnessed in a match?

I've seen my share of robots in brown out conditions. It's less severe these days than it used to be but it can still cause you some heartache at competitions. This is from one of the presentations on the control system last year:



And this is current (no pun intended) info about what happens and when:

http://wpilib.screenstepslive.com/s/...g-current-draw

EDIT: You can help prevent this by keeping happy batteries around and always keeping good ones in the robot.

[notmattlythgoe]

Originally Posted by geezloueez

What is the lowest you have witnessed in a match?

A temporary drop to 8V is a pretty common occurrence. I don't know what the absolute lowest is though. Current draw is going to be a very important thing to watch in the coming years.

The compressor turning on can drop the voltage a good 1-1.5V itself.

Edit: See Marshall's post.

[Knufire]

Originally Posted by asid61

Hence why batteries in FRC have such a low lifetime... Interesting tidbit.
We don't use batteries from 2011 or earlier. And we're probably going to start phasing out the 2012 batteries too. They start being unable to hold a significant charge.

We only use batteries in competition for a single year. After that they'll be used for practice for 2-4 years and then recycled.

[Andrew Schreiber]

Originally Posted by geezloueez

What is the lowest you have witnessed in a match?

<6V followed very closely by the robot stopping moving as the CRIO reset. Though occasionally we did see it spike that low and come back up.

[Jared Russell]

Originally Posted by geezloueez

I am a first year mentor for a rookie team. May seem like a dumb question, but I have not been involved first hand in a match yet. What is causing the batteries to drop so low? Are you using old batteries?

Internal resistance is usually on the order of ~0.01 ohms for the sealed lead-acid batteries that we use.

If your battery is charged to, say, 12.7 volts and you are running 4 CIM motors at stall (= 4 * 131A = 524A current), you can expect a voltage drop of ~5.2V just due to battery resistance (in reality, there are other losses in wiring, connectors, and speed controllers as well, so treat this as an approximate). 12.7 - 5.2 = 7.5V.

This situation happens (instantaneously) any time you rapidly change direction assuming your wheels don't slip on the ground first. Once the drive is moving, your motors draw less and less current, and battery voltage quickly recovers.

You can imagine that a 6 CIM drive, or simultaneously driving while powering mechanisms or the compressor, will only make things worse. I have seen robots drop below 6V relatively frequently in other seasons. Design with care!

[pnitin]

Originally Posted by Chris is me

Momentary drops to 9.5vs are not at all uncommon in FRC matches, depending on the number of motors on the machine, weight, traction, etc. It might not be great for batteries but I'd rather be able to drive for the rest of the match and potentially shorten a battery's life than not be able to drive to save a couple of bucks later on. Basically, I don't want the products I use to try and protect me from myself.

Actually what I am trying to say here is if you treat your battery well, it will serves you faithfully, if you abuse it, it will not serve you well. Momentary drops are called spikes and most of the filter will take care of it ( you will not see brown out for spikes) I think problem is when you want to start with 9V battery to begin with, you have problem.

That way CTRE chart is incorrect since no one ( sane person) will be turning on robot with 9V battery level in competition.
what you really want to see is how well they handles those spikes.
.

[Jared Russell]

Originally Posted by pnitin

Momentary drops are called spikes and most of the filter will take care of it ( you will not see brown out for spikes)

Citation needed.

[aldaeron]

https://xkcd.com/285/

[scca229]

As an FTAA watching plenty of matches during competitions from close behind the drivers (anyone at events where I've FTAA'd can affirm that I move around a LOT), I see MANY driver station displays with the battery section flashing red throughout the match to indicate voltage momentarily dropping below thresholds. Not sure a match goes by that doesn't have at least 1 robot where we are watching for a brownout so that we can quickly inform the team why the robot isn't moving when they tell us they lost connection to the field. I try to inform the coach as the match is still going on so that they see it happening in realtime and not just get told after the fact so they know what to look for when trouble starts.

I also kind of make a habit of looking at the battery voltage readout on each DS as I pass them verifying connection to the field to get an idea of which bots might be in trouble in the case that I see a sub-12V reading on the display. Sub-11.5V (and barring being way behind schedule) and I ask them if they have another battery next to the field that can be quickly swapped.

[Mark McLeod]

Originally Posted by geezloueez

I am a first year mentor for a rookie team. May seem like a dumb question, but I have not been involved first hand in a match yet. What is causing the batteries to drop so low? Are you using old batteries?

Here are a couple of real life examples to give you an idea of what you have to be prepared to manage in your power design.
Both Driver Station logs are from different teams that borrowed one of my laptops during competition.
Learn how to examine your own DS logs after a match. They are automatic and just brimming with useful data about how your robot performed.
Remember, too, that the power drawn during practice at home is tame compared to power drawn during a real match with competitors.

The yellow line shows the battery voltage for the duration of the match.
No roboRIO brownouts were experienced by either team during these logged events.

Both of these robots had good batteries.
These voltages are what the roboRIO and speed controllers directly experienced.
The first example is from a robot during an off-season event this past October with a large number of motors - drive, lift, tote grabbers all running.

The second example is also from an off-season event, but one held in November. It was a robot with four drive motors and one lift motor, and shows a lot less stress.

The biggest dips are when the motors are starting up from a complete stop, lifting a heavy load, or suddenly reversing.
These logs are taken from a game without active opposition. Expect much worse this coming season.

[s1900ahon]

Originally Posted by pnitin

Momentary drops are called spikes and most of the filter will take care of it

What filter are you referring to?

[pnitin]

Originally Posted by sastoller

Since you guys seem to be SD540 experts in this thread, I was wondering if you could comment on the very high output resistance of the SD540 that was measured by CTR in their tests (surely you've seen the results). The voltage drop (power lost) in the motor controller is drastically worse than the competition? Did you guys make a design choice or tradeoff that resulted in this? Or is it possible that CTR just got a bad (or damaged) SD540?

And I'll also echo Chris's question regarding a FW update to modify the 9.5V brownout feature. Could you make this voltage threshold user adjustable in the future so that users could control which motor controllers shut off first? Would it be possible to add a throttling feature to reduce output to 50% when a certain voltage threshold is reached?

http://www.ctr-electronics.com/downl...please)?<br />

All FRC the controller on market use synchronous rectification,
they are not linear converter, so what is mentioned above is incorrect.
for example, consider your cellphone charger 115V In 5V Out @ 1A current,.
According to your theory it should dissipate 110W and should melt, but it does not. It dissipate much low power since it uses switching topology.

[mman1506]

Originally Posted by pnitin

Actually what I am trying to say here is if you treat your battery well, it will serves you faithfully, if you abuse it, it will not serve you well. Momentary drops are called spikes and most of the filter will take care of it ( you will not see brown out for spikes) I think problem is when you want to start with 9V battery to begin with, you have problem.

That way CTRE chart is incorrect since no one ( sane person) will be turning on robot with 9V battery level in competition.
what you really want to see is how well they handles those spikes.
.

Are you trying to say that the SD540 looks at an average voltage reading rather than a instantaneous reading so current spikes would not cause it to brown out? Thus making a low voltage cut off to protect you from draining the battery below a 9v state of charge?

[Daniel_LaFleur]

Originally Posted by pnitin

All FRC the controller on market use synchronous rectification,
they are not linear converter, so what is mentioned above is incorrect.
for example, consider your cellphone charger 115V In 5V Out @ 1A current,.
According to your theory it should dissipate 110W and should melt, but it does not. It dissipate much low power since it uses switching topology.

Incorrect.

The output is 5VDC@1A not the input. The input is closer to 115VAC@0.05A or ~6Watts (assuming 1W inefficiency which is actually high).

In a phone charger (or most other chargers/USB power supply sources) you have a AC to DC converter (most likely a bridge rectifier). Then you have a buck converter (probably a transformer) and a switcher boost (High speed MosFET, inductor, and shottkey diode).

[pnitin]

Originally Posted by Daniel_LaFleur

Incorrect.

The output is 5VDC@1A not the input. The input is closer to 115VAC@0.05A or ~6Watts (assuming 1W inefficiency which is actually high).

In a phone charger (or most other chargers/USB power supply sources) you have a AC to DC converter (most likely a bridge rectifier). Then you have a buck converter (probably a transformer) and a switcher boost (High speed MosFET, inductor, and shottkey diode).

Exactly ,
Same is applicable for motor driven by these motor controller, input current and output current are not identical due to switching and stored energy in motor inductance. so you can not just calculate power dissipation in switch by looking at difference in output voltage and output voltage and output current