CT for current readings

[amesmich]

I have read some teams in the past have monitored current on motors. If so what CT (current transformer) or other device did you use? I was thinking that it may be a good idea to monitor total current draw and have the code throttle back before the 120a main trips out rendering the bot useless then it being too late. I am also curious as to what teams use to get an air pressure reading? What transducer. I know there are many but though some might have a favorite.
Thanks

[EricVanWyk]

Current Transformers are really only usable for AC currents.

Placing a resistor in line and measuring voltage drop is great in theory, but the currents we deal with are rather high. 120A through a milliohm is 14 watts, and your robot will draw several times that routinely (for relatively short time scales). Some teams have taken to using their existing wiring as a series resistance and met with success. As the wire heats, you will lose about half a percent of accuracy per Kelvin (ish). Likely doesn't matter for this application.

I suggest using a hall effect sensor. Allegro Microsystems make some really nice in-line ones: http://www.allegromicro.com/en/Produ...ensor-ICs.aspx

Melexis makes one that I've used before with mixed results: http://www.melexis.com/Hall-Effect-S...SA-1V-614.aspx
For this, you "simply" place the wire you want to measure near the sensor and measure the magnetic field. Your accuracy will then depend largely on placement. Great for relative readings though! It also doesn't wrap a flux guide around the current source, so it may be more prone to external noise sources than packaged solutions.

There are also things that look like current transformers, but actually have a hall effect sensor and a magnetic flux guide inside them. I don't have a part number on hand for those right now. Very nice, but typically pricier.

Be careful to read the electrical rules as to which might be legal.
Also, you might want to consider the benefit gained by the system. If you haven't popped the breaker yet, you may just be hobbling your capabilities.

[amesmich]

Thanks for that. I realize the issue with DC not moving to inductance in a transformer is not possible. I though there was something else and like you said hall effect. I found one but its expensive for the small benefit it adds. Thanks for your reply. I do appreciate it.

[mgerber]

The Allegro sensors that Mr. VanWyk linked to seem to be perfect for the job and at $6.50 a pop I'd doubt they would be breaking anyone's budget. If your team still has the flier for digikey, you could get them for free

[philso]

The LEM HASS and the Honeywell CSLA families may work better in these applications. They slip over the battery cable so it is not necessary to cut the battery cable. They are also isolated from the conductor. Lastly, the power and signal connections are made using a standard MOLEX-type connector so it is more servieable than the PCB mount types. The two I have linked to use single supplies (+5V for the LEM and +8~+16V for the Honeywell) and are not a lot more money. If you are using the Honeywell part, you may have to use a couple of resistors to divide the output voltage down to make it suitable for the Analogue input on your C-Rio.

www.digikey.com/product-detail/en/HASS%20100-S/398-1063-ND/1680529

www.newark.com/honeywell-s-c/csla2cf/current-sensor/dp/10M8410

[Al Skierkiewicz]

When WildStang first tries this we simply used 1 ft. of #10 wire that was already feeding the speed controllers. That is a pretty good .001 ohm resistor and the power loss is part of the delivery so you aren't adding additional loss.

Please remember that pressure transducers are illegal under FRC rules as answered by the Q&A earlier this season.

[Mark McLeod]

Might be an over generalization.
The Q&A only made it clear that pressure transducers were not permitted on the high pressure side.
They aren't illegal on the low pressure side, as long as they adhere to all other pneumatic rules, e.g., 125psi rating, etc.

Originally Posted by Q&A Q428

Q.Are pressure transducers (analog pressure sensors) legal for use on the high pressure side of the pneumatic system, in addition to the standard pressure switch used to control the compressor? Assuming that all specification (maximum pressure, etc) are met.

A.No. Please see [R82].

Originally Posted by R82

Only the compressor, relief valve (P/N: 16-004-011), pressure switch, pressure vent plug valve, pressure gauge, storage tanks, tubing, and connecting fittings may be in the high-pressure pneumatic circuit upstream from the regulator.

[Al Skierkiewicz]

Mark,
I have received numerous questions on this and actually saw two robots over the weekend using the transducer in addition to or in place of the pressure switch on the high pressure side. I never thought about teams using them on the low pressure side as they would only provide data when the storage pressure drops below 60 psi.

[Mark McLeod]

We have used transducers only on the low side in the past.
There are always other purposes
2012 was the last time.

For high side pressure I've always used estimation along with the regular pressure switch and that generally works out fine.