Voltage sensor

[Andrew Blair]

I'm really sorry if this is a repeat thread, but I can't find anything. I'm wondering if you can do it off the new breaker panel.

How can you give the voltage draw of a motor to the brain?

[billbo911]

Originally Posted by Andrew Blair

I'm really sorry if this is a repeat thread, but I can't find anything. I'm wondering if you can do it off the new breaker panel.

How can you give the voltage draw of a motor to the brain?

Let me see if I'm following your question.

You are trying to read the voltage applied to a motor, correct?
Or, are you looking for the current being drawn by a motor?

The voltage measurement at the breaker panel will only provide the main battery voltage. Remember, there is a speed controller between the breaker panel and the motor. So, to read the voltage applied to the motor, it would have to be done at the output side of the speed controller. Be careful with that, if I'm not mistaken, the RC analog inputs can only measure 0 to +5vdc. On top of that, to measure motor voltage correctly means the motors need to be measured under load. Therefore, motion will be involved, watch your extra body parts
Measuring current is another matter. I don't believe we have an easy way to do that with the tools provided by FIRST.
Please speak up if I'm off base on any of this.

[Andrew Blair]

Very sorry, I meant to say current. I wanted to know if we had, or could obtain a kit legal current sensor.

[Mike AA]

Originally Posted by Andrew Blair

Very sorry, I meant to say current. I wanted to know if we had, or could obtain a kit legal current sensor.

back a few years ago there was a current sensor in the FIRST KOP. I dont remember much of the specs but it was made with only about 3 components. I'm sure if you looked through some older first manuals you could find the specs and what it was made of. I'll look and see if I can find one I've got laying around.

-Mike
Team 1654

[Gdeaver]

If you check the forum from back in believe it was May or June there was a thread on Victor voltage output, PWM, h-bridges, FETS and more. You may want to review that entire thread and some the links that were posted. After you read that you'll find that current is probably a better measure of the power going to a motor. To measure current, First provided an Allegro current sensor in 2004. Your team may already have the hardware to measure current. You can look in the First 2004 archive and find the data sheet. The Allegro sensor is a hall effect device and outputs a voltage that can be read by one of the FRC analog to digital ports. There are other ways to measure current but the Allegro sensor is a good choice for the high amps our motors draw.

[Hutch]

Conveniently, the people who make the hall effect gear tooth sensor, Allegro, also make current sensors. See http://www.allegromicro.com/sf/0754/ for details. The most a FIRST team should ever need is a ACS754SCB-130-PFF (130 amps max current, and acceptable operating temp range) - those only cost $6.60 a peice.

You could always roll your own but I don't feel it's worth it (the price is low, and you could do more innovative things than current sensors ). That's not to say it isn't worth at least trying to roll your own...

[eugenebrooks]

Originally Posted by Andrew Blair

Very sorry, I meant to say current. I wanted to know if we had, or could obtain a kit legal current sensor.

As posted in this thread, a pair of 75 amp Allegro current sensors were in the 2004 KOP. See the link
http://www.allegromicro.com/demo/asek750.htm
for product information, specification sheets, and links to buy evaluation kits.

Whether or not you can actually use these on your 2006 robot depends upon whether they are in the 2006 KOP, or if they are available from an allowed vendor of electronic parts as defined by the 2006 rules. Refer to the technical documentation for a power bypass capacitor recommendation and cautions with regard to resistive and capactive loading of the output signal. Remember that your current signal is likely to be unsteady, as for the pulse width modulated control delivered by a Victor, and you will either need to condition the signal with analog electronics or you will have to process the signal appropriately with the robot controller.

[BrianBSL]

Although obviously none of us can say what the 2006 rules will be, last year they got rid of the allowed electronics vendor list and allowed all COTS parts, so unless that changes (I can't really see them going back to the very restrictive old rules), you should be able to buy these from any of Allegro's dealers or use samples (although you have to account for the cost as if you bought them from a dealer).

Make sure you get one in the right range of what you are looking to measure. If you were to use this to measure the current of something drawing 10 Amps or so, it would be pretty useless as it goes from -75A to +75A over the 5V range, and you are reading it on a 10-bit AD converter.

If you really wanted to measure voltage of the battery on the RC, a simple voltage divider (split it in 1/3 or so) would suffice.

Originally Posted by eugenebrooks

As posted in this thread, a pair of 75 amp Allegro current sensors were in the 2004 KOP. See the link
http://www.allegromicro.com/demo/asek750.htm
for product information, specification sheets, and links to buy evaluation kits.

Whether or not you can actually use these on your 2006 robot depends upon whether they are in the 2006 KOP, or if they are available from an allowed vendor of electronic parts as defined by the 2006 rules. Refer to the technical documentation for a power bypass capacitor recommendation and cautions with regard to resistive and capactive loading of the output signal. Remember that your current signal is likely to be unsteady, as for the pulse width modulated control delivered by a Victor, and you will either need to condition the signal with analog electronics or you will have to process the signal appropriately with the robot controller.