Current Sensor

[Jay Lundy]

Originally Posted by Vladimir

I already checked to make sure all the connections were made properly (end of PWM cable pins to the pins on the chip, big leads to the big leads on the chip), I did not check resistance across the big leads, I will do that today. What should it be? I know it has to be very low because it is an ammeter...

I don't have ours right now to test with, but shouldn't it be basically 0? It's just a piece of wire that when current flows through it generates a magnetic field that is read by a hall element.

http://www.allegromicro.com/sf/0750/

[CNCBoy461]

We got our current sensors to work on the first try: from my understanding of the diagram given to us, the 3 pin mount for a PWM cable seems to go with the IR Beacon kit. So, I took the shell off of a PWM cable's male side and left the 0.100" IDC terminals on it. I put those terminals through the PCB board and they held quite well.
As for testing, we used a globe motor as the inline motor so as not to draw too much current on our first try. In the dashboard viewer and debug window, we got a standard reading of 507ish. After grabbing the drive shaft (With Gloves!) we got a spike of 570ish but dropped back down to the standard 507. The globe is very hard to stop and doesn't draw that much current anyway and so that’s the reason for the low spike, but in a few days we should have our chassis done and on the floor so that we can really test the sensor's full ability.
Reminders: Make sure to use a heat sink (And maybe a can of air), Use an iron that is from 700-900F, and test all of your connections...short circuits equals no working sensor.
As for the question of "Where to buy the sensor package?"...WE DON'T KNOW!!! Will someone please post a reply to this, we would like 2 more of them...and if not, what's the voltage rating on the capacitor? 4vdc or 6.3vdc? Good luck all!

www.boilerinvasion.org and see pictures of the sensor (Soon) and a CAD model of the new Maxi Fuse Block.

[Al Skierkiewicz]

Originally Posted by Vladimir

I believe I have the current sensor wired up properly but the controller reads a number around 867 and never moves from that, no matter how much current I pass through it. I'm sure the PWM cable to the RC is aligned correctly. I tested the analog inputs with another device and they work fine (0-1023).. if I disconnect it the value floats around as it should, connect it, 867. Any ideas? Is it defective? I heatsunk it while soldering to make sure it wasn't damaged.

Vladimir,
As many have suggested, get out your multimeter. Check first that you have 5 volts at the right pin on the chip. Then check the output pin, with no current flowing it should be at 2.5 volts. If it is very low, check that the capacitor has been inserted correctly. Double check that there is no solder bridge between pins or adjacent circuit pads, top and bottom of board. If it is at 2.5 volts then check that you have 2.5 volts at the input to the RC, PWM cables are known to go bad. It is very common for a three wire cable to get reversed when looking at it upside down so double check that the right wire goes to the right hole.
Now, if anyone has any real experience with these can you express whether external magnetic fields affect the output? Thanks,

[Biff]

Originally Posted by CNCBoy461

As for the question of "Where to buy the sensor package?"...WE DON'T KNOW!!! Will someone please post a reply to this, we would like 2 more of them...and if not, what's the voltage rating on the capacitor? 4vdc or 6.3vdc? Good luck all!

www.boilerinvasion.org and see pictures of the sensor (Soon) and a CAD model of the new Maxi Fuse Block.

The Cap is limiting noise to the controler and slowing the response(slightly), voltage should be 6.3 but a higher rateing will not hurt anything and is a saftey factor. Tanyalem Caps short when they fail. Check with ohm meter, should read low to start then go to a high vaue as the cap charges. Any thing below 10 ohms after a few seconds and the cap is hosed. You allready mentioned not putting them in backward. The ACS750-75 has a link to buy more sensors at http://www.allegromicro.com/sf/0750/ don't know about the circut board Good luck.

[CNCBoy461]

I posted here...didn't want to do it again: http://www.chiefdelphi.com/forums/sh...ad.php?t=24004
Thanks Biff for the voltage on the cap.

[AustinPowers]

Make sure you are using the Get_Analog____ (Ican remeber the complete name but it is in IFI docs) routine to get the information from the Analog port. When using printf use an (INT) casting. The doc show how to read the port. I second using the mutltimeter to make sure the sensor is connect properly. Use a potentiometer on the analog port to check to make sure your program is working. Any questions just ask.

[Mercutio]

How do you tell the polarity of the capacitor? I'm holding two big mustard-colored things taped on a strip that look like capacitors. They each say

157
+16K|

in brown writing on one side. Which side has the positive lead? I've heard that bad things can happen when you hook up electrolytic capacitors backwards, so i'm a little nervous about just doing trial and error.

~Aaron

[JRHaddock]

The plus sign (+) by the number 16 indicates the positive terminal of the capacitor.

[Stephen P]

Are the current sensors necessary by FIRST rules cause we cant find ours.

[Damian Manda]

Originally Posted by Stephen P

Are the current sensors necessary by FIRST rules cause we cant find ours.

No, the current sensors ar not required, but can be very helpful in determining problems with motors, or just monitering them during competition. We have yet to build ours, we have been concentrating on making the IR work.

[Biff]

Originally Posted by Mercutio

How do you tell the polarity of the capacitor? I'm holding two big mustard-colored things taped on a strip that look like capacitors. They each say

157
+16K|

in brown writing on one side. Which side has the positive lead? I've heard that bad things can happen when you hook up electrolytic capacitors backwards, so i'm a little nervous about just doing trial and error.

~Aaron

The other way to tell is if one wire on the cap is longer it is the positive, this is kind of a standard in the industry. If the are the same or have been cut look for the marking, IE as the next post had said the lead next to the positive sign + is the postive. Some kinds of Caps are marked on the negitive side only.

[B. Back]

The current sensor pcb can be purchased from UPE-INC. Their phone number is (330)659-9287. As someone else noted, the pcbs cost $12.50 each and there is a min. qty requirement of 2 units. They are currently out of the boards, but expect more in house in about a week.

[Biff]

This is a kind of on subject question. I was reading a white paper on making current sensors by using an Op amp to get usable input voltages from the voltage drop across the fuses. The paper was written by Mr Lewis, mentor of team 811, back in 2002. I like the design, but after exhaustive research, the only acquireable parts are surface mount. Not a DIP in stock at the approved vendors or even some brokerage houses. He mentions the resistance of the 20 amp fuses at about 9 milliohms and the 30 amp at about 6 milliohms. I don't have a meter that will get me readings that low. Does any body have a resistance value for the 40 amp breakers or a test rig to get there under load. If I do the circuit with surface mount I don't want to take the gain resistors on and off to get a usable range. Thanks in advance.

[Al Skierkiewicz]

Originally Posted by Biff

I like the design, but after exhaustive research, the only acquireable parts are surface mount. Not a DIP in stock at the approved vendors or even some brokerage houses. He mentions the resistance of the 20 amp fuses at about 9 milliohms and the 30 amp at about 6 milliohms. I don't have a meter that will get me readings that low.

Biff,
Op amps are pretty "vanilla" in that you can drop almost any device in place for a given circuit and it will work. In this application you do need a low input offset voltage specification. I remember at the time that a lot of discussion occured over different methods of measuring current. While the breakers do exhibit some resistance, this a variable especially if the breakers have ever tripped. We used the resistance across a 1 foot long #10 guage wire as it is close to .001 ohms/ ft. Remember to measure the current on the supply side of the speed controller as this doesn't change polarity.

[Biff]

Originally Posted by Al Skierkiewicz

Biff,
Op amps are pretty "vanilla" in that you can drop almost any device in place for a given circuit and it will work. In this application you do need a low input offset voltage specification. I remember at the time that a lot of discussion occured over different methods of measuring current. While the breakers do exhibit some resistance, this a variable especially if the breakers have ever tripped. We used the resistance across a 1 foot long #10 guage wire as it is close to .001 ohms/ ft. Remember to measure the current on the supply side of the speed controller as this doesn't change polarity.

Thanks for the heads up about resistance variablity after tripping. The white paper I quoted was using "rail to rail" input op amps. These are findable but mostly in surface mount. If I get time I may take a run at it and go with a gain factor, for a given wire length. I am planning at this point to go with #6 right to the supply side of the Maxi 40 amps and drop to #10 or #8 to the Victors. If we can keep the wires from the Maxi's to the Motor victors the same length that would be the ideal sense point. Thanks Again.