Voltage Drop problems

[Dale(294engr]]

Hi John.. You have received lots of good advice.

Go on a current hunt..

If you don't have a clamp-on current meter..

Use your DMM to measure current as follows:

Select lowest DC mV scale (Harbor Freight use +-200 mV = fine)
Place it across (parallel to) the 120A circuit breaker terminals
(nice to mod w/alligator clips else add pin jacks to CB for stock DMM probes)

When closed, 120A CB is pretty fair 1 milliohm resistance (actually tad less)

Hypothesis: CB internal "R" must be constant even tho 'switched' a lot else trip action would likewise wander

each 1 mV read is now 1 Amp drawn, a good estimate! and Super simple.

With robot off floor (drive wheels free spin)

with nothing else activated
take note of baseline .. should be ~1-2mV =1-2A for control sys/lights
subtract this constant 'quiescent value' from all readings for more accuracy

Activate each motor one at a time..
full fwd measure / record - log,
then full reverse (like gatherer can't reverse else good to test both directions and compare)

for ea motor, "no load current" represents mechanical friction of motor, gears, chains, crooked bushings or sprocket (poor or inefficient load tensioner), etc

if you previously measured each motor alone, both Fwd /Rev and they are, & should be very close and low (~3A for CIM)
subtract this constant from measurements to isolate friction caused problem portion, record in log for future referral

During competition pit stops. this quick robot ready test reveals potential growing problem as upward creeping current long before driver is aware of pending problem

Do this for your ball gatherer as priority to isolate if its the root problem.

This catches many problems early

but if there are loose, wobbly or binding bushing/bearings, condition likely worsens when turning (lateral forces on bearings).
A bit harder to check with this method especially if robot is moving, so dangerous. SAFETY FIRST

Differential A/D channel(s) could be programmed to capture a table of values every .2 sec or so and read back from memory using debug later.

Good luck

.. hope this 'method' proves useful

.. I started using this technique with Team 294 in 1999

introduced it in workshops at Chatsworth (Team 22 - Wendy Wooten)

Been suggesting /demonstrating it to teams ever since as I perform Robot Inspections (60A CB was trickier.. due to bigger diversion from 1 milliohm)

P.S. when 120A brreaker is open and DMM sees 12-13v it will merely read overload with no harm to DMM in the mV mode.

(You can 'see charge current as lowering of quiescent draw if battery is placed on charge while in robot in this cfg.. if charger puts out 5A and Q=2A meter will read -3 mV

the math -2mV Q -3 mV dmm = -5mV = -5A = charging!)

Caution Never use DMM current scale for this test! (CB 'open' will fry internal shunt)

It's all in the math:
I=V/R example 1 millivolt/1 milliohm =1 Amp (or 1A per millivolt)
V= DMM reading
R=resistance of 'shunt' which s this case is th eclosed resistance of 1`20A CB

Measurement range with HF DMM +-199.9mV = +-199.9A

BTW can put constant load on victor or jag (old car headlight = 10A or several parallel up to ~5 for max 40A-CB hold current of 50A (=125%*40A)
More current drawn = larger voltage drop across faults so easier to 'see'
if on M+ & - PWM can be used to set a current, or on Batt+- for fixed max load to test all intervening wiring
Jiggle wires, terminals, connectors.. varying reading =problem

[waialua359]

While our team has not measured the voltage drop issues you have, we instead countered it by not having the ball travel a large distance from our feeder into the shooter head.
By keeping it a short consistent distance prior to shooting, we limited the amount of draw from the feeder in order to shoot the ball. All of our set points for aperture and motor speed has been based off the exact same feeder location of the ball prior to shooting.
I found at least a couple of teams that asked us about such issues with their own robots, when we participated in Hawaii this past weekend.

A simple solution is adding a VEX touch sensor.
We also have a custom dashboard indicator that tells our operator once the shooter wheels are at the max velocity at our particular location on the field.
If you get to watch some of our matches this past weekend on JustinTV, you'll find it works quite well, where we hit consistent shots, and quickly between shots.