Mystery L298 issue
T Chap
Posts: 4,223
I have a Seeed Studio motor sheild from Radio Shack. A few days ago, I connected it to a car lock motor, powered by a 12VDC 1.5am power supply. A Propeller sends a 3v3 signal to the input 1 to move the lock one direction, another input returns the lock. The first day, I ran over 30,000 cycles overnight. The next day, made some hardware changes and left it to run all night, working perfectly for the first hour. The next day, the lock is not moving, but is making the attempt to move but acts like it has no power. So I connected a brand new L298 driver(identical), it is doing the same issue, acts like it doesn't have enough power. So I connect the inputs to the Seeed Studio motor shield direct to 5V instead of the Prop pin, no change. I connect the motor wires direct to the power supply leads and it works great. This makes no sense. Any suggestions on what to check for? I can't check now but tomorrow I will test the cables for a bad ground between the Prop system ground and the supply to the motor driver.
Comments
While 30,000 cycles sounds OK over the life of a car door doing it in 8 hours seems a bit much.
I think these things have a max duty cycle spec. If driven to much the windings could get hot and have shorts.
Shorted windings in motors tend to make them not run.
Duane J
Ummm, have you really considered that fact that the L298 h-bridge provides 2 diode voltage drops in the process of doing its job? Provide 14 VDC to the H-bridge and see what it does.
If you carefully read the pdf for the L298, the two transistors involved seem to require 1 Volt each at saturation... so with a 12 volt input, you really are only driving the motor/coil at 10 volts at best.
And after that, if you are stalling the motor/coil with a substantial load, you may be going beyond the ability to deliver adequate power to the motor.
In all honesty, I tend to think of the L298 as a toy motor driver. Bob Blick's H-bridge with TIP120/TIP125 drivers will deliver power to bigger motors 3amps at 24 volt = 720 watts. Think about it, 75 watts is roughly a mere 1/10th of one horsepower.
1) L298 will sustain a max of 2A continuous or 2.5A repetitive pulsed (much more than your previous thread). The new measurements suggest that the design may be marginal with L298. Realistically, the current was probably a bit lower with the voltage drops.
2) The previous thread also suggested 1 second duration 10-30 times a day. I think Duane J may be on to something - maybe not the motor coils, but mechanical bits could also suffer under the constant heating. Care has to be taken with highly accelerated testing so as to not create failure modes that would not otherwise exist. A 10% or so duty cycle would help to prevent unrealistic heating anomalies and still simulate about a year of use per day.
3) You mention that the actuator was "attempting to move". Measuring the voltage output to the motor will let you know if the supply is drooping too much. If the output is, say 2V under the supply voltage, then the actuator maybe needs more voltage. If the output is, say 5V under the supply voltage, then it seems that the supply cannot keep up with the current. If it is ~0V output, then the circuit is NG.
If you change the supply and all is good, then awesome.
Per Mike & Loopy, there is a lot of voltage drop through a 298. I have used the cheap China ones with success in a higher voltage (30V), low current app on a robot chassis. Percentage-wise, that 2-volt drop is less noticeable at higher voltages.
Edit: I just saw that the Shack also carries another L298 shield for $2.48: http://www.radioshack.com/product/index.jsp?productId=12333768
Also consider the reality of automotive 12 volts. It is more than likely to be at 13.8 if the battery or higher if the battery is on full charge or being charged by the alternator.
We keep running into newbies that don't understand the automotive charge circuit can run as high as 14.2 under normal conditions, and that 12 volts is a rather silly low end figure meant to indicate something... maybe the voltage at sub-zero starting conditions. All batteries suffer from sub-zero weather.
You do have to admit that opening and closing electric door locks 30,000 cycles is something like 10 time per day for roughly a year if you figure 50 weeks of 6 days per week ( rest on Sundays and two weeks vacation).
I can seem to see where this is NOT normal wear.
MosFets certainly make the H-bridge much less wasteful and eliminate the voltage drop issue. But one always has to provide some extra voltage when using an H-bridge. The best solution is to have motors that run at 24 and 36 volts, because the wiring in less bulky and the voltage drops affect power less.
Hello!
Right!
Those electric door-lock motor-actuators are not like the electric motors that run some other items in a vehicle. Also the voltage across the storage battery in the car is usually between 10.8 to 13.8 to as high as 14 even. It requires voltage regulators to get everything set correctly. And even so there is drift. The big problem with those things is that they expect to be activated only to lock and unlock car doors. Typically at the start and stop of each trip.
So don't expect much from an albatross like that.
We soon discovered when you leave the top down and it rains that you have no electric windows or door locks. Whoever had the bright idea to put all the switches on in the armrests on the top surface just made it so easy for rain to destroy all the switches.
The design of the custom Hbridge is still in the works, but will use IRF540 and a mosfet driver IC.
As for the door lock motor, the nice thing about it is that in a power outage, the latch can still be moved manually since the door lock motors do not lock the mechanism up.
Thanks for the advice.
Very politically incorrect for 2013... but that was indeed what electric doors were about back then.
These days, it is about locking all the doors with your remote. Nobody messes with the pulls any more.
I should mention that speed is important, ideally .5 second up to 1sec for the motion. Also, experimenting with solenoids was not good, in that there is a knock at each end due to inertia slamming the device into the fixed stops. Typically when the latch hits the stops, it bounces back and landing on a fixed position after the bounce is not easy.