Hi
i have this
9751 LEGO Technic Interface B controller. It's for the older 9V Lego-Technic parts.
On the right side of its connector panel, there are 8 PWM outputs which can switch polarity and has 7 PWM levels.
The controller is programmable using a rs232 interface. The original software runs on Windows95, but luckily people made a Python and a Java API.
I want to see if i can use it to drive the
'whadda' robotarm kit. This robot kit powers the motors with 3 volts. each motor has a ceramic capacitor in parallel which says '104'.
I hope this is the right place in the EEVBLOG to post things about my ongoing project..
i have a program cycling from
1 - left, going from pwm level 0 to 7 with 1 second interval
2 - right, going from pwm level 0 to 7 with 1 second interval
When i use the dacta controller to drive the
standard 9V DC Lego motors(pics of its internals), then the voltage on the oscilloscope roughly follows the PWM modulation.(looks the line follows the initial on-spike to 9v and then comes down a bit and stays there until the pulse goes off)
But when i attach one of the DC motors of the robotarm then the signal on the oscilloscope goes to about zero with spikes looking like depending on the pwm signal. (i've got an Gould OS250TV 10 mhz oscilloscope)
i thought, the voltage drawn to 0-ish looks like a short cicruit, so
i put a 100 ohm resistor in series between the dacta-motor-connector and the 'whadda' motor, but then the motor doesn't turn.
when i put a 10 ohm resistor in place, it starts working at pwm levels 6(90%) and 7(full on)
To reduce the spikes, i had tried to a few capacitors Variations in parallel(even though it already has one). the ceramic ones(100u to 1uF) didn't seem to do much.
when i put an electrolytic 50V/820uF in parallel(hoping it wouldn't blow up when in reverse
, which it didn't) it does soften the whole PWM out to a smooth-ish line, so smooth that i don't see the pwm pulses, it's now jagged line. But it does seem to make the motor turn a bit better (at pwm levels 5 to 7).
But still not really good. And the motor isn't responding evenly for left and right turning at same pwm level. when testing i don't have the motor connected and tried making contact when at different levels. When keeping it connected, the motor makes a high-pitch sound, but only starts moving late in level 7. when i connect at level 6 or 7 only, it starts turning almost immediately. i guess it's the big capacitor first taking the energy.
i've checked some information on back-emf, but i'm not sure what to try next.
Looking at the 9V Lego motor pictures, i see things which i don't recognize.
on the
2nd picture it looks like there's some sort of diode inside.
on the
1st picture and
3rd picture it looks like there's some sort of cell inside.
i'm hoping you can tell me what i'm looking at, and how i can get the robot arm motors properly responding
Thx