Palo: preklep

2006-12-03T23:43:31Z

preklep

Palo at 17:56, 1 December 2006

2006-12-01T17:56:47Z

New page

We have been curious how do actually the signals that control the NXT motors
look like. The NXT output cable has 6 wires:

MA0, MA1 - "PWM" output signals to actuators
GND, POWERMA - optional power source for embedded devices
TACHOA0, TACHOA1 - internal rotation sensors

The MA0, MA1 are compatible with the RCX's motor, so they are not any
kind of control signal to a servo motor, they are simply connected to
two poles of a DC motor so that they can drive it forward or backward
depending on changing polarity.

When the motor is running full-speed, the MAX signals are simply connected
to ground and battery voltage to drive the motor maximum speed. When the
motor strength is decreased, the power is supplied in impulses with length
proportional to the desired power:

[[Image:nxt_motor_50_percent.png | NXT motor running half speed]] 
Plot of the MA signal when NXT motor is running half speed (50%).
 

[[Image:nxt_motor_75_percent.png | NXT motor running 75% speed]] 
Plot of the MA signal when NXT motor is running 75% speed. 
(note that the drive is supplied when the signal is low;
the other MA signal is always high). Similarly, the signal looks
the same, just inverted for 25% power.

'''So the frequency of oscillations generated by NXT is 8kHz'''.

And it is possible to drive the old RCX motors using these outputs and
the conversion cable supplied by LEGO.

It can be interesting to compare these measurements to signals generated
by RCX: we look at both the standard firmware and BrickOS firmware. Those
signals look like this:

[[Image:rcx_motor_rcx_motor_nqc_57_percent.png | RCX motor running on power 4 out of 7 with standard firmware (and NQC program)]] 
RCX motor running on power 4 out of 7 with standard firmware (and NQC program). 
 

[[Image:rcx_motor_rcx_motor_brickos_50_percent.png | RCX motor running on half of full power with BrickOS firmware]] 
RCX motor running on half of full power with BrickOS firmware. 
 

[[Image:rcx_motor_rcx_motor_brickos_75_percent.png | RCX motor running on 3/4th of full power with BrickOS firmware]] 
RCX motor running on 3/4th of full power with BrickOS firmware. 
 

The NXT output signals seem to be able to eliminate overshoot, and other ugly effects seen
in the RCX motor plots. The RCX oscillates its motor output on 500 kHz in both firmwares.
 

Finally, once we had an oscilloscope on, we also looked at the signal on the RCX's activated
light sensor, it looks as follows (with BrickOS firmware):

[[Image:rcx_light_brickos.png | RCX active light sensor sampling]] 
RCX active light sensor sampling. 

It appears that the sampling frequency is about 6.5 kHz.

@@ Line 48: / Line 48: @@
 The NXT output signals seem to be able to eliminate overshoot, and other ugly effects seen
-in the RCX motor plots. The RCX oscillates its motor output on 500 kHz in both firmwares.
+in the RCX motor plots. The RCX oscillates its motor output on 500 Hz in both firmwares.
 <br><br>

Measurements of NXT motors - Revision history

Palo: preklep

Palo at 17:56, 1 December 2006