Mindboards

tcwan wrote:When I tested my NXT motor, I removed the wheel so that it was not loaded. However, I noticed that if I dropped the Power Level to less than 60, the motor will stop turning and emit a high pitched whine. Is this supposed to be normal? I'm wondering if absolute power levels of less than 60 is actually usable on the NXT.

tcwan wrote: By searching this forum, I found a reference to Philo Hurbain's webpage, the first graph on the page which has a curve in Pink (with square markers) seem to match my observations http://www.philohome.com/nxtmotor/nxtmotor.htm. I can't get the other curves which has linear response for each specified power level.

tcwan wrote: I should note that this testing was done using NxOS, which does not implement any PID control for the motor, so the power level given to the NxOS motor driver will be the actual value sent to the AVR.

tcwan wrote: So for correct operation of the NXT motor with different power levels, would a more advanced scheme be needed? What is meant by the term 'Power Control?' Incidentally, what would be the output parameter of a PID controller in this case? Is it Power Level or RPMs?

linusa wrote: Power control, or speed regulation, is a controller which tries to hold a constant speed setpoint. So it will adjust the power level to reach this speed. If you block your motor by hand, power will increase. I wrote a bit about speed regulation here: http://www.mindstorms.rwth-aachen.de/do ... ol.html#18 , which bases on how the LEGO firmware handles this stuff.

So in this case, output is clearly power level, and input is RPM: feedback is RPM (the currently measured speed), and set point is speed (the desired speed).

tcwan wrote:

linusa wrote: By the way, in John Hansen's book the NXC Quick Reference in Appendix A shows the output functions as accepting Power levels as a parameter.
What is the mapping between the specified power level and the speed (set point)? Is it arbitrary?

gloomyandy wrote:Hi,
You may also want to take a look at the PWM control mode being used. Basically this controls what the PWM driver does during the "off cycle" of the control period. In FLOAT mode the output is not connected to the battery at all, in BRAKE mode it is connected to 0V. This setting has a huge impact on the power curve. I seem to remember that in FLOAT mode the minimum power required for the motor to start moving is approx 50% (possible more depending on the motor), in BREAK mode it is much lower, 8% or so. It looks like you are operating in FLOAT mode....

gloomyandy wrote: Not directly related to the above, but if you are interested in Lego motor control you may find the following worth reading...
http://www.gloomy-place.com/lejosmotor.htm
http://www.gloomy-place.com/images/lejospid.pdf
These mainly detail the development of the leJOS motor driver, but first also covers some of the characteristics of the Lego driver. Oh and the experimental driver described in the first link is now the released version in leJOS 0.9.0

gloomyandy wrote:Hi,

These mainly detail the development of the leJOS motor driver, but first also covers some of the characteristics of the Lego driver. Oh and the experimental driver described in the first link is now the released version in leJOS 0.9.0

Andy

tcwan wrote: However, I'm now confused with the definition: In the Hardware Developer Kit, under the section "Data Sent from the ARM7 Microcontroller",
Output Mode is defined as "Holds the output mode that can either float or break between PWM pulses. 0x00 means break and 0x01 means float".
I checked the motor setting code in NxOS, if it the output mode is set to '1' for the motor, it will work down to 10%.
Similarly, for Stopping, the output mode is set to '1' for the give motor. Is the documentation incorrect?

linusa wrote: In the Bluetooth Developer Kit, Appendix 2 Direct Commands, for SetOutputState, page 6, it says you have to set bit 0x02 to "use run/brake mode instead of run/float mode in PWM".
Not sure if that helps you, but I always got the feeling that BRAKE is something you "have to enabled", i.e. default off.

In d_output.r:
#define   INSERTMode(Motor, Mode)       if (Mode & 0x02)\
                                        {\
                                          IoToAvr.OutputMode |=  (0x01 << Motor);\
                                        }\
                                        else\
                                        {\
                                          IoToAvr.OutputMode &= ~(0x01 << Motor);\
                                        }