HP97 The journey begins

[teenix]

Hi all,

I'm still toying around with the 97 PIK chip but it still manages to offer some confusion while trying to figure out the intricacies of the print head speed control.

It is a fairly simple set up, a varying PWM signal drives the motor forward. During each time the PWM signal is turned off, the motor still spins due to inertia and briefly turns into a generator. The voltage created is sampled by a comparator and the output signal changes depending on how fast the motor is spinning. This signal is fed to the PIK which (I assume) uses the width of this signal to determine the mark to space ratio of the PWM and thus keep the speed relatively constant as the printer prints a line. As far as I can tell that's all there is to it.

What I cannot seem to understand is how the PIK varies the mark space ratio from the comparator output.

The image below shows the PWM drive to the motor transistors (top line), followed by the output from the comparator (2nd line) and finally the motor voltage appearing at the comparator input. It the motor voltage exceeds 1 volt, the comparator switches high, otherwise is low. Thus the faster the motor spins, the narrower the output pulse will be from the comparator and this tells the PIK to slow the motor down. Sounds feasible.

You can see the motor voltage mirroring the drive signal as it goes HI and LO. The motor voltage drops sharply at switch off but rises (slowly) due to the generator effect. Once this voltage goes higher than 1 volt, the comparator output switches HI, otherwise LO.

What is hard to sort out is how the PIK figures out how to vary the motor PWM signal. It does vary, but it is hard to see what is causing the change. On the second line, you can see a glitch which seems to trigger the motor drive to switch on. This pulse is only about 1-2uS wide and it does not always change the state of the motor drive so is a bit mysterious. You can see the top line PWM vary but not much from line 2 to suggest why.

My feeble brain must be low on sugar, or perhaps beer, but I cannot see anything logical in the waveforms.





cheers

Tony