Telescope drive motors that don't...

[Dodgy Geezer]

Hi,

I'm not sure if I'm in the right place, so apologies in advance if this is not the correct forum, but I'm not sure exactly what I'm having trouble with! I have a telescope motor drive that I want to drive at a slightly different speed from the commercial controller.

This site, http://telescope.marford.me.uk/Proje...controller.htm , gives a very simple circuit, and some associated code. The circuit just uses a PIC16F627 to send timed pulses to an L293D, which is then meant to drive the 2-coil stepper.

The code worked on MPLAM SiM after a minor re-jig, and the circuit does indeed turn the motor round. But with no torque. The commercial hand controller has no problems producing oodles of power, but my puny pulses produce a maximum of about 40ma in each coil.

The site owner does not respond to e-mails, so I am looking elsewhere for assistance. In fact, the circuit provides quite a jerky drive even with no load (and fails completely with even a slight load), so perhaps I should be looking for a better microstepping answer? But that mus wait until I can discover just what I am doing wrong with this simple system. Has anyone got any ideas....?

[mister_e]

Where the power is coming from? Is this possible that it's just because your power supply is not "strong" enough? AND or the step motor ask more current than the poor L293D can provide?

[Dodgy Geezer]

Mister-e, thanks for the prompt reply!

The original commercial controller for this stepper uses an Amtel micro, driving an array of 8 transistors. It runs well off a 6v battery, and is pre-programmed for a set speed.

I needed a controller with a slightly different speed, and when I saw the circuit I referred to earlier, I thought that would be ideal. It is meant to be a circuit intended to drive a telescope stepper motor of the kind I have.

I am just using the same motor and the same battery supply the commercial controller used, but substituting this PIC16F627 / L293D circuit to drive the motor. So I know there is adequate power available. The L293D is meant to provide up to 600ma continuous according to the datasheet, but I can only measure a max of 40ma when I put an ammeter on one of the coils. If the motor was overloading the L293D I would expect a greater current?

So, am I using the L293D incorrectly? Is the PIC code (provided on the circuit I referred to earlier) incorrect? It just sends a 6 bit signal to the L293D to excite a pair of coils, pauses for a measured time, then sends another signal for the next pair of coils. Surely that ought to work?

[mister_e]

Well, i'm really not a Motor Driver pro... so i would suggest you to increase the pulse length and see if it does anything better.

40mA measurement is not revealant, it's pulse measurement, it give an average of the whole thing, not the peak.. unless you're using something else fancier than a regular DVM.

6 Volts to a regular LM7805 input may cause stability problem, if my memory serves me well, Min Vin should be 'round 9 Volts.

[mvs_sarma]

Well, i'm really not a Motor Driver pro... so i would suggest you to increase the pulse length and see if it does anything better.

40mA measurement is not revealant, it's pulse measurement, it give an average of the whole thing, not the peak.. unless you're using something else fancier than a regular DVM.

6 Volts to a regular LM7805 input may cause stability problem, if my memory serves me well, Min Vin should be 'round 9 Volts.

i find that 293D needs a minimum of 7V for Vs or Vss. this should be sorted out by him first. there should be no issue to supply 8 or 9V and be happy with 7805.

If he can go for low consumption, some Low drop out regulator like LM2931-5 can be used and then 6V might work, but the current demand should be within the regulator spec

[mister_e]

TI's datasheet's says that the supply range should be between 4.5 to 36V. If that chip is connected at the regulator output, and the voltage regulator is unstable (wich i don't doubt), there will have a problem indeed. So something to try in meantime, even if i'm not a fan of it... replace the voltage regulator by a diode or two in serie.

Before i would prefer to monitor the voltage regulator output with a scope to see how bad it is...