"You need more gearing with the DC motor." Correct and this could be considered an advantage since a very small DC motor + gearing (multiple planetary stages) would have sufficient torque to move the scope. I have no idea if a RPI can keep up with 3 quadrature encoders and control 3 motors and manage a UI. However, you could make a compact unit at low cost containing a microprocessor that takes step/dir, tracks encoder position and generates PWM to run the motor. Rather like a low power, low RPM, high torque version of a Clearpath servo.
> -----Original Message----- > From: Chris Albertson [mailto:albertson.ch...@gmail.com] > Sent: Friday, July 10, 2020 10:14 PM > To: Enhanced Machine Controller (EMC) > Subject: Re: [Emc-users] off topic: pwm with a stepper driver/motor > > Stepper motors are good for this. A stepper has the most torque at zero > RPM and the torque falls as speed increases while a DC brushed motor has > very little power when moving slowly. You need more gearing with the DC > motor. > > The problem is you need to generate precise pulses. If the pulse spacing > is not accurate then you are asking a large telescope to accelerate and > decelerate instantly. So to generat pulses need either > 1) a hardware solution like a microcontroller or FPGA or pulse generator. > 2) A real time kernel on the Linux system or > 3) A PID loop to supervise a non-real-time software pulse generator > > When I build stuff, my preference is the microcontroller. I design it so > the microcontroller can work independent of a larger Linux computer or take > commands from a large Linux computer. I want some basic functionality > when > power is applied then adding the Linux PC adds more functionality. > > Linux CNC uses a slightly different idea they either go with Real-Time > Linux or use a very simple pulse generator running in an FPGA. > > Reading up to three quadrature encoders with just a Raspberry Pi might > work, or not. I don't know. A lot depends on the speed of the encoders. > > On Fri, Jul 10, 2020 at 6:45 PM R C <cjv...@gmail.com> wrote: > > > I don't think anyone did, but I know that's how most do it... > > > > But that's just too easy :) I would just like to see if I can do it, > > killing time > > > > > > On 7/10/20 7:39 PM, Ken Strauss wrote: > > > I haven't been following this closely but has anyone suggested that you > > use a > > > DC motor with an encoder? With a PID loop and PWM power to the motor > you > > could > > > run the motor continuously. DC motors with integrated encoder and > > planetary > > > gearbox are readily available for a few dollars. Just a thought. > > > > > > > > > > > > > > > _______________________________________________ > > > Emc-users mailing list > > > Emc-users@lists.sourceforge.net > > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > > > > _______________________________________________ > > Emc-users mailing list > > Emc-users@lists.sourceforge.net > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > > > -- > > Chris Albertson > Redondo Beach, California > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
