On 7/10/20 8:26 PM, Ken Strauss wrote:
"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.
Well the shafts are not rotating "that" fast if an RPI can easily do
2000rpm, with a 100ppr encoder it can do 3 400 ppr encoders at 1 or 2 or
3 rpm easily
These RPIs are pretty fast little things; with a 400 PPR encoder, with
3 there would be 1000-2000 pulses in a minute... that is easy to do
-----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.
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--
Chris Albertson
Redondo Beach, California
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