Arduino Printer DIY || Replacing Servo with brushed DC?

Hi
I have in mind to follow a guide and make an Arduino printer (Guide: https://www.instructables.com/id/Homework-Writing-Machine/)

In the guide it is used a Servo to move the pen, I am thinking if I could use a brushed DC motor instead?

The main reason is that I don't have any Servo laying around but I do get some of Brushed DC motors.

(second question) I know the DC motor will consume more current when the pen is already touching the sheet, perhaps I should place a resistor to limit the current consumption?

Servos offer very precise movements. DC motors do not. If you have an encoder on your DC motor, you will get some feedback about how much it has moved.

The servo is used to lift and drop the pen. You could probably rig the DC motor to do that. It will be trickier without a motor controller (like in a servo) because you will only be able to drive the motor in one direction. A geared motor (like in a servo) would be very helpful. Then you could drive a cam slowly enough that switches could be used for feedback. Without gearing down, the motor would likely spin a cam too fast to stop at the right place.

@johnwasser So the Servo has the driver built in?
I was thinking about using a dual pole relay to control the motor direction and using a resistor to slow down the motor.
By the way, when the servo drop the pen it stops forcing it to stay down?

F1_:
I am thinking if I could use a brushed DC motor instead?

I think this is one of those cases where if you have to ask then you should just buy a servo.

There is a huge difference between a simple DC motor and a servo.

…R

@Robin2 True, I guess I should follow your advice.
I read this in a website about servo motors:
" Servo motors receive a control signal that represents an output position and applies power to the DC motor until the shaft turns to the correct position, determined by the position sensor."
So if a servo is in place and something takes it out of the correct position it will automatically get back to the correct position?

F1_:
So if a servo is in place and something takes it out of the correct position it will automatically get back to the correct position?

The servo tries to hold the set position, as long as the applied force is weak enough. See the listed torque of your servo.

@DrDiettrich It is "1.5kg/cm", is it good enough for the task?

Which task?

@DrDiettrich to drop lift the pen

What's the weight of the arm + pen? What's the arm length to the pen? Multiply both to get the required torque for lifting. Similar considerations apply to dropping, if the pen weight is not sufficient for reliable painting.

For minimal power consumption and safe power-off it's suggested to use a spring to hold the pen lifted. Then the motor has to overcome the spring torsion for dropping.

F1_:
@johnwasser So the Servo has the driver built in?

Yes. A hobby servo has an H-bridge motor controller and a feedback potentiometer built in. You provide it with power (typically 4.5V to 6V) and a pulsed signal. The DC motor is geared down and the potentiometer measures the position of the output shaft. The pulse width (typically 1000 to 2000 microseconds) is used as a position setting (full CW to full CCW) and the electronics then drive the motor in the correct direction to move the arm to the desired position.

F1_:
I was thinking about using a dual pole relay to control the motor direction and using a resistor to slow down the motor.

The motor will likely stall before you get it slow enough to control. You could break open another CD/DVD drive and use the head stepper from that to move the pen up and down.

F1_:
By the way, when the servo drop the pen it stops forcing it to stay down?

I didn't read the Instructable in that much detail. The servo could press the pen down or it could just work against a spring/weight to lift the pen as needed and let the spring/weight apply pressure in the down position.

Understood, thanks for your answers!