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if you dont use pwm then you motor will run at a constant speed and depending on the input voltage you it may be difficult to control the motor. but you can do it without pwm.
I made a line-follower robot called Movit (commercially produced kit several years back).
It followed a dark line on the floor. It could be drawn on a large paper sheet. Or it could be the joint between floorboards.
The sensors were two light detectors close to the floor. I found it's crucial to get the sensor head right up against the line.
One sensor would not be sufficient. Two are needed for directional sensing.
It had two motors. One for each drive wheel. These were alternately switched full on and full off rapidly. Each drove four plastic gears to reduce speed.
The robot shimmied from side to side rapidly, as it followed the line. It moved a few inches per second.
The onboard circuitry must respond quickly. It has to turn the motors on and off quickly. If there was any delay then the robot would depart from the line.
The drive circuitry was simple. It didn't use PWM.
It's possible that PWM could work well. THen both right and left sides could drive at the same time. This will be complicated to get it to work.
you should not allow motor to run continuously.. you need to control the motor as per the dark line and control the speed.. if it goes beyond the dark line it has to stop and reverse the motor or slow down... DC motors while running cannot stop suddenly.. they take time to stop completely... so you run stop momentarily and scan the line and continue....
The circuit board came with all components pre-installed. I didn't try to modify how the control circuit worked.
And when you think about it... it did use PWM. At about a 6 Hz rate.
I believe I figured out why they did it that way.
The drive motors were powered by two AA cells ( 3V total). The control circuit was powered by a 9V battery.
By alternating the motors full on and full off, it ensures that the batteries only have to provide current to one side or the other. It avoids making them supply 2x current at any one time.
Moreover I suspect the manufacturer chose to economize on the motors. So they used small inexpensive motors, chosen to be most efficient at 3V supply when operated at high rpm. Then gear down the speed to obtain torque at the drive wheels.
PWM (high-frequency) should be worth a try. Seems like the robot will move more smoothly.
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