Linear actuator jitters/vibrates when getting PWM from Arduino Mega 2560 through JKR 21v3 motor controller
I got this working using the Servo library. 


You set the input method to PWM.
Connect your RX on the motor controller and hook it up to the servo pin.
Connect the ground of the Arduino to the ground of the JRK 21v3.
Then use the knob servo sample by hooking up a pot to a Arduino.
The calibrate the JRK 21v3 by selecting learn.


Now you can use the actuator as a servo.