Construction of a pneumatic system with a pulled cylinder

Hello, I need help building a closed pneumatic system with a pull actuator. The system is to replace the usual load found in an atlas-type exercise device (that is, instead of weights there will be an actuator). While the system itself is not problematic, how to solve the problem of maintaining an equal force over the entire length of the piston stroke. So the user, let's say, sets the force of 4 bars and pulling out the piston it spontaneously tries to retract with the same force all the time. This is an assignment for school and unfortunately my professor left me with this problem alone. Perhaps you know of any solutions where such a system occurs?
 
Top