From a conceptual standpoint, linear scales are the way to go when it comes to determining where an axis is in its travel, but I have to think that they will lead to resonance in the system. If there is a small amount of slop in the assembly, the motor will have to compensate for this in each direction change, but the tuning parameters may have trouble dealing with this dead zone. Even advanced higher derivative PID control loops like the one used in the UHU can have difficulty with this type of system.
Speaking from a strictly academic point of view, when the power to the motor is reversed, the UHU will take Velocity and Error into account, but will not understand the large change in required force to move the motor. With a traditional encoder, the UHU will register an edge much quicker than anticipated and will adjust power output quickley to avoid an out of control motor. With a glass scale, the UHU will not respond untill the axis moves, but the motor will have to rotate farther than anticipated with a less than anticipated force.
While this seems desirable, the problem is servos are an error feedback system and only respond when an error is registered from the encoder. When an axis is not receiving movement commands, the controller is still hunting to ensure that it stays between the two edges of the encoder. With linear scales, this means it must rotate the motor thru the entire slop of the system and move the actual mechanical components.
As far as what option is more controllable and or accurate, I have no idea. Either system could be accurate, but I highly doubt that linear scales will compensate for any significant mechanical slop without adding controll problems. If you really want an ideal system, use rotary encoders to drive the UHU and use linear scales to interface with the control software and use feedback from both the linear scales and the error parameters on the UHU to compensate for any error. The linear scales can easily be fed into a PC, but I have no idea what software can do the number crunching and motion control fast enough to be of any use.  |