Tach vs encoder

[N4NV]

I see some machines use a tach and others use an encoder to determine position. I understand how the encoder works. How does the tach work to determine machine position?

Vince

[Al_The_Man]

A DC tach is seldom used now, it was used principally with velocity style drives, as part of an inner loop feedback loop to the drive only, the outer loop was the encoder to control.
A DC tach was never used as a positioning device for CNC servo's.
Its DC ouput depends on rpm not position.
Al.

[N4NV]

I have a machine with Servo Dynamics SD1525 servo drives and SEM permanent magnet servomotors. Looking at the wiring diagram it looks like it has only tach input. How would the machine know its position? The machine is a Yamazen CNC knee mill.

Thanks

Vince

[Al_The_Man]

The Drives use the tach as I mentioned, but the controller needs feedback from an encoder of some kind, you should have one on a separate shaft or even linear scales, if it is not on the motor, On SEM they sometimes add an encoder on the end of the Tach shaft?
Al.

[N4NV]

I will have to look closer at the motor to see if it has an encoder. On the drive there is no encoder input, but there is a differential input, would this be for the encoder?

Vince

[Al_The_Man]

No, the drives do not have an encoder input, the encoder is used by the controller only.
If you are looking at the drive manual only, it may not show the motor encoder, even though there is one mounted.
Al.

[One of Many]

To clarify a bit of what Al is stating.

The Tach puts out a voltage(I.E. a small DC generator) of say 7v per 1000rpm. These are very linear( on the order of .2%) in relation to axis/motor velocity(speed). The servo amp uses this to track how fast it needs to turn to keep up with the commanded position and resulting path via a closed loop in relation to position feedback of the encoder pulses at the motion controller. The +/- 10v analog signal from the motion control is controlling the servo amp output, in sync with the feed rate and path profile, so one does not over run the other. There can be several pots on the servo amp to tune the systems electronically with the mechanical influences of the machine itself.

DC

[N4NV]

I got into the contol cabinet and the Allen Bradley 8400MP controller and found the encoder inputs. Now it makes more sense. BTW, does anyone know who works on the 8400 contollers or how I can get around the password protection. The guy who I bought the mill from sold it for someone else and he does not know the password.

Vince