Electronics Forum / Basics / December 2005

Yeah - it'll run - but not at any speed you can control ! That's what the
tacho is for.

Graham

 Feed back is needed to maintain a steady output verses
input reference which is called Tach/Reference feed back otherwise
, its called armature feed back which really does nothing
more than maintain the output voltage to the input voltage
reference ratio. in the case of the second, this is not a
steady reliable way to maintain speed because the motor may
require more voltage on to maintain speed depending on shaft
load&load Swing variations.
  Simply put, armature feed back can maintain the output
voltage but not the actual shaft speed on load..
etc...
  there are the cases where you have a feed back from some
other reference point inputting to a Trim controller which then
can regulate the input reference to the drive, this would then
be regulated from the Trim control by simply biasing the input
reference of the drive which is in armature mode.
  this all applies to DC PM/Shunts and the like motors.

The function of the tach depends on the kind of servo it is. In its simplest
case, the tach provides velocity or speed feedback and it is the speed that
is controlled. The tach element provides a voltage or count frequency that
represents the actual speed of the motor or system it is connected to.

In a position servo where a physical position is the controlled element, a
tach is often used to feedback the differential of the position, dx/dt. This
is a velocity or rate term and helps to stabilize the servo by controlling
the motion.

Thirdly, tach outputs can be integrated or counted to provide position
information. Line tacks of this kind are used on all kinds of numerically
controlled machine tools. Even the lowly mouse on your computer works this
way.