Links Steven style. I like them raw :-)
Festo Stepper motors with integral encoder
Oriental motor USA more of same
Parker Sy56 Still more of same
ADDED July 18th 2011:
In view of the almost universal comments re "not available", "doesn't exist" etc, I've referenced below a real world example. Adding a position encoder is essentially trivial - largely a mechanical issue. Some reasons as to why you may want to are explained below by a quote from Festo. Or, it may be because you want to turn the shaft by hand ;-).
As an alternative to Feto's fine but no doubt somewhat costly offering it would be easy enough [tm] to add an encoder disk to an existing stepper if required.
Here's an example of a real world product from Festo. There will be lower cost Asian emulators.
Festo EMM-ST closed loop stepper motors
- True servo performance based on stepper technology, without the traditional issues of resonance, heating, and loss of step, inherent in conventional stepper systems. This new stepper motor series offers long service life and reliable performance. The two phase hybrid stepper motor offers high torque, with options for an integrated brake and incremental encoder feedback. External gearboxes are available for all motor sizes together with adapter kits for all Festo electromechanical actuators.
- A key advantage of using closed loop feedback is that the stepper motor is always operating at maximum efficiency and cannot suffer step loss; if the motor is overloaded, the position controller automatically compensates for following error and maintains position tracking. This means that unlike an open loop stepper system, it is not necessary to oversize the motor to handle an application's peak torque demands, allowing use of smaller, less expensive units. Other operational benefits include better smoothness, lower acoustic noise levels, and absence of mechanical resonance. Although many stepper drives offer the options of closed loop facilities, these generally only offer alarm functionality in case of step error. The CMMS-ST offers true servo functionality with full control of current, speed and position.
Catalog Stepper motors EMMS-ST catalog
__ End of July 18th add on __
An electronic pulse counting position determiner could be built - with due care this would seldom if ever get lost. A comparator with very very light hysteresis will toggle when applied across a PM stepper winding as the coil voltage passes through zero. Even at VERY slow speeds (< 1 step/second) the signal will be well above what a comparator requires.
An up down counter or microcontroller can determine position by tracking transitions. Two windings (probably) required to get quadrature signal for direction.
Detent action caused by motor saliency means that rotor tends to jump in position when lightly coupled to drive (such as fingers) enhancing low speed output. So much so that depending on stepper used signal may be enough to drive logic level circuitry directly.
Start up position detection can be managed by stepping to an endpoint, as already suggested, or by having syynchronisation contacts or optical detectors.
Depending on application the need for absolute position detection may be eliminated by making the stepper the decoder but storing position information electronically. eg a volume control can show the volume level on a bar graph. The stepper drives this but its absolute position is of no relevance. In other systems the absolute position matters 0 eg use as a postion encoder for a "vernier caliper"* or "dial gauge"* or machine table etc. Traditionally synchronisation is achieved with a reset button.
*Interestingly - no vernier on the vernier-caliper and no dial on the dial-gauge due to electronics invasion.
Many digital "vernier" caliper users will have absolutely no idea what a "vernier" is or that the concept even exists.