Electronic – Inferring back emf from oscilloscope

In addition to the links supplied by @suha in the answer to the question linked by @Scott Winder, as well as my own excellent answer to the same question (shameless self plug), the following Freescale paper is also worth reading as it specifically references position sensing:

AN1913 3-phase BLDC Motor Control with Sensorless Back-EMF ADC Zero Crossing Detection using 56F80x

In a nutshell, it comes down to counting the number of zero-crossings and measuring the timing between them. For a 3-phase BLDC motor, one phase will be high voltage, one phase will be low voltage and one phase will be off. You will know which is which because you will be providing it. You will also know how many poles the motor has. Combining this information with your measurement of when the off phase crosses the mid-point between the high and low phases (the zero crossing), you can determine speed and position.

By looking at each phase state, you can determine what your electrical angle is:

You can then convert the electrical angle to a mechanical angle by taking into account the number poles using the following equation:

$$\theta_{electrical} = \dfrac{p}{2}\theta_{mechanical}$$

Where: \$p= \text{number of poles}\$

Then, by timing and counting the number of zero crossings, you can determine how long it takes to complete 360° of mechanical rotation which can then be converted into a speed in RPM.

You don't say it, but your problem may be that you're trying to start the stepper at full speed. If so, there is a maximum speed (which varies somewhat with load) beyond which a stepper will not accelerate, and this speed is normally well below what you can reach with a gradual increase in speed. Google on "stepper motor torque curve".

The problem is that, with a 4-phase stepper such as you are using, if the shaft angle lags more than 2 step angles behind the commanded angle, the torque reverses and the motor sits and vibrates and makes horrible noises. This is not, technically speaking, a stall condition, where the shaft does not move at all.

The torque-speed curve for your motor can be found at http://www.kelinginc.net/KL34H2120-42-8AT.pdf and indicates that the motor can be run at 5000 steps/second, which suggests that your problem is your attempt at fast start.

If, on the other hand, you've already tried a slowly increasing step rate while running, then you probably do need a heftier motor. However, my calculations for a .144 hp motor at 10 rps gives about 2 Nm torque, and the linked torque curve for your stepper is about the same, so I wouldn't expect a problem. Unless, of course, you've modified your mechanical setup somehow.