QSH2818 Manual (V1.04 / 2010-OCT-19)

11

Copyright © 2010, TRINAMIC Motion Control GmbH & Co. KG

6.4 Back EMF (BEMF)

Within SI units, the numeric value of the BEMF constant has the same numeric value as the numeric

value of the torque constant. For example, a motor with a torque constant of 1 Nm/A would have a

BEMF constant of 1V/rad/s. Turning such a motor with 1 rps (1 rps = 1 revolution per second =

6.28 rad/s) generates a BEMF voltage of 6.28V.

The Back EMF constant can be calculated as:

The voltage is valid as RMS voltage per coil, thus the nominal current I

formula, since the nominal current assumes a full step position, with two coils switched on. The

torque is in unit [Nm] where 1Nm = 100cNm = 1000mNm.

One can easily measure the BEMF constant of a two phase stepper motor with a (digital) scope. One

just has to measure the voltage of one coil (one phase) when turning the axis of the motor manually.

With this, one gets a voltage (amplitude) and a frequency of a periodic voltage signal (sine wave).

The full step frequency is 4 times the frequency the measured sine wave.

6.5 Choosing the commutation scheme

While the motor performance curves are depicted for fullstepping and halfstepping, most modern

drivers provide a microstepping scheme. Microstepping uses a discrete sine and a cosine wave to

drive both coils of the motor, and gives a very smooth motor behavior as well as an increased

position resolution. The amplitude of the waves is 1.41 times the nominal motor current, while the

RMS values equal the nominal motor current. The stepper motor does not make loud steps any more

– it turns smoothly! Therefore, 16 microsteps or more are recommended for a smooth operation and

the avoidance of resonances. To operate the motor at fullstepping, some considerations should be

taken into account.

Driver Scheme Resolution

Velocity range

Torque

Comments

Fullstepping

200 steps per

rotation

Low to very high.

Skip resonance

areas in low to

medium velocity

range.

Full torque if dam-

pener used,

otherwise reduced

torque in resonance

area

Audible noise

especially at low

velocities

Halfstepping

200 steps per

rotation * 2

Low to very high.

Skip resonance

areas in low to me-

dium velocity

range.

Full torque if dam-

pener used,

otherwise reduced

torque in resonance

area

Audible noise

especially at low

velocities

Microstepping

200 * (number of

microsteps) per

rotation

Low to high.

Reduced torque at

very high velocity

Low noise, smooth

motor behavior

Mixed: Micro-

stepping and

fullstepping for

high velocities

200 * (number of

microsteps) per

rotation

Low to very high.

Full torque

At high velocities,

there is no audible

difference for full-

stepping

Table 6.3: Comparing microstepping and fullstepping

 

 

A

I

Nm

ngTorque

MotorHoldi

s

rad

V

U

NOM

BEMF













2

/