Dan Jones, President, Incremotion Associates, Inc.
New Market Needs
Stepping motors are used in hundreds of position based applications that need more torque with each new generation.Â While the 2 phase 1.8Â°/step hybrid step motor was invented in the early 1960’s, it has undergone a number of performance improvements that have more than tripled the step motor torque within the same motor volume (torque density).Â
The electronic drive also plays a major role in providing low ripple current that reduces unwanted noise.Â The driver is suitable for applications that require the drive to be mounted on the step motor body.Â The 2 phase CVK family members range in sizes from the 20 mm (0.866 inch) square up to the 56.3 mm (2.22 inches) square step motor.Â The CVK was developed along with its driver to replace earlier models.
Higher CVK Torque
The size 24 CVK 2 phase step motor (in red) has been redesigned to outperform its preceding CMK (in blue) two phase hybrid step motor. (See Figure 1.) The 1.8Â°/step CVK running torque at 2,000 rpm reaches 14 oz-in (0.1 Nm) or about 9.9 percent higher than its previous CMK step motor counterpart.Â The CVK torque margin grows to 22 percent or 30 oz-in at 1,000 rpm.
The New CVK Driver
The 2 phase CVK drive employs a bipolar drive system used for the first time by Oriental Motor. The drive helps furnish the CVK step motor with 30 percent more low speed torqueÂ (Figure 2) of 64 oz-in (0.45 Nm)Â at 300 rpm than the older Size 24 CMK 1.8Â°/step 2 phase step motor and drive.
A new lower phase resistance and low phase inductance winding is utilized in order to allow the motor current to riseÂ more quickly.Â A reduced number of turns in the motor winding also leads to lower torque.Â The electronic drive corrects for this motor condition by being able to draw more current thereby developing more lower speed torque.Â This condition illustrates the importance of matching the 2 phase step motor to its associated driver.Â Figure 2 aptly illustrates this condition.
Step motor vibration is caused in the CVK two phase 1.8Â°/step motor by the low internal damping characteristics.Â The electronic driver has ameliorated this condition in the 2 phase step motor.Â The speed sensor that is a DC tachogenerator, which transforms speed into voltage. This condition provides the ability to measure both speed and speed fluctuations or ripple.Â Figure 3 shows the speed ripple in peak-to-peak voltage (Vp-p).Â A comparison of the new CVKÂ 2 phase step motor vibration signature shows a significant performance improvement against the previous CMK 2 phase step motor.
The Step Motor Driver
The CVK series runs with Oriental Motor’s unique smooth microstep driver that reduces vibration at low speed (less than 50 rpm).Â The new control strategy corrects the current phasing between 50 and 200 rpm thereby extending the lower vibration zone.Â The redesigned step motor winding in combination with the smooth drive results in much lower motor vibrations and enhanced position accuracy.
Figure 4 compares the new 2 and 5 phase CVK step motors’ overall vibration signatures.Â While the new 2 phase CVK outperforms its 2 phase CMK predecessor in both torque and step accuracy, it takes second position in step accuracy to its 5 phase CVK counterpart.
The compact CVK driver is thinner than the conventional CMK 1.8Â°/step driver.Â The CVK drives use the line driver method for motor control.Â The line driver is capable of handling higher frequency pulsesÂ easier than the more traditional open collector method.Â It can be operated at high speed range even when high resolution is set.Â Also the line driver method has superior electrical noise resistance over the open collector method.
Step Motor Accuracy
The step motor accuracy for the 2 phase step motor has been reduced from 0.078Â° performance for the earlier CMK step motor to 0.025Â° for the CVK step motor.Â See Figure 5.
TheÂ CVK family of 1.8Â°/step motors in combination with its associated drivers has raised the performance levels for torque, power, step accuracy above previous models.Â The CVK product family covers a wide variety of motor sizesÂ and performance benefits for an ever increasing range of applications.