Motion control in all its various forms has been a core competency of Cornerstone since inception.
AC and DC drives and motors
These are the workhorses in industrial applications. Whether it is for moving a conveyor or extruding plastics, these products can handle the job. Today’s AC drive can be divided into simple open loop V/Hz drives, open loop vector drives, and closed loop vector drives. Although the AC drive continues to replace the DC drive on more and more applications, there are still benefits to be found in both technologies.
AC drives are the right choice when/because:
- They use conventional, low cost, 3-phase AC induction motors for most applications.
- AC motors require virtually no maintenance and are preferred for applications where the motor is mounted in an area not easily reached for servicing or replacement.
- AC motors are smaller, lighter, more commonly available, and less expensive than DC motors.
- AC motors are better suited for high speed operation (over 2500 rpm) since there are no brushes, and commutation is not a problem.
- Whenever the operating environment is wet, corrosive or explosive and special motor enclosures are required. Special AC motor enclosure types are more readily available at lower prices.
- It is desirable to use an existing constant speed AC motor already mounted and wired on a machine.When the application load varies greatly and light loads may be encountered for prolonged periods.
- DC motor commutators and brushes may wear rapidly under this condition.
- Low cost electronic motor reversing is required.
- It is important to have a back up (constant speed) if the controller should fail.
DC drives are the right choice when/because:
- DC drives are less complex with a single power conversion from AC to DC.
- DC drives are normally less expensive for most horsepower ratings.
- DC motors have a long tradition of use as adjustable speed machines and a wide range of options have evolved for this purpose.
- Cooling blowers and inlet air flanges provide cooling air for a wide speed range at constant torque.
- Accessory mounting flanges and kits for mounting feedback tachometers and encoders.
- DC regenerative drives are available for applications requiring continuous regeneration for overhauling loads. AC drives with this capability would be more complex and expensive.
- Properly applied brush and commutator maintenance is minimal.
- DC motors are capable of providing starting and accelerating torques in excess of 400% of rated.
- Some AC drives may produce audible motor noise which is undesirable in some applications.
- No restrictions on cable length while AC drives have distance limitations due to reflected waves.
Servo and Stepper drives and motors
Servo and stepper systems are the workhorse of the performance motion market. Understanding the differences and pros and cons of each is crucial to the selection process.
Stepper motors are permanent magnet motors where the rotor and stator have teeth on them. These teeth cause the motor to have multiple positions per revolution where these teeth line up. These steps are used for positioning. A typical stepper motor will have 200 steps per revolution. To go 2 revolutions, the stepper drive would tell the motor to go 400 steps. Steppers don't require position feedback because it's assumed the motor will go the number of steps the drive commands it. Stepper systems are incredibly reliable and repeatable and do not miss steps when sized correctly.
Servo motors are usually brushless motors with some type of feedback device (encoder or resolver) mounted directly on the motor. Direct Drive systems are another type of servo motor. The servo drive continuously reads the actual position of the motor and compares that to the commanded position. The drive then adjusts to compensate for that error.
The simplest stepper drive is nothing more than an amplifier. A step and direction control signal is sent to the drive and the drive sends the appropriate current to the motor to turn that number of steps. With a standard 200 step/rev motor, a drive can make the stepper motor have many more steps/revolution than just 200. This is called microstepping and is a function of the drive. With a microstepping drive, you can achieve over 25,000 steps/revolution for precise, smooth motion. Features like like encoderless stall detect further enhance the positioning reliability of this technology.
The simplest servo drive is also just an amplifier. The method of control for a servo is usually a -10V to +10V DC analog signal. With a drive in torque mode, the -10V to +10C control signal tells the drive how much torque the motor should put out. With a drive in velocity mode, this signal tells the drive how fast and in what direction to go. For example, a -5VDC signal to a drive in velocity mode tells it to go at 50% of max speed in the negative direction. The drive will constantly monitor the actual speed of the motor and compare it to the commanded speed. If the load on the motor suddenly increases, the drive will increase the torque to maintain the commanded speed.
Servo Drives can do much more and can be controlled in many other ways such as via fieldbus networks. They can also contain programming capabilities and IO.
Which is Right for You
Stepper systems are less expensive and simpler than servo systems because they do not require feedback. Even though they are open loop, they can be very reliable and repeatable positioning systems. They will not however produce the speed, acceleration and torque of a servo system.
Servo systems are often used because steppers just can't provide the performance needed for a given application. They are very responsive and powerful and respond better to more dynamic loads. They guarantee that you are at your desired position or velocity at all times because they are a closed system. Servo systems also have the ability to control and limit torque where steppers can not.