Comparison of Stepper Motor Servo Motor

Selection of Steeper Motor or Servo Motor.

Stepper Motors, DC brush Servos and brush less servos each have their respective benefits and drawbacks. No single motor technology is ideal in every application, despite what some manufacturers may claim. 

This section reviews the relative merits of each technology and lists the application types most appropriate to each.

The following section gives some idea of the applications that are particularly appropriate for each motor type, together with certain applications which are best avoided. It should be stressed that there is a wide range of applications which can be equally well met by more than one motor type, and the choice will often be dictated by customer preference, previous experience or compatibility with existing equipment. With the increased requirement for intelligent drives, the real cost differential between brush and brush less servo systems are diminishing. In the majority of new applications the choice is therefore between stepper and brush less servo. 

Cost conscious applications are always worth attempting with a stepper, as it will generally be hard to beat on cost. This is particularly true when the dynamic requirements are not severe, such as “setting” type applications like periodic adjustments on printing machines.

High Torque, low speed, continuous duty applications are appropriate for direct drive servos and frequently also for stepper motors. At low speeds the stepper is very efficient in terms of torque output relative to both size and input power. A typical example would be a metering pump for accurate flow control.

High torque, high speed, continuous duty applications suit a servo motor, and in fact, a stepper should be avoided in such applications because the high speed losses can lead to excessive motor heating. A DC motor can deliver greater continuous shaft power at high speeds than a stepper of the same frame size. 

Short, rapid repetitive moves may demand the use of a servo if there are high dynamic requirements. However the stepper will offer a more economic solution when the requirements are more modest.

Positioning applications where the load is primarily inertia rather than friction are efficiently handled by a servo. The ability to overdrive a servo motor in intermittent duty allows a smaller motor to be used where the main torque demand only occurs during acceleration an aeceleration.

Very arduous applications with a high dynamic duty cycle or requiring very high speeds will normally require a brushless servo.

Source: Parker Automation

Additional Information can be found at www.compumotor.com

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Stepper motor Application and uses

 Stepper Motor Applications and Advantages Disadvantages

 • Lowest-cost solution

• A stepper motor will always offer the cheapest solution. If a stepper will do the job, use it.

Rugged and Reliable:

Steppers are mechanically very simple and apart from the bearings (like in servos) there is nothing to deteriorate or fail.

No Maintenance:

There are no brushes or other wearing parts requiring periodic checking or replacement.

Industry-standard ranges (Nema or metric):

Steppers are produced to standard flange and shaft sizes so finding a second source is not a problem.

Few environmental constraints:

A stepper may be used in just about any environment, including in a vacuum. Special magnets may be needed if there are very large magnetic fields around, e.g. in evaporation chambers.

Watch heat dissipation in a vacuum (there is no convection cooling).

Inherently failsafe:

There are no conceivable faults within the drive to cause the motor to run away. Since current must be continually switched for continuous rotation most faults cause the motor to stop rotating. A brush motor is internally-commutated and can run away if continuous current is applied. A brushless servo relies on the feedback signal. If the signal is damaged, or absent the motor will run away.

Not easily demagnetized by excessive current:

Owing to the perpendicular planes of the permanent magnet and alternating flux paths stepper motors will more often melt the windings before demagnetizing the permanent magnet, as would happen in a brushed motor.

Inherently stable at standstill:

With DC flowing in the winding, the rotor will remain completely stationary. There is no tendency to jitter around an encoder or resolver position. This is useful in applications using vision systems.

Can be stalled indefinitely without damage:

There is no increase in motor current as a result of a stall or jam as in a servo system. There is no risk of overdriving a stepper system due to large loads, or high speeds.

High continuous torque in relation to size:

Compared with brushed servos of the same size, a stepper can produce greater continuous torque at low speeds. 

Only 4 leads required:

This minimizes the installed cost, particularly important in applications where connections are expensive (e.g. vacuum chambers).

 

Stepper Motor Drawbacks:

 Ringing, resonance and poor low speed smoothness:

These are criticisms generally leveled at full-step drives. These problems may be almost wholly overcome by the use of a higher-resolution drive.

Undetected position loss in open loop:

This should only occur under overload conditions and in many applications it causes few problems. When position lost must not go undetected, a check encoder may be fitted which then results in a very secure system. The encoder is not needed for positioning, only for confirmation. If a positioning encoder is desired a servo system should be used.

Uses full current at standstill:

Since current is needed to produce holding torque, this increases motor heating at standstill.

Noisy at high speeds:

The 50-pole rotor has a magnetic frequency of 2.5 kHz at 3000 rpm. Magneto-striction causes a correspondingly high-pitched sound.

Excessive iron losses at high speed:

Again due to the high pole count, hysteresis and eddy current losses are higher than in a servo. A stepper is therefore not recommended for continuous operation at speeds approximately above 2000 rpm.

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Modbus Communication

Modbus is a serial communications protocol published by Modicon and generally used with PLCs (programmable logic controllers). It has now become a factory standard for communications protocols in industry, and is the most commonly used means of communicating industrial devices.

The main reasons for the extensive use of Modbus over other communications protocols are:

•  It is open source protocol and it’s free.
•  It is simple to understand, easy to implement.
•  It moves raw bits or words without placing many restrictions on vendors

 Modbus allows for serial communication between many third party devices connected to the same network, for e.g. a system that measures temperature and humidity and communicates the results to a computer or number of VFDs communicating to PLC over a single Modbus serial line. 

Modbus is often used to connect a supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems. Modbus protocol is available for serial and Ethernet ports.

Each device intended to communicate using Modbus is given a unique address. Maximum of 247 stations can be connected on a Modbus network. Maximum length of wire is limited to 1200mtr. Generally Master/slave configuration used for Modbus. 

Any device can send out a Modbus command, although usually only one master device does so. A Modbus command contains the Modbus address of the device it is intended for. Valid range for address is 0 to 247. An address equals to 0 means a broadcast to all devices in the network. Only the intended device will act on the command, even though other devices might receive it. All Modbus commands contain checking information, ensuring that a command arrives undamaged. The basic Modbus commands can instruct an RTU to change a value in one of its registers, as well as commanding the device to send back one or more values contained in its registers.

  

Specification	Modbus
Governing Standard(s)	EN1434-3 ,IEC870-5
Communication Methods	Master/Slave
Communication Speed	9.6 -115.2 kb/s
Max. Data Size	250 bytes (125 register)
Max. Stations	247
Maximum Cable Length	1200 mtr.

For Reference only

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