User Rating: 5 / 5

Star ActiveStar ActiveStar ActiveStar ActiveStar Active

Almost any type of motor can be used in motion control applications a brief summary of motor types follows:


The Basic Motion Control System
The basic motion system consists of a motion controller and a motor (or mechanical energy converter such as a hydraulic motor or pneumatic  cylinder ) with a positional feedback system. The motion control system also requires a Drive that interfaces between the motor and the motion controller.


DC Brushed Servo Motors
This type of motor is the best for applications that require very high precision at lower speed this is because this type of motor is the most stable and has the lowest "cogging factor".  Brushed motors offer the lowest system cost of all servo motors. The motor costs about the same as other motors but the amplifier is relatively simple and is therefore around half the cost of ac servo types. The system controls the motor by the application of a variable pulse width modulated signal. In 4 quadrants where at rest the motor has full power applied the armature. For 50% of the time the power is applied in one direction and for the remaining 50% in the other direction. When the controller commands the motor to move, the pulse timing is changed to apply a high percentage to one pole of the motor, causing the motor to apply a force in one direction. This ratio is based upon and analogue calculation and is infinitely variable duty cycle changes such that the timing applies more time. It is possible to over stress the motor for a limited time is some cases it is possible to obtain as much as 200% of the rated torque from the motor. Careful design is required to ensure that the motor is not over stressed which can cause the motor to overheat and damage the motor.



DC Brushless and AC Servo Motors
DC brushless are in reality AC servo types. There are two basic types of AC servo motor depending upon the type of feedback system. These are the hall effect type where the motor is fitted with three hall  effect devices that measure the magnetic field strength to determine the exact position of the armature and resolver feed back type. The resolver type is the most expensive but offers the best performance since the exact position of the armature can be determined. Within the hall effect types there is a further division where the encoder is used with the Z pulse to determine the position of the armature. For this type of motor to be effective and precise the exact position of the motor armature must be known. The hall effect type of motor uses an encoder to provide positional information for the drive the (the latter type) and the controller. The resolver type provides sine and cosine information to the amplifier which uses this information for internal control purposes and to provide a simulated encoder pulse stream for the motion controller. AC servo motors have one major advantage over DC brushed types in that it is possible to induce more current into the armature than is possible with bushed types. It is also possible to reduce the revolving mass of the motor which results in better performance due to better acceleration/de-acceleration. AC servos are often used because they have no brushes to wear out but often it is the bearing life that is as important as the brush wear. The resolver type offers the lowest cogging as the armature position is exactly known (cogging is higher than with DC servo motors). Hall effect types often need to move so that the armature position can be determined especially with the type that uses both the encoder. The resolver type can resolve the armature position to 1 in 64,000 per revolution but using conventional encoders the position can only be determined to 1 in 10,000. It is possible to over stress the motor for a limited time is some cases it is possible to obtain as much as 200% of the rated torque from the motor. Careful design is required to ensure that the motor is not over stressed which can cause the motor to overheat and damage the motor. AC servo motors need to be controlled by the application of three phases using a 6 transistor bridge. Typically the bridges are of MOS-FET or IGBT types. The best quality servo amplifiers produce synthesized sinusoidal 3 phase. The sinusoidal wave form is simulated by the application of Pulse Width Modulation ( PWM ). The frequency of the PWM is normally from 10KHZ to 50KHZ. Transistor and IGBT types need to be run at lower frequencies due to limits imposed by bi-polar transistors. The other common type of amplifier is the lower cost trapezoidal type. This type produces a trapezoidal wave form which is easier to produce and needs the use of lower PWM frequencies. The disadvantage with this technique is that it is less precise, more prone to cogging and poor wave shape decreases the motor efficiency and therefore causes the motor to heat. Some amplifiers even use the application of square waves and make no attempt to assimilate the correct sinusoidal wave form.

Stepper motors
There are many applications for stepper motors the majority are for applications such as printers where there are fixed points of operation. The major advantage of stepper motors is the price they are far cheaper than other motor types. Most stepper motors are used in lower power applications. Stepper motors have a fixed number of magnetic poles on the rotor that align with the poles on the stator. When the motor is excited electrically the armature moves to align the poles. When the power is applied to the other direction the armature is forced to move 1 pole to align with the magnetic flux. Movement is caused by controlling the application of the power through the motor windings in a complex pattern, as the motor moves through a series of steps. The number of steps is determined by the number of poles in the motor. Currently the number of steps for a typical motor is 60 per revolution, however it is possible to obtain motors with 300 or more steps per revolution. Stepper motors can be "micro stepped " by controlling the phase angle with respect to each winding it is possible to cause the motor to align between poles this method is reasonably reliable for "half stepping" where the motor moves between steps but when quarter or eighth steps are used the results are less well defined. One major problem with steppers is that they tend to run very hot as they have to apply full torque all the time they are used. Servo motors only need to apply torque when required as the position is maintained by the motion controller. Stepper motor do not normally need or use an encoder as the position is controlled by the application of "step and direction" information from the motion controller. This step and direction is normally via TTL output. Stepper motors are normally not suitable for applications where there is a need for very high accelerations or very high speeds, or where there is the possibility of sticktion (i.e. where there is the possibility of instantaneous loads that might exceed the maximum torque rating of the motor). If the motor is subject to sticktion there is possible of loosing position as the motor may stop the point of sticktion while the controller continues to demand a positions ahead of the point of sticktion. Where very smooth movement is needed consider a DC brushed servo motor.


DC Motors
Motion controllers can control DC motors directly via a suitable amplifier. DC motors can only be used for less demanding applications where great precision is not needed for example the velocity of a conveyer, controlling small jacks and other small low precision machines. The PMC range can supply PWM for the direct control of motors via a suitable bridge arrangement. Other control methods can include Thyristor for unidirectional applications. DC motors have a higher mass than servo motors and therefore suffer in performance levels and efficiency. For high performance applications consider the use of a servo motor.


AC Motors
Single and three phase motors can be controller in several ways from simple ON/OFF control via the controllers outputs to general motion control. For single phase motors is not possible to provide positional control other than on a crude basis. It is however possible to control the speed of the motor using one of the controller's outputs to fire burst fire or phase angle control a triac. Three phase motors are suitable for control by various AC drives like the ABB range. These drives use single or three phase mains supply, this is applied to a rectifier and smoothing capacitors. The drive produces a three phase output using a similar PWM technique to that used in the AC servo motor amplifier. The major difference between AC servo motors and AC motors is the level of accuracy and performance for example AC motors do not have high level of armature control and therefore it is not possible to high level of precision offered by the servo motor. AC motors are designed for a specific mains frequency they have a high armature mass and therefore the performance will be much lower than servo motors. But there are many applications where their performance is adequate for example controlling the velocity of conveyors, pumps, jacks and other low precision applications. It is possible to obtain drives that range from simple unidirectional velocity controls to bi-directional positions systems with breaking. Breaking normally requires an braking resistor but regenerative braking is possible.

Hydraulic Systems
Another system in common use is the hydraulic system. Hydraulic systems are in general more difficult to implement and maintain and are less accurate than electrical systems however they are very powerful. Common uses of hydraulic systems are for bending machines where the motor needs to supply a large amount of torque. hydraulics are more difficult to control because of the characteristics of oil. When the oil is cold it's viscosity is a problem and the system is lethargic and unresponsive. However with the proper valves and proportional valves it is possible to achieve reasonable accuracy. Many hydraulic systems these days are being replaced by servo motor systems this is because modern servo motor are more precise and the torque has increased due to recent developments in motor technology.


Pneumatic systems
Pneumatic systems are in general used for fixed position applications such as robots pick and place machines. It is possible to control the position of some pneumatic cylinders but this is difficult and not very accurate. TRM has a system that allows the position control of certain types of pneumatic cylinders however the system is not ideal where accurate control is needed.


TRM International Ltd.
86 Moss Road
Southport, PR8 4JQ, UK.

Tel: +44 (0) 1704 563777

Go to top