DC motor tips
Selecting DC motor position and speed control units. PC servo control, PLC motor control, speed control and torque control considerations. Selection criteria for controllers: The motion controller is the central element in higher-level motor drive systems. It is where all the threads come together and thus, the controller must satisfy a disparate number of requests. It Must; -Be able to control the manipulated variable with sufficient accuracy in a reasonable amount of time. -Be able to process the information provided by the feedback sensor. -Understand the set values and commands of the higher level system. -Provide the required electric power. -Compliment the motor type (DC brushed or Brushless) Control variable and feedback sensor.(encoder, tacho, resolver) The motor controller and feedback type are largely determined by the selection of the control variable and system accuracy. First it must be possible to configure the electronics for the control of the desired command function, i.e current, speed and position control. Second the controller must understand the signals of the required feedback sensors. Current Controller. A current controller with the motor is considered a torque controller. Without sufficient friction and static load, the current controllers tend to continuously accelerate the motor to a "run away" condition. The imparted torque results in a continuous acceleration unless it is prevented by a higher level command signal, sufficient inherent system friction, a mechanical stop or a voltage limit. Speed and position controllers usually include a current controller in the form of a fast, lower-level control circuit to insure a dynamic response. Current controllers do not require separate sensors as the motor current can be measured directly at the controller output using a low-impedance resistive circuit. Many maxon controllers can also be operated in pure current control mode. Speed Controller. Speed controllers can use the signals from a variety of feedback sensors such as a DC tacho, encoder or resolver for speed evaluation. In simple applications, the signals from the motor itself are also used, e.g the motor voltage with IxR compensation or the outputs of hall sensors in the case of BLDC motors. A distinction must be made between 1, 2 and 4 quadrant controllers. 1-Q controllers are used in simple applications to drive BLDC EC motors. the response not being very dynamic. Highly responsive servoamplifiers are 4-Q controllers, 2-Q controllers fall somewhere in between with respect to control dynamics. Some speed controllers can also be used as open loop controllers, for control without feedback, an input voltage which can be adjusted by means of a set value is simply applied to the motor. This is often the case with brushless motors when the electronics are only used for commutation. A particular example is EC motors with integrated electronics (two wire brushless DC motors). Position Control. Position controllers must be four quadrant controllers because of the requirement to accelerate and decelerate in both directions of rotation. This is essential for dynamically moving to or holding a position. maxon position controllers require signals from a digital incremental encoder providing position feedback. The higher the pulse rate of the encoder the better the theoretical position resolution. maxon position controllers exhibit digital communication and signal processing. Thus, they can be connected to a bus system and controlled with a PC or a PLC. Communication, Set values and data processing. As the local connection point for the drive, the controller has to be able to understand the command signals of a higher level system, process them appropriately in response to the local sensors and give feedback to the higher level system. There are two basic types of systems: controllers with analog signal processing and controllers with digital signal processing. Analog set values and signal processing Analog control is simpler and frequently found in older systems for speed or current control. Command signals (typically set value voltages) are input and processed in the form of an analog voltage in electronics circuits made of passive elements and operation amplifiers. It is important to note that all signals must be relayed in the form of voltage values and that the various signal ranges, e.g. of the set value and feedback signal, must match. The pulse frequency of the encoder, for example, is first converted in the controller to an analog voltage and amplified as necessary. The voltages are calibrated with the potentiometer n max which is also used to set the speed range. It must be ensured that the command voltage present in the application is within the set value range of the controller. Frequently used command signal ranges are -10 ... + 10VDC and 0 ... 5 VDC. Analog controllers have a high bandwith of several kHz. The output value is corrected very quickly in response to a change to the set value or feedback. Disadvantages include possible voltage drifts due to temperature changes and the time consuming, individual adjustment of the potentiometers e.g. for setting the control parameters (gain) of the current limit. Digital commands and signal processing More modern and high-quality speed controllers and most position controllers work digitally. Here the input signals are converted to new output value via digital signal processor (DSP). Digital controllers require a local set of operating instructions called firmware to function properly. In addition to the control function the firmware can also be used to perform additional tasks such as electronics commutation for BLDC motors. Ideally a digital controller receives a digital signal command via a bus. The command input structure must be delivered in a format, referred to as device protocol, so that the controller can communicate. There is a wealth of such systems which are often closely coupled to the bus system used. maxon is a user of the CANopen protocol, a standardised platform well known with larger drives. Still some microdrive manufacturers use their own proprietary command protocols. Less complex digital controllers also use analog command signals and setting parameters and can therefore be operated similar analog controllers (see above). The input signals must first be digitised for further processing in the DSP. Consideration must be given to the voltage range and resolution of the analog-digital converter. Today's DSPs have so much processing power that there is essentially no limit to the bandwith for drive systems. Digital controllers are unaffected by temperature drifts they can be configured identically in large numbers and they can even be individually reprogrammed for special tasks. Cumbersome calibration using a potentiometer is not required. The parameters of digital controllers are in defined formats and ranges. This can lead to performance limitations. The maxon EPOS motion controller family, for example, can only process integer speed values (in rpm) up to maximum speed of 25000 min-1.
Power - The controller must be capable to deliver the required components of electrical power i.e. voltage and current.
Voltage - It is critical to be able to operate the controller with the existing supply voltage V cc, i.e. Vcc must be within the controller's specified supply voltage range. If the supply voltage is too low, the electronic components will not function properly; if the voltage is too high, the electronics may be destroyed. The power supply should provide a constant voltage with residual ripple so as not to exceed the maximum rate value. The controller must also be capable of outputting the required motor motor voltage. The motor voltage will generally be smaller than the supply voltage since a portion of the voltage will be lost in the output stage or cannot be transferred for control reasons. Such a voltage drop typically 5-10% has already been taken into consideration in the selection of the winding.
Current - It is also essential that the controller be capable of supplying the required current. The controller must therefore have sufficient continuous current capacity and the necessary maximum current capabilities. An important aspect with maximum current is time, namely how long an overload situation can be maintained. The overload capacity is addressed differently in different controllers. Refer to the data sheets and operating instructions for details. In most cases, high maximum currents in controllers are limited to durations of less than one second. This must be taken into account when calculating overload operation. The controller and the motor can be protected against thermal overload by limiting the maximum current and its duration. Possible implementations of this protection mechanism are:
- The ability to set continuous current limits and maximum current limits to protect the motor, in some cases with consideration for the thermal history.
- Monitoring of controller temperature
- Temporal limitation of the maximum current in steps: the longer the duration, the less current
- Consideration of the maximum power loss: integration of I2 over time.
Analog control maxon DC motor
maxon DC motors are high
quality motors fitted with powerful
permanent magnets. The "heart" of
the motor is the worldwide
patented ironless rotor. This means
using cutting edge technology to
produce compact, powerful and
low inertia drives. These DC
motors have very fast acceleration
thanks to their low mass moment of
inertia. The modular construction
of the A-max and RE-max programmes
offer countless options and top
performance at competitive prices.
maxon EC motor
Our electronically commutated DC
motors are characterised in
particular by favourable torque
behaviour, high performance, an
extremely wide speed range and
unprecedented service life. Their
outstanding control features help
create precision positioning drives.
Similar to the ideology of the A-max
programme, a modular motor range is
available with the EC-max programme.
The EC-powermax range provides top
performance per volume unit, pushing
the boundaries of technology.
maxon flat motor
The flat design of the brushless
DC flat motor makes them the ideal
drive for many applications.
Designed as internal or external
rotor motors they are often the
ideal solution when space is limited.
The well thought-out and simple
design means that production is
largely automated, helping keep down
the price.
Characteristics of a maxon coreless
DC servo motor.
The principle of the bell-type
winding without an iron core, I.e.
without slotted laminations, is responsible for the liner response
and dynamic characteristics of the
maxon DC motor.
In conventional DC motors, the soft
magnetic teeth of the winding are
polarized and attracted by the
permanent magnets located nearby.
A position change to the next
magnetic pole requires a
re-magnetization. The motor’s rotor
has a tendency to stop in these
preferred magnetic positions.
As a result of this cogging torque
the torque generated has a
pronounced ripple.
maxon DC motors are free of this
cogging torque, because their rotors
contain no iron. This design has
the following advantages:
Smooth running, even at low speed.
Low vibration.
Low noise.
Simple control of rotor position.
“maxon academy”
Maxon motors are supplied as a
Piece-part system with gearhead
(Gearbox), encoders (feedback),
and a full range of speed and
position control units.
"Servomechanism"
A mechanism which is used to convert
a low powered mechanical motion
into one which requires much greater
power. The output power is usually
proportional to the input power and
the device is often electronically
controlled.
"Servo motor"
Any motor which provides the power
for a servo mechanism.
“Macquarie”
The term servo motor is often
misused by motor companies,
particularly hobby motor suppliers.
A motor needs to be part of a
servomechanism to be called a servo
motor.
The key factor for a servomechanism
is that the system is closed loop.
The most common method for closing
a loop in a servo mechanism or part
of the mechanism is to use an
encoder or resolver on the motor.
A motor without linear performance
characteristics is not capable of
achieving a true servo response.
Rhombic windings in coreless
DC motors have a distinct advantage
over skew wound motors in that a
skew winding has an overlap which
creates a larger air gap and reduces
the motors efficiency. maxon is the
original inventor and world wide
patent holder of the ironless winding,
System maxon.
Author: MMAU/19.10.2009
