Example motor and control selection -Controller Information - maxon academy Dr Urs Kafader
A perforated strip with labels to be wound into a spool by DC motors at constant tension. Uniform tension is critical to prevent the perforation from tearing. Therefore, a non cogging, uniformly rotating ironless DC motor should be used to drive the spool application. The DC motor torque has to be adjusted to account for the changing diameter of the wound strip. Gentle startup of the strip is critical.
Evaluation and selection control concept and feedback sensor.
- The control variable is the torque and therefore a current controller (torque control) is required. The command signal has to increase linearly with increasing spool diameter to satisfy the requirement of constant load tension.
-Particular attention must be paid to the conditions during motor start up. The motor current value must be slowly increased to the target torque.
- When using an EC motor (brushless), smooth, uniform sinusoidal motor commutation is a good choice: maxon DES 50/5 or DES 70/10 controller.
- It may be necessary to take additional measures to prevent an uncontrolled acceleration of the spool at no load (e.g. if the strip breaks). This can be done by: - Limiting the maximum voltage - Monitoring the speed by means of IxR compensation with DC motors or hall sensors with EC motors - Monitoring the speed by means of an additional speed sensor (e.g. encoder; already present with sinusoidal commutation).
Example motor application -Positioning table with drill
An appliance for drilling holes in circuit boards requires two motor drives for positioning the circuit board (xy-table), one motor drive for lowering the drill (z-axis) and the drill spindle motor. The four drives are controlled centrally by a PLC in conjunction with additional I/Os (e.g. a bit break dectector) via a CAN bus.
The xy-table is positon actuated by two lead screws.The motors are equipped with encoders for positioning. Synchronization of the two DC motors is unnecessary, all that matters is to reach the end position. Once in the target position, the position controlled z-axis lowers the drill which itself is speed controlled.
The obvious choice here - for all axes - is the use of the EPOS family of motor position contollers with CANopen interface for communciation with the PLC. If the periphery of the machine is not overly complex, a cost effective, programmable EPOS P can repalce the PLC.
The drill itself could also be operated with a pure motor speed controller. If different speeds have to be communicated to the drill, a digital speed controller with CAN interface (e.g. maxon DES or maxon EPOS) makes sense. If the drill is only switched on and off and the speed remains constant, a less complex brushless speed controller will suffice.
This cmotor and controller example should also be considered representative of other handling and automation systems.
PID Controller settings
A PID controller is a combination of proportional, integral and derivative controllers. It describes how the error signal is amplified to produce an appropriate motor response to reduce this deviation. With speed controllers, a closed loop control circuit with a simple PI alogorithm is usually used. With positioning systems, a derivative term is also necessary. Each of the three terms influence each other and understanding this interaction is of particular importance for the fine-tuning of a motor positioning system. For optional system performance, the coefficients Kp, Ki, Kd have to be set depending on the specified motion and load inertia (Literature Feinmess).
P, Proportional controller: The error signal e (difference between actual and set value) is multiplied by a user-specified factor Kp and then transmitted as a new current command. An elevated Kp value accelerates error correction.
If Kp is too large, severe overshoot will occur, at even higher values of Kp the system may begin to oscillate, which in turn can result in instability if the damping in the system is insufficient. Kp cannot completely eliminate the error e, since the proportional correction value (Kp . e) becomes smaller as the error e decreases. The result is a residual error value that is particularly important in systems that require a high current simply to mainitian motion (e.g. because of high friction).
I, integral conroller: Here the error is summed over a longer period, multiplied by a user-specified factor K , and then added to the new motor current command. Because this method also considers past errors, the current command value does not necessarily approach zero as the error value e approaches zero. However, this eliminates the residual error. There is an inherent disadvantages to this method. The factor K, destabilizes the entire control circuit. Without appropriate damping, high Ki values can cause severe system oscillations as past errors have a delayed effect.
D, derivative controller: The D controller considers the change in the error which is multiplied by a user-specified factor Kd and added to the current command. Sudden errors such as set value jump, can be very quickly corrected with this method. Properly set, this type of control can improve control stability, but a certain residual error remains as the derivative of a constant is zero.
Feed-forward
With the PID algorithms there is only a correction signal if there is an error. For positioning systems, this means that there always is - in fact there has to be - a position following error. The objective of the feed forward control is to minimize this following error by anticipating future system behaviour and communicating the expected error to the control circuit in advance. There are generally two correction variable two correction variables available for this purpose and they have to be determined for the specific application and motion task.
- Speed feed-forward correction: This component considers the mass interia and provides sufficient current to accelerate this interia.
- Acceleration feed-forward correction: This component considers the mass interia and provides sufficient current to accelerate this inertia.
1-Q Amplifiers
Incorporating these features reduces the average following error when accelerating and decelerating. By combining a feed-foward control and PID, the PID controller only has to correct the residual error remaining after feed-forward, thereby improving the system response.
- Can only accelerate the motor (speed and torque point in the same direction).
- Work either in the first or third quadrant.
- Have no controlled braking behaviour. Controlled braking means that a requlated torque is generated opposite the direction of rotation.
- Use the inherent friction in the drive system for braking and decelartion. The motor current cannot flow in a manner that kinectic energy is removed from the drive.
- Generally react relatively sluggishly to load to load and set value changes from high to low (depending on friction).
- Can often br used in both directions of rotation. Some manufacturers call this 2Q but in amplifier vernacular this is called 2x 1-Q. The direction of rotation can only be set as either CW or CCW. e.g. by means of a digital signal (switch).
- Are available from maxon only for simple brushless DC motor drives.
2-Q Amplifiers
- Control the speed in one direction if rotation in the event of a load change, both during acceleration and braking. Operation in the other direction of rotation is acheieved by means of a digital signal (switch) and not via a negative set value, for example.
- Work either in the first and second or third and fourth quadrants.
- Reduce speed in an active manner. Active means that a torque ir current can be applied opposite the direction of rotation.
- Regenerative energy can flow back from the motor into the amplifier (power suppy) during braking.
- Rapidly control the speed, both with increasing and decreasing load.
- React quickly to changes the speed set value.
- Are available from maxon only for brushless DC motors.
4-Q Amplifiers
- Control the speed in both directions of rotation in the event of a load change both during acceleration and braking.
- Work in all four quadrants.
- Actively reduce speed. Active means that a torque or current can be applied opposite the direction of rotation.
- Regenerative energy can flow back from the motor into the amplifier (power supply) during braking.
- Rapidly control the speed, both with increasing and decreasing load.
- React quickly to all changes to the speed set value.
- Are mandatory for position control systems.
Selection criteria for feedback sensors
The feedback sensor (encoder, DC tacho or resolver) must be appropriate for the control task and comply with the other components. It is selected and mounted on the motor according to the maxon modular system. It must measure the correct control variable (speed, position, direction of rotation) with sufficient resolution. As a rule of thumb, albiet one that cannot always be observed, the resolution if the sensor should be at least four times higher than the specified accuracy if the control variable. The controller mustr be able to process the signals from the sensor, and the voltage and frequency signal ranges of the controller must not be exceeded. The controller must provide the sensor with the required energy in a suitable manner (e.g. suppy voltage for encoders, HF supply for resolvers).
Feedback sensors are usually mounted directly on the motor and measure the motor behaviour. Due to the mechanical play in the drive, the load responds with a delay to changes in direction are usually mounted directly on the motor and measure the motor behaviour. Due to the mechanical play in the drive , the load responds with a delay to changes in direction and position errors can occur. In certain cases it my be appropriate to attach the senor directly to the load, e.g. in the form of a linear encoder. The problem of the mechanical play has now been shifted to the control circuit where it causes dead time and phase shifts. Some control algorithm of very high quality control systems allow for compensation of a known amount of play.
Author: maxon academy/25.08.2010
