Brushles DC motor with local control intelligence from maxon motor's compact drive

For the bottle cap not to become a bottleneck.
The example of a motor driven machine producing bottlecaps for wine and liquor bottles shows a typical case of a local intelligent and highly integrated small power DC motor drive (60W). The PLC functionality built into the motor drive coordinates the local inputs and outputs such as program selection, motor start and stop signals, light barrier, state and control of the scissors and error signaling. The workload of the main PLC of the machine is relieved. The programmable maxon compact DC motor drive complies with the usual drive technology standards and is designed for harsh industrial motor use.

Modern motor drives in handling systems and industry machines are designed as networks of single modules. Despite the ever increasing complexity and trend towards miniaturization, the complete system should exhibit a high degree of robustness. The application that follows describes a typical example of how these demands may be addressed.

In wine and liquor bottling machines a film is shrunk over the bottleneck and the cork or cap, a so-called bottle cap. Two different modes of operation are used to cut the film to the desired length. The first method advances the reel-fed band by a fixed distance after a starting signal. Then, a digital signal activates a pair of motor controlled scissors cutting the band. The second method uses a marking on the band which is detected by an optical reader and initiates a relative motor position move. Again, the scissors are activated. The selection of the mode of controller operation is set at the beginning by a digital signal.

 

Figure 2: Schematic representation of the appliance using the programmable MCD EPOS P.

The filling machines have an expected life of several decades if properly serviced and maintained. During the life cycle faulty modules and sub functionalities may be replaced, improved and modernized. This is made easier by a basic modular conception of the machine. Traditionally, a stepper drive controlled by the main PLC was used to cut the shrink-film. A redesign of this function uses a programmable maxon compact drive MCD EPOS P. The underlying goal is to put the modularization one step further by transferring intelligence to the local drive.
Another important aspect is to reduce the outer dimensions of the drive and at the same time achieve a higher force. The requirements are at least three motor working cycles per second. These conditions are optimally fulfilled with the use of a brushless DC motor as found in the MCD EPOS combined with a maxon ceramic planetary gearhead GP 32 C (reduction ratio 18:1), to help provide higher life expectancy. The MCD is smaller than the existing motor controller and fits perfectly below the feeding roll with the gearhead. A toothed belt transmits the torque from the gearhead to the roll with an additional reduction of 2:1.
The local PLC functionality is built into the programmable MCD EPOS P 60W and controls the complete cutting operation. The motor drive receives a digital signal from the main PLC defining the mode of operation. Upon the starting signal, the complete process is controlled locally. This means the signals of the optical sensor and the state of the scissors are read, the position moves with the motor executed and the scissors activated. The digital outputs are used as well to provide error signals and to activate the inputs. When replacing an existing design with a redesigned module it is important that the existing interfaces for the rest of the machine may continue to be used. It is very helpful that the inputs and outputs can be freely configured on the MCD EPOS P.

 

Figure 3: Simplified process sequence for illustration of the input (I) and output (O) control.
Programmable drive unit
The small and light weight compact motor drive MCD EPOS P with 60W power rating perfectly fits the requirements of the application. Motor, encoder and motion controller are all integrated into an aluminum housing allowing protection up to IP 54. All internal connectors are vibration proof and suitable for a harsh industrial environment. The brushless DC servomotor impresses by the high power density and the long life expectancy. Due to the high starting torque, an excellent dynamic behavior is obtained. The slotless winding allows smooth operation without cogging even at the lowest speed. This is further supported by the sinusoidal electronic commutation scheme of the controller. The digital incremental encoder with 1000 counts per turn results in 4000 positions of one motor revolution or a nominal resolution of 0.09°. Combinations with gearheads are foreseen to increase the torque.
The motion controller used is a member of the maxon EPOS family, a modular positioning system with CANopen filed bus interface. It can be used as a speed, torque and position controller. The motion controller is integrated in the slim and compact housing without increasing the outer dimensions of the drive with any protruding parts. The sophisticated heat management of the power stage is made with a printed circuit board specially mounted on an aluminum support. This allows keeping the electronics small and yet get the full rated power from the motor. The power stage is optimally adapted to the power requirements of the motor.
Due to the opto-coupled configurable inputs and outputs all kind of signals from the drive periphery can directly be connected, i.e. end switches or reference switches.

The programmable version of this compact drive - MCD EPOS P 60W - includes a PLC functionality with a CANopen master. Decentralized drives can be realized in a most simple manner. At the touch of a button an autonomous movement may be executed, and at the same time local digital outputs and inputs can be set, read and used for controlling the program sequence. In addition, there is the possibility to create networks of multi axis motor systems by means of the CANopen communication.

The programming of the integrated PLC complies with the international standard IEC 61131-3 as opposed to many other small drives which are programmed by some proprietary language. Software tools and programming libraries are included for free.
Advantages in the industrial environment
The advantages of the new solution for cutting the shrink film using the maxon compact drive (MCD) EPOS P 60 W consist of the compact size, the built-in intelligence and the strict application of industrial standards which simplify the integration and the flexible replacement of existing modules. This application example illustrates a typical and growing trend towards miniaturization. This trend can be found in drive technology as well where it is tackled by two strategies. In appliances made in larger quantities the drive components are designed to fit into the overall system. Mechanical and electronic interfaces are adjusted precisely to match the application. For example, the lead screw and the gearmotor may be one entity, or the motion control may be integrated into the electronic controller of the complete appliance. In the industrial field modularization is very important which means that compact drives are achieved by integrating motor, sensor and motion controller into one housing maybe even supplemented by an additional PLC functionality. Typical examples for this are the two versions of the MCD EPOS. In this field, robustness plays an important role as well. Robustness is not only to be understood in terms of mechanical durability or resistance against external disturbances and interference, but also in terms of simple concepts and networking by means of known and proven components fulfilling the usual industrial standards. The interfaces must be clearly defined, the wiring minimal, the range of functionality sufficiently extended. All this supports modularization where complex tasks can be executed autonomously by a local intelligence, controlled by very simple commands from the main control. Standardization could stand for using the CANopen field bus system with defined device profiles to build up networks, for free configuration of inputs and outputs or for PLC programming following IEC 61131-3.
Conclusion
Drives with less than 100W power are used in many industrial machines for a variety of tasks: Feeding foils and films, adjustable stops, handling, automation to mention just a few. Over the last few years, more and more drive systems with integrated intelligence may be found on the market. Larger motors with power rating of 1 kW and above mostly fulfill the usual industrial standards, whereas smaller drives come along very often with proprietary solutions which are difficult to integrate into the overall system. For the machine designer, compliance with standards is extremely important because it allows using a uniform concept. When designing the modules, one may fall back on well-known interfaces, hence reducing the development time. The maxon compact drives open up new possibilities in such applications. Be it a drive axis in a CANopen network or as a programmable drive unit autonomously controlling an appliance as a whole or in combination with a higher level PLC as shown here in the example of the film cutting device.

compact drive MCD EPOS P

Author: MMAG maxon Academy Dr Urs Kafader /23.08.2010