Ball Mill Control System Upgrade
The American biotechnology product development company; Thermo Fisher Scientific approached us for help with the design and installation of a new control system for three new ball mills. The mills were to replace an existing system, which was reaching the end of its lifecycle and did not comply with modern safety standards. As such, for this ball mill upgrade the new control system had to mimic a validated process.
Adbro’s scope in this GAMP project, as well as providing the control system, was to consult on the safety concept on the machines, ensuring compliance to EN ISO 13849-1.
The customer was also interested in future development, making further improvements and process automation, therefore it was imperative that the solution had the flexibility to achieve this.
CAD drawing of Ball Mills
For both the ball mill wiring and final installation we used 304 grade stainless steel basket for securing cables.
A stainless-steel trolley was designed and fabricated, which was required to empty the alumina balls from the mills. The trolley needed to be bespoke as it was part of the guarding concept on the machine, it also needed to be made with FDA compliant materials. A coded actuator was mounted on the trolley, which allowed the safety system to detect when the trolley was fully inserted onto the ball mill outlet.
The positioning system required an absolute encoder to be mounted onto the ball mill, this was a challenge as unfortunately, no facility was allowed for mounting the encoder by the machine builder. The initial prototype was mounted onto the shaft end of the drum, this was not an ideal position as it left the encoder exposed on the end of the machine. The clearance between the machines was too close. This meant that the only other ways to mount the encoder was either on the motor, where gearbox backlash would be an issue or perpendicular to the drum shaft, through a guard. We decided that the latter would be the best approach, however, this meant that a completely bespoke coupling arrangement needed to be developed. A helical face gear split in half was mounted onto the shaft as a collar. The encoder was integrated into a bracket assembly with a supported shaft and pinion on the end, which passed through the shaft guard. The entire assembly (excluding bearings) was 3D printed using FDM technology using a high-performance carbon/nylon composite material.
The new control system was a Siemens based solution. Using current generation hardware, the controller was an S71500 CPU, integrated into the ET200SP IO system, an ultra-compact form factor which allowed us to keep panel sizes down. The PLC was also a failsafe type controller, which enabled direct integration of the safety system with the control system.
The safety devices used in the system were Pilz based. Emergency stop buttons with integrated lights allowed for rapid diagnostics in the field if a switch had been pressed. A solenoid locking system was implemented on the infeed hatch, with a rundown timer that prevented access to the system until the machine had come to a standstill. On the outfeed of the ball mill a coded magnet switch was used, which monitored if either the trolley or valve was inserted on the outlet, again with LED’s to indicate if the switch was healthy.
A distributed concept was used in the control system. Siemens ET200SP remote IO was mounted on each ball mill with all instruments connected to it. This greatly reduced the amount of field wiring and reduced the final installation time significantly.
The drives used in the system were Siemens series G120C inverters. These were selected as they enabled PROFINET integration with the PLC and could be configured from the same software package and as the PLC was a failsafe type, it also meant that drive safety could be implemented over the communication channel, using PROFISAFE. Using positioning technology objects and MC Open function blocks in the PLC, the drive was closed loop controlled with a PROFINET encoder. The system was able to position the hatch at predefined stopping points and accurately control the rotational velocity. An electromagnetic holding brake was used to keep the drum in position, when power was removed from the drive.
The ball mills were controlled through a Siemens HMI, compliant to CFR21 part 11. This allowed flexibility for the user, enabling them to control elements such as timers, positioning control, data logging, user tracking, diagnostics alarms of equipment in the field, and holding recipe systems (future scope). PDF’s were also stored on the HMI, allowing operators to view the control system manual and technicians to reference electrical schematics.
The ball mill upgrade was a success.
The new control system was able to mimic the validated process, ensuring that speed was maintained at the correct level.
Operation of the machine was simplified. Operators no longer had to manually move the hatch to the top and use a bolt to secure the drum. A key interlock was no longer required for access as the hatch would automatically unlock when the mill was at a standstill. The controls were simplified on the mill, with ergonomics in mind.
Traceability; the HMI recorded all operator interactions, as user must be logged into the system to operate it. Electronic signatures with also implemented on process critical values.
Diagnostics; alarms were logged by the HMI, monitoring drives and I/O. This allowed the technicians to fault find the system easily.
Operation was made safer by a combination of both mechanical guarding and safety switches. The inclusion of a global emergency stop philosophy allowed any E-stop on a ball mill to halt the entire system.
Consideration was made for future upgrades to the system, meaning hardware will not need to be replaced due to unsupported functionality or obsolescence.