Motion Control Provides Pharmaceutical Prescription
| By: Kristin Lewotsky, Contributing Editor
Pharmaceuticals are big business -- according to industry analysts, in 2007 alone, the global pharmaceutical market reached $712 billion. Manufacturing is only part of the process. Once pills, capsules, ointments and so on are produced, they must be packaged, cartoned, then sent to pharmacies where they are dispensed to the public. That’s where motion control comes into play, not only speeding the process but allowing manufacturers and retailers to trim staff and leverage lean manufacturing practices to remain competitive.
The industry presents several challenges, among them a stringent regulatory environment. Pharmaceutical packaging involves highly regulated substances. The U.S. Food and Drug Administration (FDA) rules mandate that no operation is considered to be to be done properly unless it is validated. Each product must be checked and verified, for example with barcode scanning and vision systems, to ensure that when the product gets to the consumer it is safe.
One means of packaging pharmaceuticals is in blister packs consisting of an array of plastic domes, each enclosing an individual pill. The packaging process does not end there, however, which is where MGS Machine Corp. (Maple Grove, Minnesota) comes in. The company produces machines for secondary packaging operations, for example enclosing the blister packs in card stock, loading multiple blister packs into cartons with literature pieces, or packing the cartons into cases in specific pack patterns.
“We use servos because our applications are more and more demanding and the servos seem to be able to do more and do it smoother and better and quicker, ” says Alex Vigdorovich, principal engineer at MGS. Reliability is also key. “Some of our machines produce up to 450 cartons per minute. Stoppage even for a short period of time affects the production of our customer quite significantly.”
MGS produces intermittent-motion and continuous-motion equipment controlled by PLCs. In a typical machine, an element dubbed the bucket collects the products to be placed into the carton. The machine architecture splits the operations into 360-degree cycles enabled by encoders and programmable limit switches (PLSs). Certain events need to take place within this 360 cycle of the encoder disk rotation: for example, a bucket needs to move, another component such as a blister pack needs to be added to the bucket, a barcode needs to be scanned, etc. The PLSs can be set to trigger different events within the cycle, such as moving the bucket from 10 degrees to 150 degrees, adding the blister pack from 150 degrees to 200 degrees, scanning the barcode from 200 degrees to 250 degrees, and so on.
Problems can occur if mechanical backlash causes the system to dither over the zero-degree point. In such a situation, the system would report an error. Consumer safety can’t be compromised, so regulations mandate that in the event of error, every product in the system must be cleared out. This can result in significant scrap loss. One way to address this problem is to add another PLS channel and some additional code to the PLC, but that adds cost and engineering time, as well as complexity.
Motion control provides an alternative. MGS uses integrated servo systems with control logic. “The controller resides within the PLC,” say Vigdorovich. “The PLC has what is called a virtual master that the servo follows with some kind of ratio or camming off of it. This virtual master doesn't have any backlash because it's virtual, so it is perfect. If it's stopped at zero, we don't have these debouncing problems.”
Motion Control Moves In
Servo motors provide more benefits than just in controls. MGS has limited the use of pneumatics because of concerns about noise, vibration, accuracy, and reliability. Even mechanical systems have their drawbacks. “A few years back, we basically used a single motor and then a lot of drive components, chains, and belts to drive different components and synchronize them,” says Vigdorovich. When we went to multi-axis servo systems, we eliminated all those drive components and the problems and maintenance and dirt that come along with them.”
In pharmaceutical manufacturing, cleanliness is a huge issue, but servo motors bring more than reduction of contaminants. System designs become simplified, which pays added benefits. Once upon a time, companies simply built a dedicated line to package each product. In today's more competitive environment, however, the same production line needs to be able to switch rapidly from product to product. To avoid cross-contamination, regulations require that during each switchover, all traces of the previous product be removed. Motion-control systems eliminate many of the cams and gearheads of mechanical designs, minimizing the places for product to be hidden.
The high torques provided by servo motors yield high-speed operation, but the capabilities of electronic camming are also important, says Vigdorovich. Electronic shafting can introduce a ratio between the speed of axes but electronic camming allows different axes to move differently, for example one axis slowing down intermittently to allow a vision system or a barcode scanner to snap a picture while the other axes operate uninterrupted. “We allow the camera more processing time to see the code on the product better,” says Vigdorovich. “This increases the reliability and efficiency, which are extremely important to our customers.”
Electronic camming also enables axes to accelerate and decelerate as necessary, for example allowing the bucket that delivers the product to the carton loading area to enter and exit the space quickly enough that there is no mechanical interference while moving slowly enough at the other end to allow blister packs to be loaded up. “E-camming helps us deal with that,” Vigdorovich notes. “We can change the motion of the displacing mechanism, accelerating when needed and decelerating if needed again.”
Safety also plays an important factor in pharmaceutical packaging machines. Certainly operator health is a concern, but safety drives additionally increase reliability and reduce downtime by providing a safe-torque-off option to replace removing drive power in the e-stop condition. “We try to avoid jams but if they happen several times within a certain period of time, there is a wait for the system to start,” says Vigdorovich. “Any wait for our customers when they're producing 300 parts per minute is a lot a lot of production loss. What safe off allows us to do is basically keep power on the components but not allow components to start unless they are told to start.”
Of course, packaging the items is only half the battle. Once they reach the pharmacy, medications must be dispensed, often hundreds of times per day. A pharmaceutical equipment supplier named ScriptPro has developed a solution -- a line of machines that stores and automatically dispenses pills, capsules, and unit of use medications like lotions and ointments. Designed to have the same form factor as a pharmacy shelving unit, their machines consist of an array of pill cells fitted with dispensing chutes (see figure 1). A gantry arm fitted with a gripper unit moves across the array to carry a pill vial to a selected dispensing chute within seconds (see figure 2). A singulation system delivers pills to the chute, where they are counted by optical sensor before dropping into the vial, to an accuracy of 99.7%.
Motion control, in conjunction with careful engineering, allowed the company to achieve its desired performance, footprint and robustness in a wall-plug compatible unit. Each machine incorporates just four servo motors: one to power the gripper unit that holds the vials, one to power pill dispensing, and two to position the gantry arm along the x- and y-axes.
The pills in each cell sit directly on top of a 3 in. diameter, flat plastic gear (see figure 3). The gripper unit incorporates a pivot arm with a 1 in. servo-motor-driven gear that engages with the plastic gear. As it turns, it pushes pills into the singulation funnel that leads to the dispensing chute. A separate actuator on the pivot arm opens the door to the dispensing chute, allowing them to enter the vial.
Servo motor muscle
As with packaging operations, the torque offered by servomotors provides big benefits. Pills consist primarily of active ingredients held together by inert binders. Some pills may be chalky; all tend to be heavy in bulk, both of which require a significant amount of torque to move the plastic gear. “The motor that turns the gear for our counting apparatus is actually fairly torquey because there’s a lot of stuff pushing on [the gear],” says ScriptPro software engineer Bradley Lockard.
Of course, all that pill dust raises the issue of cross contamination, as well as motor contamination. The ScriptPro team addressed the issue by never exposing any part of the system to a given pill except the pill cells and the vials.
Servo motors supplied the torque the team needed in a form factor small enough to allow the use of encoders, which guaranteed performance. “We have over 99% accuracy on all of our pill counts and that's almost impossible to do without feedback on the control motor,” Lockard adds. Here again, the flexibility of motion control paid off. “For the pill count motor, say you want 50 pills. At the very beginning, you want to just dump them in as quick as you can, but you want to make sure that you have good control on the last few. That’s why all the motors we use are servo motors with optical encoders.”
Torque, accuracy, flexibility and reliability: motion control fills the prescription for pharmaceutical success.