PID Tuning for Catheter Reflow Heater

What if you needed very precise heat control for your catheter reflow process and commercial solutions weren't cutting it? Maybe you need to reflow different polymers at different temperatures along the length of your device. What do you do? Maybe creating your own PID controller and tuning it is the answer. This article will walk you through a tuning methodology.

The Basics: We'll assume that you're familiar with PID control but let's go over some basics. The purpose of a PID controller is to reduce the error between a process set point and the actual process parameter. This difference is called the error or "e" in the equation. This error is what drives the whole control loop.

The "P" coefficient is multiplied by the error to affect the output. The higher the error, the more the proportional term reacts. The "I" coefficient is multiplied by the integral of the error over time. Simply put, this term grows over time the longer there is error in the system. Finally, the "D" coefficient is multiplied by the derivative of the error over time. How fast the error is changing affects this term.

Setting a goal. It's important to define what an acceptable outcome will be. If you don't define the goal, you may never finish the tuning process. In this case we want the process temperature to reach the target temperature in 10 seconds or less. We want to limit overshoot to less than 10º C and we want the process temperature to stay within 5º C of the target.

Seeing the Process. It can be daunting to think of where to start tuning a PID system. The first thing you need is a way to visualize the process parameters and change the process setpoint. Here we've programmed the heater to alternate the target temperature between 300º C and 400º C (red line). We're plotting the actual air temperature in blue. The heater output is shown in green and varies between 0-100% power.

Is the Process Controllable? This may sound like a stupid question but answering this upfront can save you countless hours and frustration. If you were to turn on the heater element, could the temperature reach and surpass the target? Does it come down with the heater off? If the heater doesn't have enough power to reach the target, or has way too much power and blows past the target, you'll never be able to control the system. (I've learned this the hard way). The heat should also react proportional to the power applied to the heater. Say the first 80% of power commanded did nothing, and then suddenly the heater turns full on at 81% commanded power. This would make controlling the system impossible.

---

P=10, I=0, D=0

P=20, I=0, D=0

Adjusting the P coefficient. The first thing to do is set the I and D coefficients to zero and start adjusting the P. The images above shows a P at 10 and 20. When P is set to 20, there are large oscillations in the temperature. With P set to 10, the Oscillations are dissipating quickly. We could pick a P term that doesn't produce any Oscillations or overshoot, but we will take care of a little overshoot later with the D coefficient. Notice that in either case, the temperature is consistently under the target. This will be resolved with the I coefficient.

---

P=10, I=1, D=0

Adding I coefficient. Now that we have the P coefficient producing some output, we can add I to the controller. With I set to 1 in this case, you can see that already the temperature is running much closer to the target. Some of the oscillations from the P term are reduced as a result of the smaller error too.

---

P=10, I=3, D=4

Adjusting the D coefficient. Now that P and I are set up, we can increase the D coefficient and attempt to remove overshoot and oscillations. The image above shows the response after adjusting the D coefficient. Notice the extra I term keeps the temperature very close to the target. But there is too much overshoot and there are still some oscillations.

---

P=5, I=1, D=5

Refining. In this image, we see a much more predictable response. Overshoot is reduced and the process quickly finds the target without oscillating. The overshoot and oscillations were reduced by reducing the P and I coefficients. As we saw from the initial P adjustment, this term was producing some oscillations. It was too much to fix with the D term, so we backed off of the P.

---

P=4.5, I=0.7, D=5

Finishing up. With a little more refinement, we have a process that quickly reaches the target temperature without overshoot. This type of controlled response is critical when changing temperatures along the length of a catheter. In this extreme example, the temperature setpoint changes 100º C and the process temperature follows in less than 10 seconds

How to Decide Between 3DPrinting vs Machining for Your Prototype Needs

Once you own a 3D Printer it can be easy to get carried away with all the cool things you can print. But is it always the best option? Let's compare machining (turning) four parts vs. 3D printing the same parts.

These spools are for a new product that removes Zeus FluoroPEELZ®​ heat shrink after reflowing catheters. They are 1" OD and feature a knurled grip and threaded holes. They are well suited to either machining or 3D printing. We'll consider these prototype components for this study.

Time to Make This is how long it took to machine or print the parts once the parts were designed. The machine parts beat the printed parts in this category by 30 minutes.

Material Cost The raw material cost. The printed parts pictured here were made with generic PLA and cost $0.58. But if your printer uses proprietary material, this can be as high as $3.84 for the set.

Additional Modeling Time These parts needed to be knurled so extra time was required to "cut" the knurls in SolidWorks. The machined part doesn't require the knurl to be modeled.

Labor Cost This is where the magic of leaving the printer unattended comes into play. This figure represents labor to model the extra features in CAD, and a quick printer setup. The machined part requires a machinist to spend the full 2 hours on the parts.

Total Cost The machined parts are over 3X more expensive when taking into account labor. This can vary, but 3D printing is a much more hands-off process and will usually be cheaper than machining for prototyping.

​Robustness of Part Here is where the machined aluminum parts shine. They are of course much more robust. They are also a user touch-point and feel much better to the touch.

In this case, machining is the better option for these parts. The parts are stronger and feel better in the hands of the user. The decision to machine these parts was made with the user needs in mind. There is no single answer to the print vs. machine question. Each part must be evaluated on a case by case basis.

3D Printing End-Use Parts for a Wire Feeder

Most of us think of 3D printing as a great way to build prototype parts quickly. We then think of conventional manufacturing methods like molding or machining for final parts. But what if you could skip that step altogether?

Using 3D printing for end-use parts requires us to think of 3D printing as a manufacturing process. And like all manufacturing process it has its strengths and limitations. But once these strengths and limitations are understood, parts can be designed for 3D printing. Just like a part that is well designed for machining may not be easily injection molded, such a part won't be ideal for 3D printing. So we need to understand the process when designing.

There are two ways of designing parts for 3D printing. One is to start with parts that are already designed to be machined or molded, and convert the design for 3D printing. This can allow for reliably printed parts that are able to meet the requirements of the originals. But this approach does not take full advantage of one of the key strengths of 3D printing which is that complex parts can be made. The second way to design parts for 3D printing is to conceive of the design from the beginning with 3D printing in mind. This allows for a high integration of features onto a low part count. This allows us to simplify the overall design by adding complexity to the components.

For this wire feeder, a gear needed to be added to a timing pulley. With a conventional machining approach, two or more parts would have been made and put together as an assembly. But by printing the parts as one piece, the gear, pulley, and flanges can be integrated into a single part. This saves cost, complexity, and assembly steps.

This feeder will be used in a ISO class 6 clean room, and will be used on a medical device manufacturing line. For this reason, carbon fiber-filled nylon was chosen. It can be run without lubrication and is stiff enough to withstand the loads needed. In future posts, print orientation, material selection, other 3D printing specifics will be explored.

You may want to consider 3D printing for your next design. Or maybe you're already a 3D printing design hero. Let us know in the comments if you've made end-use parts that are printed.