Final Design

Final Design Solution

Figure 1: Final Setup

Figure 2: CAD Model Figure 3: Annotated Setup

The final design solution is shown in the figures above, and consists mainly of an incubation chamber, an external heating unit, and a PID controller. The incubator is an enclosure that will secure the adapter plate and keep heated air trapped within the chamber. The adapter plate is secured to the chamber with magnets, and the cell culture is secured to the adapter plate with a metal clip. The chamber also contains a temperature sensor for use with feedback control, and a magnetic switch that detects when the cover of the chamber has been removed. The external heating unit will provide the heated air for the incubator. The decision making for the method of heating can be found in the Risk Reduction Efforts section of the Appendix. The fan is connected directly to the 12V power supply, while the heater is connected to the solid state relay. The relay is switched on and off for periods of time to vary the power output to the heater and control the temperature. Heated air is blown through three tubes connected to the chamber. The PID control is performed using a Raspberry Pi. The Raspberry Pi reads the temperature from the sensor and uses the measurement to deliver a pulse width modulation (PWM) signal to the solid state relay. There are also two buttons to raise and lower the temperature setpoint, a power button, a display for viewing the current temperature, and a magnetic switch to detect when the cover of the chamber is removed. The switch is there to turn off the heater when the cover is opened, in order to prevent the resulting temperature drop from causing the PID controller to increase power output.

Design Decisions:

Heating Apparatus:

Figure 3: Compact Fan Heater

Justification of Final Choice

In the end, although the bulb radiant heater could have worked well for the project, updated size constraints from the sponsor made heating through radiation with the bulbs unlikely as they no longer could fit within the confines of the enclosure. As a result, the compact fan heater was selected and utilized as an external unit. Testing showed favorable temperature distribution ranges and demonstrated that the heater was more than able to provide the necessary amount of heat required. Additionally, it was both readily available and easily controllable due to the screw terminal wiring.

Heater Adapter:

Choices Considered

Justification of Final Choice

The final decision was to use a configuration with three inlets of 0.25 inch inner diameter and a single heater. This was decided after performing a series of tests with different tubing configurations and found that using three of these tubes is enough to achieve a good temperature distribution in the enclosure. Although the configuration with 10 inlets and 2 heaters had the lowest temperature distribution, it was very bulky does not fit well on top of the microscope stage. The offset from 37oC in the first two configurations shown was believed to be due to using an inaccurate thermocouple along with an off-the-shelf PID controller for feedback control. The third test was performed using more accurate sensors and the Raspberry Pi for feedback control. Soft rubber feet were used to dampen the vibrations from the fan heater to allow for clearer images.

Enclosure:

Figure 4: Enclosure Setup

Justification of Final Choice

The final decision of material for the enclosure was aluminum. Tests on an aluminum and plastic enclosure showed there was not much difference in the required power output to maintain the chamber at 37°C. Optical quality is also important to the sponsor, so a sturdier material that won’t move on the microscope will be important. Extra care will have to be taken during machining to avoid wasted work. The magnets are to make sure the adapter plate is secured. The magnetic sensor is used to detect when the enclosure cover is opened.

Adapter Plate:

Figure 5: Adapter Plate Annotated

Justification of Final Choice

The adapter plate will fit in the enclosure and be secured with the magnets. A clip will be used to secure the cell culture to the adapter. The area where the cell cultures sit is machined thin as to allow the microscope lens to get as close to the sample as possible. The magnetic securing feature was requested by the sponsor, due to the ability to pop off incase the microscope lens were to hit the adapter plate. Inexperienced microscope users tend to hit adapter plates often with the microscope lens, and allowing the adapter plate to pop off avoids damage to both the adapter plate and lens.

PID Controller:

Justification of Final Choice

The final decision was to use the Raspberry Pi as the temperature controller. The Raspberry Pi can hold larger programs and run multiple tasks together, which makes additional logic and controls easier to implement. Some extra care will have to be taken for turning on and off the Raspberry Pi.

Figure(s) 6 & 7: Final PID Performance Graphics

Imaging Results:

Figure 8: Sample Imaging With Fluorescence