The aim of the project is to demonstrate the feasibility of a cheap and miniature ophthalmoscope based on versatile and portable MEMS scanning mirrors, capable of three-dimensional measurements of different structures of the fundus, including the optic nerve head, and functional measures requiring excitation. Such a device could be acquired by ophthalmologists, but also by general practice or paramedical staff (orthoptists, nurses and opticians) for the systematic and remote screening of glaucoma. This page describes the electronic system. For details about the optical system see link to ISI.
The proposed electronic system forms a bridge between the optical system with the mirrors and the sensor on one side and a processing system to visualize the captured image on the other side. It can be split into three separate parts:
- a system controller
- the mirror controller
- the data acquisition
The System Controller links the processor system to the analogue electronic circuitry. It controls the angle of the mirrors and samples the collected data. The light data is packaged into images of 100 x 100 monochrome pixels and then sent on to the processor system.
The processor system is used to analyze and visualize the collected data. To be as flexible as possible, it uses a USB connection.
The electronic circuitry transforms the signals from the digital domain into the analogue domain of the mirrors and also from analogue to digital for the image sensor. The image sensor is connected with an microfiber cable to the optical system. The mirrors use a 9-pin D-sub connector.
To power the system, a supply with both ±5 Volt and ±12 Volt is needed.
In order to verify the functionality of the system, we wrote the little Python application PySerial2PNG. When it runs on the processor system, i.e. a computer with Windows, it generates Portable Network Graphics from the data received over USB.
Thanks to using existing hardware extensively, we have been able to build a working prototype in a short time-frame.
The System Controller is realized on a HEI FPGA-Rack FPGA board. This is a standard FPGA board used at the Infotronics Unit for various projects designed to fit into two rack units (2U). The controlling is implemented in the Xilinx Spartan-6 LX FPGA. The VHDL design is analyzed in the chapter Hardware Configuration.
A USB connector, which acts as serial COM-port on the receiving end, is used to send the collected data on to the processor system. The low-level protocol is RS232 with 1 start bit and 8 data bits. The targeted bitrate was 3 Mbit/s. Test on a standard PC showed that at this speed, some data bits are not received (~5%). Therefore a version with a bitrate of 1 Mbit/s has been generated. Here all the bits are received correctly.
To transmit the images, a transfer protocol has been developed. The goal of this protocol is to be able to verify the good reception of an image on the processor system. It should also be able to show an image even if there are some pixels missing in a line. Therefore a marker for each image as well as each line have been introduced.
The RS232 splits the data into words of 8 bit. In our image transfer protocol, the most significant bit is always a control bit. It defines if the following seven bits of the word are control bits or bits of a pixel. There exist 3 different words with control data:
- Start of Image (SoI):
0b10000001 = 0x81
- End of Line (EoL):
0b10000011 = 0x83
- End of Image (EoI):
0b10001010 = 0x8a
Two data words are used to transmit the luminance value of one pixel:
- MSB (13:7):
- LSB ( 6:0):
As a result, there are 14 data bits per pixel.
A shown in the graphic below, at the beginning of an image the SoI word is sent. Then follow all the 100 lines of the image. Each line of 100 image pixels and therefore 200 data words is terminated with the EoL word. The whole image is wrapped up with the EoI word.
|Data + EoL||0xxxxxxx||…||0xxxxxxx||10000011|
|Data + EoL||0xxxxxxx||…||0xxxxxxx||10000011|
This protocol adds some overhead, so that we have
For each word, there are
- 8databit + 1startbit + 1stopbit = 10bits
which have to be transmitted. This results in
So for one pixel we have
To increase the transmission speed of this protocol, the EoI or even the EoL marker could be omitted. However the capacity to display images with missing pixels would be greatly reduced.
At the targeted bitrate of 3'000'000 bits/sec the maximal image rate is
and at the lower test bitrate of 1'000'000 bits/sec
On the other side, the VME 96-pin connector connects to other electronic components used for signal transformation. The pin assignment is based on the HEI VME Backplane Bus.
The mirrors' angle is proportional to a current of ±10 mA and ±15 mA, depending on the mirror, and controlled by an FPGA. For this we use a DAC to transform the digital signal from the FPGA into a voltage, and then a Howland Current Pump to transform the voltage into a current.
Due to how the mirrors are installed in the optical system, the maximal angle they should reach is ±3.5° for the fast mirror and ±4.5° for the slow mirror. This results in a maximal applied current of ±3.5 mA and ±2.6 mA respectively.
The DAC in use is the PCM1793 24-BIT, 192kHz Stereo D/A converter on the HEI PP Audio ADC DAC board. This DAC was chosen because it fulfils the requirements of this project and we had it ready to use on our PP Audio ADC DAC board. This board is designed for audio applications and therefore cancels out any DC-offset. We soldered a 6.4 kΩ pull-up resistor on the positive input of the OpAmp on the Analogue Output Stage. This resistor forms a voltage divider with the 3.3 kΩ resistor to halve the supply voltage.
As we want to define the DC current, i.e. when there is no differential voltage applied:
- V − − V + = 0.
Therefore we can simplify:
R3 can be calculated
As it is a stereo Audio-DAC, it is furnished with 2 channels: left and right. In this application we use one channel per mirror.
Howland Current Pump
A Howloand Current Pump is used to convert the voltage signal from the DAC into a current signal for the mirrors. Thereby we use the "Improved Howland Current Pump" with trimmer (for more information please refer to AN-1515 A Comprehensive Study of the Howland Current Pump).
We did a simulation in P-Spice to better understand the circuit and to specify the resistors' values.
An Avalanche Photodiode (APD) module captures the light reflected by the eye. We used the Hamamatsu C5460 APD module. It has to be powered by ±12V and it's output varies between -10 V to 0 V. As the controlling FPGA works best with signals between 0 V and 5 V, a general inverting amplifying circuit (HEB_GIA) has been developed based on the PP board design. The resulting signal is digitized by the PCM1804 full differential analog input 24-BIT, 192-kHz stereo A/D converter on the HEI PP Audio ADC DAC board and sent on to the controlling FPGA.
To provide a compact and transportable system, we invented the ISI toaster containing a FPGA Rack Backplane Stack board. It allows to mount up to four rack cards in a 3D printed housing with a screwed on front panel. For USLO it holds a PP-Backplane board with all the small extension cards explained above mounted on, and an FPGA rack board with the controlling FPGA.
This design is also separated into two distinct parts, one to control the mirrors and the other one for data acquisition.
Here we have four blocks, where there are simple counters and the fourth one controls the DAC.
The first counter block called Prescaler generates a pulse every time the mirrors can move to a new pixel location. We are able to modify the image rate by adjusting the frequency of this pulser. All the rest of the design is coupled to this pulser and therefor adapts automatically to any chosen image sampling frequency. If we like to fully use the speed of the serial link to the processor system of 1 Mbit/sec we get
To allow for some protocol overhead and to be able to implement an effective Prescaler, we settled for 42500 pixels/sec.
Together with the overhead of the protocol this leads to ultimately
To compensate some mechanical shortcommings of the mirrors, also a slower pixel rate of 425 pixel/sec has been tested:
Together with the overhead of the protocol this gives
Then FastMirrorCnt generates the controlling signal of the fast moving mirror. On every pulse from the Prescaler the counter increments by one. When it arrives at 100, it decrements by one until it reaches 0. At each inversion of the counting direction a pulse is sent to the next counter. So to speak it counts the pixel on each line, or in other words, it's the vertical counter. In the normal scanning mode the triangle has therefore a frequency of:
The SlowMirrorCnt works exactly the same way as the FastMirrorCnt. It increments on each pulse from the FastMirrorCnt until 100 is reached, then it counts down to 0. A pulse is sent out at every counting direction change. This means it counts the lines on each image, or in other words, it's the horizontal counter. In the normal scanning mode the triangle has therefore a frequency of:
ADCctrl interfaces with the ADC and sends the counter values from FastMirrorCnt and SlowMirrorCnt over a 24-bit I2S connection. There they are sampled at 192 kHz which equals to about
Here we also have four main blocks. The data flows from the DAC to the serial port. It is also received over a 24-bit I2S connection.
The DACctrl puts it into a parallel format. As the DAC is primarily intended for audio applications, its sampling speed is per default 192 kHz. Like above this means the ADC produces 4 samples/pixel. The average value of these 4 samples is calculated in the Filter.
When using the slower pixelrate, the ADC produces about
but we still take only the last four samples to calculate the average in the Filter. This gives the mirrors some time to settle at the desired position, before the samples are taken.
The DualBuffer is a memory with storage space for 2 complete images. This storage space is separated in two distinct buffers. During writing the image data to one buffer, the other one is sent on to the Serial Port.
Serial Port reads the image from the DualBuffer and adds the control words from the protocol. Then it serializes this data and sends it over the USB connection to the processor system.