- Jacob Palmer
- Yuyao Tang
- Vato Chulukhadze
The objective of this design project was to create an analog, customizable circuit capable of detecting a jam in a high-speed IBML optical scanner. The imageTrac high-volume scanning machine, developed by IBML, is a document handling device capable of ultra-high-speed batch processing with its conveyor belts moving as fast as 125 inches per second. Though the system is equipped with complex error-detecting circuitry, there is always room for improvement. A peculiar problem that is known to cause issues with the system is papers being stuck at the paper tray deposit due to pressure from the paper tray onto the paper clips which place the papers in it. The circuit proposed in this senior design project will be capable of detecting such pressure and provide a high-speed output for an error-correcting circuit. The imageTrac system has a significant acoustic signature with a well-defined tone. A jam in the system introduces a broadband signal to the acoustic spectrum, covering up the original operating tone. By placing a piezoelectric microphone close to the motor controlling the paper tray clips, at a location where the tone of regular operation can be detected at a large enough SNR, accurate error detection is possible using a FET input amplifier, a tone decoder, a loudness detector, and an AND gate.
The design process began with recording audio from the piezo into a Digital Audio Workstation (DAW). After spectrally analyzing the collected audio, the signal revealed two quantifiable characteristics of a jam: There was a frequency peak at 2044 Hz, and the VRMS of the signal increased.
While the initial plan was to base this project in Digital Signal Processing (DSP), both of these characteristics could be detected with analog circuitry. The frequency peak could be detected using a tone decoder circuit, and the increase in volume could be detected using a comparator circuit. Because both of these circuits give a logic high or low as an output, the outputs of these circuits could be connected together with an AND gate.
The Tone Decoder
The tone decoder circuit uses an LM567. This chip contains Voltage Controlled Oscillator (VCO) and phase detection. The frequency of the VCO can be set with an RC circuit. The frequency of the oscillator is determined by the equation f = 1/(1.1 x R1 x C1).
By using a 50kΩ potentiometer in conjunction with a 0.22µF capacitor, the VCO is able to produce a frequency of 2044Hz, as well as a range of other frequencies for other cases. When the VCO is set to 2044Hz, the LM567 outputs a logic low when it detects an input frequency within the range of the bandwidth. When no frequency is detected, the LM567 outputs a logic high.
A comparator is a circuit that can be used to check whether an input voltage is greater or less than a reference voltage. This circuit can be created by placing a reference voltage on the positive input of an op-amp and an input signal on the negative input. The op-amp chosen for this task was the TL032. When the input signal is greater than the reference signal, the op-amp outputs +Vcc, which in the case of this circuit is 5V. When the input signal is less than the reference voltage, the op-amp outputs -Vcc, or -5V. For this implementation, +Vcc and -Vcc can be treated as logic high and low, respectively. By creating a voltage divider from +Vcc to ground with a 10kΩ potentiometer, it becomes possible to adjust the required threshold for the input voltage.
One problem that arises from this implementation is that the signal from the piezo has a range from +Vcc to -Vcc, meaning that even when the signal is loud enough to pass the comparator’s threshold, the negative parts of the signal could make the comparator inconsistent.
In order to mitigate this problem, an envelope detection circuit was placed in front of the input to the comparator. An envelope detector is a diode in series with a resistor and capacitor in parallel. The diode blocks out any negative signals, and the RC time constant smooths the output, resulting in a more consistent input for the comparator.
Putting It All Together
In order for the tone decoder and comparator to give accurate results, the signal from the piezo requires amplification, as well as de-coupling between components. The first stage of gain in the circuit is a TL072 op-amp set in a non-inverting configuration. A 100kΩ potentiometer connected to the negative input and ground allows for this first stage to be adjusted. While this first stage feeds directly into the tone decoder, a second stage of gain is added to the signal path going to the TL032.
Using the second half of the TL032, this second stage is another non-inverting amplifier with a set gain of 2. The second stage was added because for lower gain signal, the amplitude of the input signal was not enough to forward bias the diode, meaning that no signal was reaching the comparator.
The outputs from both the TL032 and LM567 lead to a NAND gate. The output of this NAND gate is then used as both inputs for a second NAND gate. This inverts the signal, meaning that the circuit outputs a logic high when a jam is detected.
The circuit was initially created on a breadboard before being transferred over to a protoboard for a more streamlined final product. Leads were placed at various places along the signal path of the circuit to demonstrate how the signal is used along that path. Figure 1 provides a block diagram of the signal flow, Figure 2 provides a schematic for the entire circuit, and Figure 3 shows a picture of the completed circuit on a protoboard.
For the provided model of imageTrac printer running at a speed of 50 inches per second, this circuit was successful in detecting jams in the collection tray as soon as they happened. The circuit can be seen in action in the following videos. Video one shows a tone decoder output with the yellow line and the piezoelectric microphone signal with he blue line. Note that the upper range of the signal is only limited by the range of the tone decoder rather than the dynamic range of the amplifier – clipping does not matter. In the second video, one can see the output of the loudness detector to the jamming signal. Note that the blue line is the rectified signal and the yellow line is the output of the comparator. Finally, one can see the output of the AND gate, which is the blue line.
Given that different versions of the imageTrac system will have differing acoustic signatures at different speeds, a circuit has been designed to be used with a circular diaphragm piezoelectric microphone as an input with customizable tone detecting frequencies, amplifier gain, and loudness threshold, so that an engineer could tune this given circuit to any system.
The piezoelectric microphone signal can be monitored using a 3.5mm TRRS plug attached to the piezo input. The signal can then be recorded directly into a DAW for further analysis. An example of this can be seen in figure 4.
When looking for an optimal place to place the microphone, use a spectrum analyzer. When running the machine, look for a location close to the motor controlling the paper clips, with a clear operating tone. An example of the spectrum as seen from a piezo placed at an optimal location can be seen in figure 5.
In our case, one can notice a clear tone at 2044 Hz which lies at least 10 dB higher than the rest of the signal. This tone gets covered up in the case of a jam – inspect figure 5 – the 2044 Hz signal is now covered up. At the same time, the introduction of all this noise increases the true loudness of the system. In figure 6 we can see the spectrum analysis of a piezo with a recording of normal operation followed by an intentional jam, The min. RMS level is observed during times of normal operation, whereas maximal RMS level is observed during the jam.
On top of a 10 dB RMS difference, we can observe a difference in true peak loudness as well. If we configure a tone decoder and the amplifier to provide a logic low output while the ‘tone’ of normal operation can be detected, and a rectifier connected to a comparator to provide a logic high output when a reference voltage is surpassed, we can use an AND gate to detect each time pressure is applied to the paper clips, internally, or externally. Furthermore, the tone decoder output essentially provides a logic high whenever anything goes wrong with the circuit – this could be used as an input elsewhere in the system for future use, the same goes for the loudness detector.