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Sensor Hardware Integration

A project log for GePS

standalone gesture controlled musical instrument

Zed CatZed Cat 10/07/2018 at 16:470 Comments

Goals

The first issue we attack is the integration of the sensor unit:

So far we had good results with using the custom Arduino design described in the last project log. But the bottleneck is at the clock rate of the MCU: It runs at 12 MHz and with all optimisations this gets us to a latency of ~4.5ms. This is already pretty good. But performing many concerts with this instrument showed us that the whole experience will benefit from decreased latency, even if it's only 2 or 3ms.

If you transmit the sensor data through wired serial connection, you can experience the difference, as latency drops to ~3.2ms. But who wants to perform with strings attached?! :o)

The first step we took is to replace the XBee Explorer Dongle by the DFRobot XBee USB Adapter V2 to hook up the XBee to the computer. This adapter features the Atmega8U2, allowing a faster and more reliable connection. Only by replacing this adapter we already gained 1.5ms!

XBee USB Adapter
XBee USB Adapter V2 - Atmega8U2 with the XBee attached, receiving sensor data

Finding a new platform

The real issue is clock speed and the corresponding possible baud rate through wireless transmission. Coming from the Arduino Pro Mini (8 MHz, ~6ms, 38'400 baud) to the Sense/Stage Arduino (12 MHz, 4.5ms, still 38'400 baud) led us to increasing both clock speed and baud rate. The third parameter that could be an obstacle is the sensor sampling rate.

More recent Arduino boards show great specs but when it comes to the form factor, they disqualify. But one module showed amazing specs: The DFRobot Cure Nano.

Fantastic!

The only drawbacks to the Sense/Stage: There are no integrated XBee headers and no power switch. The missing XBee headers we use as an opportunity to move the antenna to an other position on the glove. By that we achieve better radiation characteristics (in other words: we reduce shielding the radiation by the hand or body of the performer). The power switch we also sew to the glove (for the prototype we go with the Adafruit switched battery connector "JST-PH").

If you want to add another 3 degrees of freedom: The Curie Nano even features a 3 axis magnetometer (compass)! That would allow to determine absolute direction during performance (i.e. for events where you point a objects on stage / in the room).

The new unit, before assembly
Cure Nano, XBee headers, Xbee, power switch and LiPo battery

Reprogramming the sensor

We update the sensor unit code to run on the Curie Nano. This includes the following changes:

Great! With these changes alone we drop to a latency of 3.2ms!!

Increasing baud rate

The baud rate has to be changed in the settings of both XBees, the sensor unit code and testing sound module!

Increasing the baud rate delivered great latency improvements: at 115'200 baud we reached a latency of 1.3ms! But sadly the XBee connection became unstable (bytes got dropped frequently) to a degree that is not acceptable for performing on stage. Waiting for the serial buffer to clear (Serial1.flush()) and adding a millisecond after the transmission of each complete sensor reading brought back stability to the connection.

The latency is now at around 2.3ms which is still a fantastic improvement: half the latency of the Sense/Stage version and almost a third of the Arduino Pro Mini solution.

Improved Latency
The Pure Data test patcher measures the time that passes between each complete sensor reading.
Layout of sensor unit and glove
Preparing to sew the new sensor unit to the glove. This time, the XBee will be placed outside the glove, on the back of the performers hand

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