October 18, 2018: Percussion update

Solenoid mounts under construction

Ah, well. The percussion bass drum is pretty much finished (see earlier updates here and here). The solenoids are mounted, the drivers are tested and installed under the drum, and everything is ready. Unfortunately, the solenoids are not quite as strong and powerful, as I had hoped and, indeed, designed for. In the current mechanical configuration, they are not able to pull the mallets nearly as hard, as I had hoped. Some adjustments can be made to make them sound off consistently, but it’s certainly not going to be mistaken for thunder. I’m afraid we’ll have to make some other changes to the setup to better utilize the pulling force of the solenoids.

Bass drum pedals and solenoids

Some changes are quite easy, like pulling the pedal mounts a bit away from the solenoids. The setup is basically a class 3 lever, with the fulcrum (pivot) at one end, the resistance (mallet head) at the other end and the effort (solenoid) between the two endpoints. The solenoid has a stroke length of 10-15mm, so that’s a fixed measure. The closer the solenoid is to the fulcrum, the farther the mallet can be moved by the stroke length, but it also means it takes more effort (force) to move the mallet. By moving the pedal mounts (and thus also the fulcrum) back means that the solenoids will pull at the mallet arm further from the fulcrum, making better use of the force, though at a cost of moving distance. Some experiments must be made to determine, if we can reach a point where we get a bit more sound out of the drum.

If that doesn’t work, we are goin to have to look at the entire mechanical setup and perhaps discard the pedal mounts altogether and build something else.

Finished bass drum installed

What works is, as predicted, to move the pedal mounts a bit back, but combined with replacing the steel mallet rods with even longer wooden dowel rods. Using wood greatly reduces the torque (rotational force) needed to swing the mallet, while the extra length gives the mallet head more momentum as it hits the drum vellum.

So, a bit of reconstruction, but not much. The drum still won’t drown out a jet engine, but it will have to do. The bass has been mounted on the EnsembleBot cabinet, and now we need to calibrate the drum to determine how many milliseconds of solenoid actuation is needed, and then re-program the master controller to accept the newest member in the ensemble.

October 14, 2018: Percussion update

New MOSFET solenoid driver

As metioned in an earlier update we are going to need a better MOSFET driver for our oversized linear solenoids used for the bass drum. To help a bit with the violent current changes when the 2.5 A solenoids are actuated, we place two 10,000 uF capacitors in parallel. The MOSFETs are IRL540 logic-level N-channel MOSFETs, ensuring a fully opened channel (0.077 Ω) when activated. The flyback diodes are rated for 3 A, which should more than enough.

Even though we technically only need two drivers, we fitted the module with four in case we’re going to need more in the future. For now, we’ll use the two spare drivers for some LED light effects inside the bass drum. We made a small auxillary module accepting 2 x 12V inputs directly from the MOSFET module. Each channel has three LEDs in parallel, coupled with a 100uF capacitor.

The plan is to have the LEDs light up when the bass drum solenoids actuate, just like we’ve done it in both the tubular bells, the PipeDream organs and several of our other percussion instruments. There we just placed an LED (and resistor) over the solenoid, making the LED light up as long as the solenoid is actuating, which is usually only about 10-30 milliseconds.

However, we are going to take advantage of the separate driver channels to make a bit of effect here. The 100uF capacitor makes the LEDs light up instantly, but fade away over somewhere between 0.5 and 1 second, instead of the short blinks of only a few tens of milliseconds.

Double-pedal bass drum under construction

The bass drum is actually just an open-backed bodhrán, so we are going to hide the driver module and the LED module behind the drum. Placing the LEDs in there as well, lets us illuminate the drum vellum from the inside.

The bass drum itself is also under construction. As shown on the image, it’s based on actual bass drum pedals, though from a childres drum kit. They were cheap, okay?

However, we’ve removed the pedals, replaced the expansion springs and replaced the mallet arms with a longer steel rod. Mounts for the solenoids has been more or less designed, but not yet built (or tested).