4 Firmware versions prior to 1.9.13 used acceleration to detect
5 launch. Starting with 1.9.13, the firmware switched to using
6 pressure to trigger recording. Altus Metrum strongly recommends
7 upgrading all EasyMotor boards to current firmware, the
8 behavior of which is described here.
10 EasyMotor needs to be rigidly attached in the airframe, in any
11 convenient orientation.
13 === Trigger to Start Recording
15 Starting with firmware version 1.9.13, a pressure sensor must
16 be attached, and pressure change is used to initiate and conclude
17 data recording. Pressure and 3 axes of acceleration are recorded
20 EasyMotor looks for a 50 psi rise in pressure from ambient to
21 start recording. Recording stops when the pressure drops
22 below 50 psi and shows variation of less than 30 psi for at
23 least 10 seconds. These conditions are intended to capture all
24 actual burn data including anomalies such as "chuffs".
26 === Power Switch and Battery
28 In addition to the circuit board itself, EasyMotor needs
29 a power switch and battery to operate. Like many Altus
30 Metrum products, EasyMotor requires a single-cell LiPo
31 battery, and has an on-board charging circuit. The 5V
32 needed for inexpensive pressure sensors is generated
33 onboard using a switching regulator to boost the voltage.
35 Because he often mounts EasyMotor to the motor's forward
36 bulkhead instead of to the airframe itself, Bdale often
37 uses a length of “shooter wire” from an e-match or used
38 motor igniter as a power switch, routing the wire out of
39 the typical fin can vent hole and using “twist and tape”
40 to power up the board. Whatever works!
44 The primary motivation for designing EasyMotor was to have
45 a reliable way of recording motor chamber pressure during
46 flight. To that end, EasyMotor supports attachment of a
47 low-cost analog pressure sensor. The board provides 5V
48 to power the sensor, and an input for measuring and
49 logging the output voltage from the sensor.
51 The kind of sensor EasyMotor is designed to work with
52 takes 5V in and has a linear analog output that ranges
53 from 0.5V at 0 to 4.5V at the maximum pressure supported
54 by the sensor. Very inexpensive sensors that have a
55 “1/8 NPT” threaded input, a “Buick-style” 3-pin connector,
56 and typically ship with a short cable and mating
57 connector, are readily available through various vendors
58 including Amazon, eBay, and AliExpress.
60 To log in-flight chamber pressure, a typical approach
61 might be to drill a 1/8" sampling hole all the way
62 through the center of the motor's forward closure, then
63 drill and tap partially through the closure with a “1/8
64 NPT” pipe tap. Fill the touch hole with grease, screw in
65 the pressure sensor, and attach the sensor leads to
68 If you aren't up for machining closures yourself,
69 link:http://lokiresearch.com[Loki Research] sells
70 54mm and 75mm "experimental bulkheads" with threaded
71 sensor ports that can be used with snap ring cases.