Umbra infractus has used several different mechanical and electronic configurations since its early development. As the remaining works within the existing edition and additional exhibition versions were prepared, some components used in earlier versions had become discontinued or were no longer the most practical choices for fabrication and long-term maintenance.
The purpose of the new development was not to change the behavior of the artwork, but to establish a compact, reliable, and maintainable hardware system using components that could be replaced or serviced more easily in the future. This involved reconsidering the motion system, servo choice, battery supply, voltage regulation, and microcontroller as parts of a single system.
Original Motion System
The first study version of Umbra infractus used two geared DC motors together with a custom mechanical power-switching system operated by a servo motor. Rather than controlling all aspects of the motors electronically, part of the switching and directional behavior was produced mechanically.
Solarbotics GM2 and GM3 geared DC motors were used during this period. These motors were also used in other works in the Species series. Their low rotational speed, relatively quiet operation, and ability to operate at low voltage made them useful for small kinetic mechanisms.
The GM2 and GM3 use a 224:1 gearbox and are designed for operation at approximately 3–6 V. At 3 V, Solarbotics specifies a speed of approximately 24 RPM and a free-running current of approximately 40 mA. These characteristics made the motors particularly suitable for mechanisms in which movement needed to remain slow and relatively unobtrusive.
The mechanical switching system worked, but it also introduced additional moving parts. As the artwork developed, continuous-rotation servos provided another way to produce bidirectional continuous movement with much of the motor-control hardware integrated into the actuator itself.
Transition to Continuous-Rotation Servos
By 2012, a version of Umbra infractus was using two Hitec HSR-1425CR continuous-rotation servos controlled by an Arduino Duemilanove.
The continuous-rotation servo configuration simplified the motion system. Each servo contained the motor, gearbox, and control electronics in a single unit. Direction and approximate rotational speed could be controlled through the servo signal generated by the microcontroller, eliminating the need for the earlier mechanical power-switching mechanism.
Some later editions of Umbra infractus used Parallax continuous-rotation servos. The use of continuous-rotation servos was therefore already established within the history of the work before the current hardware redevelopment began.
Returning to the Hitec HSR-1425CR
For the new editions, the Hitec HSR-1425CR was selected again. The decision was not a transition to a new actuator type, but a return to a servo that had already been used successfully in an earlier version of the artwork.
The HSR-1425CR is an analog continuous-rotation servo designed for a 4.8–6.0 V supply. Hitec specifies a no-load rotational speed of approximately 43.4 RPM at 4.8 V, stall torque of approximately 2.8 kgf·cm, a running current of approximately 150 mA under no-load conditions, and a stall current of approximately 640 mA. The servo measures approximately 40.6 × 19.8 × 36.6 mm and weighs approximately 41.7 g.
The new system was planned around a regulated 5.0 V supply. Because 5.0 V is close to Hitec's published 4.8 V test condition, the expected performance is also close to the 4.8 V figures: roughly 44 RPM and approximately 2.8–2.9 kgf·cm of maximum torque. These are only approximate values, since Hitec does not publish a separate performance specification specifically at 5.0 V.
The current specifications also require some interpretation. The approximately 150 mA figure represents operation without a significant mechanical load, while the 640 mA figure represents a stall condition. The actual current inside the artwork falls somewhere between these values and changes with mechanical resistance, acceleration, direction changes, and the load attached to the servo.
Choosing the Battery System
Once the servo configuration was established, the next question was how to supply power to two servos and a microcontroller in a way that would remain straightforward to maintain over the life of the artwork.
A four-AA battery pack was selected as the preferred power source. AA batteries are widely available, standardized, and easy for a gallery, collector, technician, or conservator to replace. Conventional alkaline and NiMH AA cells are also generally more chemically stable and present a lower fire and thermal-runaway risk than rechargeable lithium-ion battery packs, while avoiding the need for dedicated lithium battery charging and protection circuitry. For longer exhibitions, rechargeable NiMH AA batteries can also be used. A four-cell NiMH pack has a nominal voltage of approximately 4.8 V, which can still be regulated to the required 5.0 V by the step-up/step-down converter.
Establishing a Regulated 5 V System
A regulated 5.0 V supply was chosen to make the electrical environment more consistent. Both the continuous-rotation servos and a 5 V microcontroller could then operate from the same regulated voltage.
A conventional step-down regulator alone was not ideal. A fresh four-AA alkaline pack begins above 5 V, but during discharge its voltage can eventually fall below 5 V. A step-down regulator can reduce the initial battery voltage but cannot maintain 5 V once its input becomes too low.
A boost-only regulator creates the opposite problem: it can raise a low battery voltage but is not intended to reduce a fresh battery pack that begins above the desired output voltage.
A step-up/step-down, or buck-boost, regulator was therefore more appropriate. It can maintain the same output voltage while the battery pack moves from above the target voltage to below it during discharge.
Selecting the 5 V Converter
The regulator selected for the new system was the Pololu S13V30F5 5 V step-up/step-down voltage regulator.
The S13V30F5 provides a fixed 5 V output from an input range of approximately 2.8–22 V. Pololu specifies a typical maximum output of approximately 3 A when the input voltage is close to the output voltage, although the actual continuous-current capability depends on input voltage and thermal conditions.
This provides substantial headroom for the two HSR-1425CR servos. Even the unlikely case in which both servos simultaneously approached their published 640 mA stall-current value would represent approximately 1.28 A of servo current. Normal operation should remain substantially below this condition.
The resulting power architecture became:
4 × AA batteries → Pololu S13V30F5 → regulated 5 V rail
The servo power is supplied directly from this regulated rail rather than being routed through the microcontroller.
Power Stability and Servo Current
Servo motors can create short current transients when they begin moving, reverse direction, encounter resistance, or rapidly change speed. Although the average current may be relatively modest, these brief changes can disturb the supply voltage if the power system has insufficient reserve or if the wiring introduces significant resistance.
A 4700 µF electrolytic capacitor was therefore added across the servo 5 V and ground rails near the servo power branch. The capacitor provides a local energy reserve during short current changes and helps reduce sudden voltage variation at the servos.
The controller, regulator, and both servos share a common ground. The higher-current servo supply path is kept separate from the servo signal wiring as much as practical before the grounds join at the common power system.
Initial Microcontroller Replacement
The earlier 2012 version used an Arduino Duemilanove. Because the Duemilanove has long been discontinued, a new microcontroller was necessary for the new editions.
The Adafruit ItsyBitsy M0 Express was initially selected for testing. It was physically much smaller than the Duemilanove, provided more than enough processing power for the artwork, supported the Arduino programming environment, and included onboard USB programming.
The initial integrated system therefore consisted of four AA batteries, the Pololu 5 V step-up/step-down regulator, two HSR-1425CR continuous-rotation servos, and the ItsyBitsy M0 Express.
During testing, an unexpected difference appeared between USB-powered and standalone operation. The continuous-rotation servos stopped correctly near their expected neutral pulse while the controller was connected by USB, but their behavior changed when the controller operated only from the artwork's battery supply.
This led to a separate investigation of servo pulse timing, the SAMD21 clock system, USB clock recovery, and possible replacement controllers. That investigation is documented separately in Umbra infractus - Microcontroller Selection.
Final Planned Hardware Configuration
Following the controller investigation, the planned controller for future editions became the Adafruit ItsyBitsy 32u4 5V / 16 MHz.
The final planned electronic configuration is therefore:
4 × AA alkaline batteries → Pololu S13V30F5 5 V regulator → ItsyBitsy 32u4 5V + two Hitec HSR-1425CR servos
This configuration keeps the entire operating system around a regulated 5 V supply while remaining considerably smaller than the Arduino Duemilanove-based electronics used in the earlier edition.
Estimating Battery Operating Time
The expected operating time cannot be calculated precisely from the published component specifications because servo current depends strongly on the actual mechanical load. It is nevertheless useful to establish an approximate range before measuring the completed artwork.
The current movement program operates only one servo at a time. A servo is selected randomly and moves for approximately 3–10 seconds. Both servos then remain stopped for approximately 1–5 seconds before the next movement begins.
Using the midpoint of these ranges gives an average movement period of 6.5 seconds and an average stopped period of 3 seconds.
6.5 / (6.5 + 3) ≈ 0.68
One servo is therefore moving approximately 68% of the time, while the other remains at neutral.
Using Hitec's approximately 150 mA no-load running-current figure and approximately 8 mA idle current as a theoretical low-load case, the two servos together would average roughly 110–120 mA. Adding the microcontroller and converter losses places the complete system somewhat above this value.
This represents a best-case calculation because the servo in the actual sculpture is mechanically loaded. If the moving servo averages approximately 250–300 mA during normal operation rather than the published 150 mA no-load figure, the average load of the complete 5 V system is approximately 200–250 mA.
A four-AA alkaline pack with approximately 2,000 mAh of useful capacity under this type of load would therefore suggest an operating time in the general range of several hours rather than days. After accounting for converter losses, mechanical loading, battery-voltage decline, and variation among batteries, a preliminary planning estimate of approximately 6–10 hours of continuous operation is reasonable.
This estimate should not be treated as a specification for the artwork. Servo load, friction, commanded speed, battery chemistry, battery condition, temperature, and converter efficiency can all affect the result.
The final operating-time value should therefore be established experimentally after the mechanical system is complete. A useful test will be to install four new alkaline AA batteries, operate the artwork continuously under its normal mechanical load and movement program, and record the elapsed time until reliable operation can no longer be maintained. This measured runtime can then become part of the maintenance documentation for future editions.
Current Direction
The development of the new electronics began as an effort to replace discontinued components while preserving the established movement of Umbra infractus. In the process, the power system was reorganized around a four-AA battery pack, a regulated 5 V supply, two HSR-1425CR continuous-rotation servos, and a compact 5 V AVR controller.
The resulting system is simpler to reproduce than the earliest mechanically switched version and substantially more compact than the Arduino Duemilanove-based configuration used in 2012. Just as importantly, the individual components are clearly defined and can be documented independently, making future maintenance and replacement more straightforward.
Wonbin's YouTube Channel