Refining a robotic subsystem to be “flight-ready” for a space mission pushes the design to exhibit lower mass, smaller volume and simpler control architecture, all important factors in effective space system design.
For engineers at Honeybee Robotics working on a Sample Manipulation System (SMS) for a 2009 mission aboard NASA's Mars Science Lab. (MSL), these goals led to a strict development philosophy of functional concentration to achieve an optimized flight design.
The objective of the functional concentration approach is to optimize design by incorporating concentrated functionality only at discrete, well-defined points. The ability to deliver mechanical energy and provide sensing resolution at pre-defined points leads to high reliability and refined performance.
“The main goal of our latest redesign was to make the SMS flight-ready and to fit within the design constraints of the overall system,” says Erik Mumm, a systems engineer for Honeybee. “As the overall system evolved, so did the SMS. As requirements emerged and refined, the design changed to fit within the new requirements.”
The SMS is a robotic subsystem in the Sample Analysis at Mars (SAM) instrument suite. SAM analyzes gases evolved from solid samples or the Martian atmosphere. Processing solid samples requires transport of the sample from an inlet port to an oven where the sample is hermetically sealed and pyrolized. The function of the SMS is precise manipulation of samples from the sample inlet device to one of two pyrolysis ovens. When inserted into the ovens, sample cups require the SMS to command forces up to 300 lb-ft to achieve a hermetic seal with the pyrolysis ovens.
According to Mumm, one of the biggest challenges with the redesign focused on maximizing the number of samples SAM could process. This meant reducing the number of actuators and increasing the number of sample cups. And like all space mechanisms, the design team needed to make the system as simple and as reliable as possible.
“We reduced the system to only two actuators for a three-degree of freedom system,” Mumm says, “and packed 74 sample cups into our allotted volume, up from eight to 10 sample cups in the initial design. We also removed 1.5 kg from the mass of the mechanism.”
The most valuable optimization of the flight SMS is the shift from five actuators to two, a carousel and elevator actuator. The new system accomplishes this by populating the sample cups in two polar arrays and by using the elevator actuator to lock and unlock the sample carousel disk from the REF via a toggle lock.
The toggle allows the sample carousel disk to be grounded or actuated using a rotating elevator frame. The same actuator positions a sample cup to a given location and positions the elevator actuator beneath the selected sample cup, so it may be elevated to accept a sample or inserted into an oven.
“The system actually achieves two degrees of freedom from one actuator,” Mumm says. “By toggling the latching mechanism, the system can either rotate the carousel or lock it to the ground and move the elevator independent of the carousel.”
The elevator mechanism is capable of delivering a force of 300 lb for sealing a cup in the pyrolysis oven. This actuator also drives the cam to open and close the toggle lock mechanism. A jack screw transforms the motor output into a linear motion. Multiple function actuators are critical in reducing the burden on control electronics without sacrificing system performance.
The SMS incorporates . Stepper motors were initially examined for use on the SMS because of their simplicity. The decision was then made to shift to brushless because of the dual functionality of the elevator motor. The need to deliver the high torques at the end of the stroke during the preload along with a time limit imposed on the duration of the move created the need for a high-speed and high-torque actuator. While the rotational axis actuator could use a stepper motor as well, the use of a brushless was chosen for qualification and procurement simplicity, as well as control commonality.
Feedback sensing achieves the specific positioning requirements of the SMS while minimizing the sensor's effect on the electromechanical system. Honeybee decided to design an ad hoc incremental encoder at the output of the mechanism utilizing features of the mechanical system. This approach takes advantage of the tooth pattern in the grounding ring and utilizes it as an optical vane for a pair of positioned infrared LED and phototransistor pairs.
According to Honeybee, this design allows the SMS to accurately position the rotational/carousel axis without components with large mechanical and electrical overhead such as absolute encoders. Though absolute encoders provide this same level of precision, they would need to be perfectly aligned during integration.
The SMS control algorithm lends itself to a simple state machine. The control scheme has been specifically tailored to the mechanical system to provide a predictable state transition of a single switch at each step in the SMS control algorithms. If a state transition in any other switch is sensed, a fault is detected before the mechanical system is compromised.
Complex decision-making algorithms are not necessary with this approach. The mechanical system design creates a set of discrete move termination conditions for a given command. Switch state transitions are referenced to predicted hall counter values for verification.
The functional concentration design approach is used to determine the set of rules to govern the motion of the SMS. When any of these simple rules are broken, a fault is easily detected. The cause of the fault can be deduced based on what rule is broken and when it is broken in the command sequence.
The philosophy of concentrated functionality applied to actuators, sensors and mechanical structure has led Honeybee to a design optimized for mass, volume and control simplicity. This design tactic has proven to be an excellent method for flight systems where fully defined requirements and specific interfaces allow for highly customizable design.