InSight Instrument Deployment Arm

Overview

The Instrument Deployment Arm placed the InSight lander’s two primary instruments on the surface of Homestead hollow in western Elysium Planitia, then spent the rest of the mission doing work it was not built for: pressing on regolith, pinning and hammering a stuck penetrator, and throwing soil across the solar arrays to let the wind clean them [1]. It is a four-degree-of-freedom backhoe arm, a refurbished Mars Surveyor 2001 unit, with titanium links and a scoop and grapple at the wrist.

The mole recovery campaign the arm carried out is described on the InSight HP3 mole page.

Specifications

Parameter	Value	Source
Degrees of freedom	4: shoulder azimuth, shoulder elevation, elbow, wrist	[1]
Reach	1.8 m
Link material	titanium
Payload capacity	9 kg at 1.65 m extension in Mars gravity, equivalent to 33 N
Peak joint torque	35, 120, 65 and 10.5 N·m, joints 1 to 4
End effector	scoop with a 250 cm³ chamber, plus grapple
Scoop chamber	approximated as a 7.5 × 5 × 4.5 cm box plus a half box; bottom blade 7.5 cm wide
Qualified temperature range	-110 to +70 C structure and joints; -60 to +70 C motors

Every row in that table is a design specification or a design capability, not a measured performance. The same is true of the two numbers most likely to be mistaken for measurements: the 1 cm positioning accuracy and the 1.5 cm instrument position knowledge are requirements the deployment system had to meet, not a demonstrated repeatability [1]. What was measured against them is the cross-check: an IDC stereo orthophoto and the monocular fiducial localization agreed to 1 to 2 cm on the surface.

Running the motors below their qualification floor

From sol 550 the joint heaters were not run at night and the motors dropped below -90 C against a -60 C qualification floor. No degradation and no operational impediment was observed through sol 1442 [1]. That is 30 C of margin below a ground qualification limit, established by flying it rather than by testing it, and it is a single vehicle over one mission rather than a bound on the design.

Force without a force sensor at the wrist

No contact force reported from the surface was measured on Mars. Each one comes from reproducing the flight arm pose in a JPL testbed and reading a three-axis load cell at 0.5 Hz, in Earth gravity [1]. The weakness of that method is that force direction is strongly pose dependent, and the paper shows it. A flat scoop push near the HP3 hole in the sol 240 pose gave a median Fx of 56.3 N and Fz of 77.2 N; the same push with the forearm vertical gave 17.7 N and 54.4 N; extending the arm 48 cm farther raised Fx back to 55.4 N against Fz 61.6 N [1]. The Fx to Fz ratio rises with extension. A single contact force figure for this arm does not exist, and the 80 N average downward force quoted as the end effector’s capability is a nominal figure carried over from the arm’s own design description rather than a per-activity result [1].

The pre-flight planning estimate for the sol 240 first regolith interaction was a maximum applicable force of 40 N at that location [1]. That was an engineering expectation used to plan the activity, and it is roughly half what the load cell reproduction later gave.

Cleaning the solar arrays with the scoop

Seven experiments dumped soil from the scoop onto the lander deck so that wind would carry it across the arrays, abrading dust off them. Experiment 1 on sol 884 dumped 112.6 ± 10.4 cm³ from 35 cm above the deck into 6 to 13 m/s winds for an instantaneous gain of 4 W, a same-sol gain of 24 Wh/sol and a long-term gain of 35 Wh/sol [1]. Across all seven the total was 80 Wh/sol, about 15 percent of output in that period, which extended the mission by roughly 242 sols.

The bounding case is experiment 7 on sol 1238: 99.1 ± 15.3 cm³ dumped from 42 cm over the east deck in low winds, which produced no dispersion and no measurable change at all [1]. The technique depends on wind the operator does not control. About 30 percent of dumped soil disperses 1 to 2 m downwind, measured by differencing digital elevation models of the scoop and the resulting pile [1], and the campaign as a whole is correlational: the 80 Wh/sol is not attributable dump by dump.

Soil properties inferred from arm contact

The arm’s contact record is the input to a soil model rather than a set of soil measurements, and the source treats it that way. Duricrust cohesion at the landing site is 2 to 15 kPa, inferred from mole penetration resistance through cone penetration theory, the range spanning the assumed internal friction angle; an independent scoop-based estimate gives about 6 kPa [1]. Bulk density runs 1200 kg/m³ in the uppermost layer, 950 to 1100 kg/m³ in the duricrust, 1300 to 1500 kg/m³ in underlying sand and 1600 kg/m³ in the deepest sand and gravel [1]. That is a derived model of the first 40 cm.

One deliberate mechanical experiment was run on the regolith itself. On sols 1074 to 1075 the arm pushed to four sequential commanded depths of 2.5 to 4 cm from a 1 cm standoff, with the arm current and torque limits set for maximum downward force, and the imprint depth was read from digital elevation models built from IDC images [1].