Cold motor causes Spirit to remain parked for a day

Posted: February 11, 2004

A missed communications window caused by a cold antenna motor on the Mars Exploration Rover Spirit prevented the robot from racking up any additional distance on its odometer Tuesday night.

"Yesterday was an operational issue day with Spirit. We did not get the morning high-gain antenna pass. As a result of that, we did not get sequences loaded up (into the rover)," project manager Pete Theisinger told reporters in a teleconference today.

Spirit has remained parked since its Monday night workday. It will examine the dune features to its left before continuing to the Bonneville Crater. Credit: NASA/JPL
As the Sun rose for the start of Spirit's 38th workday on Mars, the Pancam Mast Assembly was creating a shadow on the high-gain antenna gimbal motors. The motors have heaters to ensure they are warm enough to move. But the cold temperatures in the shadow were too great for the heaters to overcome, causing the motors to stall when trying to point the lollipop-shaped antenna to face Earth.

"The colder you get, the more current you have put into the motors to get it to move. So we set those limits. Because we were in the shade, we did not set high enough current limits for the motors," Theisinger explained.

"When we first started to do the high-gain antenna session yesterday, we started out by going to a (calibration), which goes to a hard-stop. The way the motor knows it has gone to the hard-stop is it stalls against the hard-stop. Because we had set the current limits so low, it stalled immediately, thought it was at the hard-stop. Then when it continued on with its high-gain antenna session, it was pointed off in a different direction than we expected. So we got no data down.

"We did a high-gain antenna session later in the day when things had warmed up. That went just fine. We looked at all of the telemetry, and everything is just perfect. It was just this failure to understand that we are going to run cold in the morning and we need to either wait till later in the day when the shadow left the actuators or apply more heat with the heater we have.

"We've taken steps to fix that operationally now so we don't have those issues in the future."

Without having the high-gain antenna session in the morning, controllers weren't able to load the day's driving commands into the rover.

"That situation plus diagnosing what situation we had and making sure we were really okay took all of the day. So they chose not to drive yesterday," Theisinger said.

Once the communications trouble began, controllers began troubleshooting to narrow the possible cause.

"You really don't try and prejudge until you get to the end of the story," Theisinger said.

"They got a beep from the low-gain (antenna) and that told them the sequence didn't get in, and it also told them that at least at the time of the beep the telecommunications channel was working just fine. And so they thought maybe they had a high-gain antenna pointing problem.

"They commanded a low-gain session and they got that, and so they knew once again the telecommunications was fine. And that comes down with a whole bunch of fault information, which said we were not in fault protection and there were not fault responses running. So that was a whole bunch of good news. The question then became why did the high-gain antenna session not happen?"

Engineers determined the problem had to be a mis-pointing of the high-gain antenna. Since the rover was not in X-band fault, Spirit didn't know the antenna was facing the wrong direction. In addition, controllers determined that Spirit had marked all of the telemetry as sent, meaning the craft thought it had completed a communications session with Earth using the antenna.

"That got them pretty much focusing on thermal pretty quickly. So I don't think they were too alarmed as they walked through the possible scenarios. They got on the trail pretty quickly."

On the upcoming Sol 39 workday at begins Wednesday evening (U.S. time), Spirit will snap microscopic imaging of tiny dunes to its left and then drive upwards of 25 meters on its continuing trek to Bonneville Crater.

Meanwhile, the Opportunity rover remains healthy as it drives along the bedrock outcropping, taking imagery and science data.

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