TYNGSBOROUGH, Mass. — The knock on Brian Hart's door came at 6 a.m. An Army colonel, a priest and a police officer had come to tell Hart and his wife that their 20-year-old son had been killed when his military vehicle was ambushed in Iraq.

Brian Hart didn't channel his grief quietly. Committed to "preventing the senseless from recurring," he railed against the military on his blog for shortcomings in supplying armor to soldiers. The one-time Republican teamed with liberal Sen. Edward Kennedy to tell Congress that the Pentagon was leaving soldiers ill-equipped.

And then Hart went beyond words to fight his cause. He became a defense contractor.

He founded a company that has developed rugged, relatively inexpensive robotic vehicles, resembling small dune buggies, to disable car bombs and roadside explosives in hot spots like Iraq and Afghanistan.

Now, Hart has won over the military brass he criticizes. Three years after starting Black-I Robotics Inc., Hart and his four employees won a $728,000 contract from the Pentagon in June to further develop the "LandShark" robot.

Technology to protect troops is a subject uncomfortably close to home for Hart, who says the death of his son, Army Pfc. John Hart, left him in "total devastation."

Brian Hart can't forget the call he got from his son in Iraq a week before he was killed by a gunshot Oct. 18, 2003.

"He asked me to help him: 'Get us body armor and vehicular armor,' " Brian Hart said. "He thought he'd be killed on the road in an unarmored Humvee. And a week to the day later, he was."

The Pentagon contract requires Black-I to supply three of its six-wheeled, electric-powered vehicles this year and provide support.

The military will test two units, while Boston's Logan Airport will get one for bomb-disposal duties. If tests go well, soldiers in Iraq could be using the robots as soon as next year, Hart said.

His company also is trying to secure an additional $1.5 million in Pentagon funding next fiscal year.

At 275 pounds and about 4 feet long, Black-I's LandShark looks like a dune buggy without a seat for a human driver. Hart hopes to make them available for commercial sale to law enforcement next year, with expectations that the cost would be $65,000 to $85,000 per robot, including the chassis and add-on bomb-disposing equipment. The vehicle can pull tilling equipment to plow up soil where an explosive or trip wire may be hidden.

Or it can drop off "disrupters" that can be maneuvered near a bomb and set off, with jets of water disabling the bomb.

Hart contends LandSharks will be far less expensive than many of the Pentagon's current bomb-disposing robots, including models made by two larger Boston-area companies, iRobot Inc. and Foster-Miller Inc. Those models have more sophisticated electronics, but also are more fragile than LandSharks, which use car batteries rather than lighter and pricier lithium-ion batteries.

"We want to make robots affordable, so that a private first class or a lance corporal could get this equipment," Hart said.

While many Pentagon critics, including families of soldiers, have spoken out about better gear for soldiers, Brian Hart stands apart for his decision to launch a company focused on troop protection, said Bill Thomasmeyer, president of the National Center for Defense Robotics. The Pittsburgh-based nonprofit organization helps robotics firms like Black-I compete for government contracts.

"I don't know of any other similar company that is headed by someone who has had such a personal loss as he has," Thomasmeyer said. "His company has had to overcome a lot of obstacles to get to this point, without having a lot of resources."