Robb, 57, testified Monday that he argued with his wife about a trip she and their daughter were taking over the holiday break. He did not want the 12-year-old to miss any school.
"We started a discussion about that. The discussion was tense," Robb said. "We were both anxious about it. We both got angry. At one point, Ellen pushed me. ... I just lost it."
Robb said he picked up the chin-up bar, which was lying nearby, and struck his wife with it repeatedly.
He later threw the weapon in a trash bin in Philadelphia and tried to make their home look as if it had been burglarized. Detectives were suspicious from the start, though, because the scene was poorly staged and nothing was missing.
The 49-year-old homemaker was found dead in the kitchen, near the partially wrapped presents.
Robb's trial had been scheduled to start Monday. He could have faced a life sentence if convicted of first-degree murder, but prosecutors felt there were no guarantees given the circumstantial evidence.
The professor pleaded guilty to one count of voluntary manslaughter, which is defined as an intentional, unlawful killing, with provocation, in the heat of passion.
Montgomery County prosecutor Bruce Castor called the case "a classic heat-of-passion killing."
Robb adored his daughter Olivia and feared he would see less of her in a divorce, both sides agreed.
University spokesman Ron Ozio said a Penn official spoke to Robb's lawyer after Monday's hearing and asked for his resignation.
Ellen Robb's brothers, Art Gregory of Haddonfield, N.J., and Gary Gregory of Boston, said their sister suffered verbal abuse throughout the marriage, eroding her self-esteem.
"What kept them there was their undying love for their daughter Olivia," said Art Gregory, who is now raising the girl. "Both of them put Olivia first, beyond anything else, unfortunately to a very tragic end."
Rafael Robb apologized to Olivia, who was not in court, and said he was "very remorseful."
"I know she liked her mother. ... And now she doesn't have a mother," he said, stifling tears.
Robb, who has been held without bail, talked to his daughter by phone over the weekend and admitted that he was responsible for her mother's death. They have not seen each other since his arrest in January.
Sentencing will likely take place in a few months. Guidelines call for a prison term of 4 1/2 to seven years, but Castor said the statute allows for anything from probation to 10 to 20 years.
