WORCESTER — A former Cambridge police officer was placed on probation for four years Monday after admitting to robbing an 86-year-old Southboro man he followed home from an ATM machine. Timothy Caulfield, 49, received the sentence in Worcester Superior Court after pleading guilty to charges of unarmed robbery of a victim 60 or older and assault and battery on a person 60 or older. Authorities said Caulfield, on Jan. 2, followed an 86-year-old man home from a Framingham ATM and accosted him in his Southboro driveway. They said Caulfield robbed the man by taking his wallet and cellphone from his pocket. He also threatened the man, police said, but did not hurt him.