The former chief executive of Bixby Energy Systems, Bob Walker, will appear before a federal magistrate judge this afternoon to answer allegations of witness tampering in an investment fraud case.

Walker, 70, of Anoka, is best known for starting Select Comfort Corp. and inventing its signature product, called the Sleep Number bed. But he left that firm in the 1990s and later became involved in Bixby, a coal technology venture that in 2011 became embroiled in allegations of investor manipulation.

Earlier this month, Walker was arrested and accused of trying to raise money for the venture in the past year despite the investment fraud case pending against him.

Walker's lawyer, Peter Wold of Minneapolis, said earlier this week that the arrested was an "abuse of power" and he will contest every allegation "with full force."

Walker has been in custody pending the hearing Friday afternoon before U.S. Magistrate Judge Tony Leung in St. Paul.

In a complaint filed Monday, Walker is accused of using at least three "unwitting" shareholders to continue directing Bixby and its board of directors even though he was forced out in early 2011 and had been charged with defrauding about 1,800 shareholders who invested more than $40 million.

Walker's activities apparently involved a lot of e-mail, according to the 16-page affidavit by FBI special agent Jared Kary.

In some, Walker acknowledged that his lawyer advised him to stay out of Bixby's business and that his friends would have to be ­Bixby's public face.

In other e-mails, Walker allegedly discusses soliciting at least $2,000 from his brother-in-law for Bixby activities. He also allegedly discussed how a prospective investor named Bharat Katari had been poised to invest about $100 million in the company.

The FBI accuses Walker of using the three unwitting Bixby shareholders to pass information via e-mails to a principal of Global Partners United, a company in Beijing that had a contract to market Bixby's technology in Asia.