You have summarized the usual problem with course surveys, Harry. Filling them out every 3 weeks only brings more attention to the process. To be effective, students have to believe that it is confidential and there won't be any retribution for negative comments. They also have to see that it is a useful tool - that something will be done if comments are given. Too often students complain about poor teaching practices but don't see anything done to improve the instructor's performance. Have you considered actually meeting with students for 15 minutes to get their thoughts? Again, confidentiality is critical as is an honest response to their concerns. If you are unwilling or unable to take action, you must tell them that and explain why. You must always be ready for any answer to the questions you ask.