There is a very simple solution. Create and RSS feed with the pages that are changing.
That is much better than pinging or an xml sitemap. However, Google does read your sitemap only with the frequency it deserves. So if you have bursts of changes, lets say you add ten pages in two days and then nothing for three weeks and then 5 pages in two days. Chances are Google figures a daily check or every three days is enough. However if you add about three pages a day, almost every day, it might check the RSS feed every two hours (especially if these are new pages and not just changes to existing ones.)
If you then want to also ping Google, read the
Google Ping URI. But this needs a small program and programming knowledge.
Good luck
K<o>