Problem calling Postgres function
Define the table name parameter as text and then pass a string:
create or replace function field_summaries(gap interval , _tbl text, _col text)                                
returns table(hour text, average numeric, minimum numeric, maximum numeric) as
$func$   
begin
 RETURN QUERY 
  EXECUTE format('select time_bucket($1, time)::text as hour,                                                                                      
                         avg(%I), 
                         min(%I),
                         max(%I)                                                                                                         
                  from %I d                                                                                                         
                  where d in (select device_id from sensors)                                                                                           
                  group by hour', _col, _col, _col, _tbl)
     USING gap; 
END;
$func$ 
language plpgsql
stable; 

Then call it like this:
select *
from field_summaries('5minutes', 'm_09', 'current');