สร้าง Trigger บน MySQL แล้ว Error Unknown column 'col_name' in 'NEW'

DELIMITER $$
CREATE TRIGGER `update_data` AFTER UPDATE on `tb_partctrl`

FOR EACH ROW

BEGIN
	DECLARE col_name CHAR(10);
	DECLARE num_rows int(4);
	DECLARE	i int(4);
	DECLARE col_names CURSOR FOR SELECT COLUMN_NAME FROM INFORMATION_SCHEMA.COLUMNS  WHERE table_name = 'tb_partctrl';
	
	SELECT FOUND_ROWS() into num_rows;

	SET i = 1;
	the_loop: LOOP

			IF i > num_rows THEN
					CLOSE col_names;
					LEAVE the_loop;
			END IF;

			FETCH col_names INTO col_name;     

				IF NEW.col_name <> OLD.col_name THEN  //มันบอกว่าไม่รู้จัก col_name ค่ะ
						INSERT INTO tb_hispartctrl (modifydate, Fieldchange, Oldvalue, Newvalue) VALUES (NOW(),col_name,NEW.col_name,OLD.col_name);
				END IF;
				
			SET i = i + 1;  
	END LOOP the_loop;

END$$

DELIMITER ;