Warranty Accrual (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Oct. 31, 2012	Oct. 31, 2011	Oct. 31, 2012	Oct. 31, 2011	Apr. 30, 2012	Apr. 30, 2011
Warranty claim activity
Beginning accrued warranty balance			$ 1,400	$ 2,300	$ 1,300	$ 1,800
Provision	(175)	(184)	24	(176)
Costs incurred	(125)	(216)	(424)	(724)
Ending accrued warranty balance	$ 1,000	$ 1,400	$ 1,000	$ 1,400	$ 1,300	$ 1,800