Discontinued Operations (Details)			12 Months Ended
	Feb. 22, 2019 USD ($)	Jun. 29, 2018 USD ($)	Apr. 30, 2021 USD ($)	Apr. 30, 2020 USD ($) contract	Apr. 30, 2019 USD ($)	Apr. 30, 2018 USD ($)
Discontinued operations
Amount of alleged damages	$ 6,500,000
Statement of operations
Net loss from discontinued operations					$ (2,964,000)	$ (3,887,000)
(Loss) gain on sale of business, net of tax (benefit) expense of $(76) and $2,444 for the year ended April 30, 2020 and April 30, 2019, respectively				$ (265,000)	8,490,000
Tax expense				(76,000)	2,444,000
Net (loss) income from discontinued operations				(265,000)	5,526,000	(3,887,000)
EES Business | Disposed of by sale
Discontinued operations
Cash consideration received		$ 31,994,000
Gain on sale of business		11,420,000			(486,000)
Working capital dispute		$ 922,000
EES Business | Disposed of by sale | Other income (expense)
Statement of operations
Net Sales				551,000	2,758,000
EES Business | Discontinued Operations
Statement of operations
Net Sales					4,256,000	38,411,000
Cost of sales					5,097,000	33,384,000
Gross margin					(841,000)	5,027,000
Selling, general and administrative					1,515,000	7,825,000
Research and development					1,072,000	3,526,000
Other (expense), net					(1,000)	(27,000)
Loss from discontinued operations before income taxes					(3,427,000)	(6,351,000)
Benefit for income taxes					(463,000)	(2,464,000)
Net loss from discontinued operations					(2,964,000)	(3,887,000)
(Loss) gain on sale of business, net of tax (benefit) expense of $(76) and $2,444 for the year ended April 30, 2020 and April 30, 2019, respectively			$ (265,000)		8,490,000
Tax expense				(76,000)	2,444,000
Net (loss) income from discontinued operations			$ (265,000)		$ 5,526,000	$ (3,887,000)
Holdback | Disposed of by sale
Discontinued operations
Cash consideration received				$ 6,500,000
Number of remaining contracts | contract				2