Property, Plant and Equipment - Summary of property, plant, and equipment (Details) - USD ($) $ in Thousands	Mar. 30, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	$ 46,441	$ 43,203
Less: accumulated depreciation and amortization	14,580	12,676
Property, plant and equipment, net	31,861	30,527
Manufacturing equipment
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	28,866	25,314
Research and development equipment
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	7,111	6,088
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	7,166	7,080
Capital leases
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	883	882
Software
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	125	60
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	203	195
Vehicles
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	210	210
Assets not yet placed in service
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	$ 1,877	$ 3,374