PEH　(piezoelectric　energy　harvester)　outputs　a　higher　voltage　when　the　natural　frequency　matches　ambient　vibration　frequency.　How　to　widen　working　frequency　band　is　a　hot　issue　in　study　of　PEH.　In　this　paper,　an　auto-tuning　PEH　is　presented　by　attaching　a　box　to　each　corner　of　its　free　sides　and　a　ball　is　placed　in　each　box.　Change　of　the　balls　position　makes　natural　frequency　of　PEH　variable,　which　expands　working　bandwidth　of　the　device　to　accommodate　complex　ambient　vibration.　Fixing　the　equipment　on　experimental　platform　by　designing　fixture,　hammering　experiments　are　carried　out　to　measure　natural　frequency　of　the　structure　when　balls　are　in　different　positions.　It　is　found　that　the　maximum　and　minimum　natural　frequencies　exist　in　the　auto-tuning　PEH,　which　form　a　resonance　bandwidth.　Moreover,　there　are　multiple　position　combinations　of　two　balls　at　the　same　natural　frequency　in　the　range　of　resonance　bandwidth.　Fixed　the　apparatus　on　exciter　by　the　fixture,　and　a　constant　excitation　frequency　is　selected　in　the　bandwidth　to　perform　auto-tuning　experiments　on　the　instrument.　It　is　observed　that　location　of　balls　is　automatically　changed　to　make　natural　frequency　of　the　system　coincide　with　the　excitation,　and　the　law　of　variation　in　balls　position　is　obtained.　The　converter　is　subjected　to　sweeping　experiments　through　an　exciter　and　an　oscilloscope.　Using　voltage-frequency　diagrams,　it　is　shown　that　the　auto-tuning　PEH　outputs　a　higher　voltage　and　its　frequency　band　is　wider　when　the　ball　rolls　freely　compared　to　two　cases　where　balls　are　fixed.　©　2019　IOP　Publishing　Ltd.　All　rights　reserved.